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ADVANCES IN

AGRONOMY VOLUME 42

This Page Intentionally Left Blank

ADVANCES IN

AGRONOMY Prepared in Cooperation with the AMERICAN SOCIETY OF AGRONOMK

VOLUME 42 Edited by N. C . BRADY Science and Technology Agency for International Development Department of State Washington, D . C .

ADVISORY BOARD G. H. HEICHELR. J . KOHEL G . E. HAM E. L. KLEPPER R. H. FOLLEIT D. R. BUXTON E. S. HORNERJ . J. MORTVEDT N . L. TAYLORR. J. WAGENET

R. D. HARTER

ACADEMIC PRESS, INC. Harcourl Brace Jovanovich, Publishers San Diego New York Berkeley Boston London Sydney Tokyo Toronto

COPYRIGHT

0 1989 BY ACADEMICPRESS, INC.

ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMI'ITED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

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ISBN 0-12-000742-8 (alk. paper)

PRINTED IN THE UNITE0 STATES OF AMERICA 8 9 9 0 9 1 9 2

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CONTENTS CONTRIBUTORS. ...................................................... PREFACE.............................................................

ix xi

BIOLOGICAL EFFICIENCIES IN MULTIPLE-CROPPINGSYSTEMS

Charles A. Francis I. 11. 111.

IV. V. VI.

Introduction .................................................... Importance of Multiple-Species Systems .......................... Efficiency of Resource Use by Multiple Species.. ................. Pest Management in Multiple-Cropping Systems. .................. Biological and Economic Stability of Cropping Systems. . . . . . . . . . . . Future Applications for Multiple-Cropping Systems ................ References .....................................................

1

4 7 17 25 35 36

SEED COATINGS AND TREATMENTS AND THEIR EFFECTS ON PLANT ESTABLISHMENT

James M. Scott I. 11. 111.

Introduction ................................ The Seed-Coating Process ....................................... Coatings to Facilitate Planting. . . . . . . . . .

........................................... IV. V . Protective Coatings. ......................... VI. Nutrient Coatings ............................................... VII. Herbicide Coatings ......................... .................................... VIII. Other Coatings.. . . . IX . Treatment Processes. ............................. ......................................... X. References ..................... .............

44 48 53 55 57 61 70 71 73 75 77

CONSERVATION TILLAGE FOR SUSTAINABLE AGRICULTURE: TROPICS VERSUS TEMPERATE ENVIRONMENTS

Rattan La1 1. 11.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conservation Tillage and Sustainable Agriculture ..................

86 89

vi

CONTENTS

I11 . Mulch and No-Till Farming for Different Ecological Environments ........................................ IV . Pros and Cons of the No-Till System: Tropics versus Temperate Zones ................................. V . Noninversion and Minimum Tillage .............................. VI . Subsoiling as Conservation Tillage ............................... VII . Conservation Tillage for Problem Soils ........................... VIII . Why Conservation Tillage? ...................................... IX . Environmental Pollution and Conservation Tillage ................. X . The Systems Approach to Conservation Tillage and Supportive Cultural Practices ..................................... XI . Soil Guide to Conservation Tillage ............................... XI1 . Research and Development Priorities ............................. XI11. Conclusions .................................................... References .....................................................

95

110 117 122 126 140 160

163 177 182 183 185

MICROBIALLY MEDIATED INCREASES IN PLANT-AVAILABLE PHOSPHORUS

R . M . N . Kucey. H . H . Janzen. and M . E . Leggett I . Introduction .................................................... I1. Sources of Plant-Available Phosphate in Soils ..................... Ill . Mycorrhizal Effects on Plant Phosphate Availability . . . . . . . . . . . . . . . IV . Phosphobacterins and Organic Phosphate Mineralization ............ V . Inorganic Phosphate-Solubilizing Microorganisms .................. VI . Sulfur Oxidation and Rock Phosphate-Sulfur Mixtures ............. VII . Future of Technologies .......................................... References .....................................................

199 200 202 207 209 220 222 223

ENZYMOLOGY OF THE RECULTIVATION OF TECHNOGENIC SOILS

S . Kiss. M . Dr5gan.Bularda. and D . PaSca I. I1 . Ill . IV . V. VI . VII . VIII . IX . X.

Introduction .................................................... Technogenic Soils from Coal Mine Spoils ......................... Technogenic Soils from Power Plant Wastes ...................... Technogenic Soils from Retorted Oil Shale........................ Technogenic Soils from Iron Mine Spoils ......................... Technogenic Soils from Manganese Mine Spoils ................... Technogenic Soils from Lead and Zinc Mine Wastes ............... Technogenic Soils from Sulfur Mine Spoils ....................... Technogenic Soils from Lime and Dolomite Mine Spoils ........... Technogenic Soils from Refractory Clay Mine Spoils ..............

230 230 252 253 257 259 260 263 264 264

vii

CONTENTS

XI. Technogenic Soils from Bentonitic Clay Mine Spoils . . . . . . . . . . . . . . XII. Technogenic Soils on Sand Opencast Mine Floor Drift and Spoils.. . XIII. Technogenic Soils from Overburdens Remaining after Pipeline Construction. ...................................... XIV . Recultivation of Soils Remaining after Topsoil "Mining". . . . . . . . . . xv. Concluding Remarks ............................................ References .....................................................

266 267 269 272 272 274

EFFECTS OF NITRIFICATION INHIBITORS ON NITROGEN TRANSFORMATIONS, OTHER THAN NITRIFICATION, IN SOILS

K . L. Sahrawat 1. 11. 111.

IV . V.

Introduction ............... ........................... Effects of Nitrification Inhibitors on Physical and Chemical Processes Relevant to Nitrogen Transformations ............. Effects of Nitrification Inhibitors on Biological Nitrogen Transformations. ........ ............... Other Effects ................................................... Perspectives ............... ........ References .....................................................

279 280 290 303 305 306

COMPACTION EFFECTS ON SOIL STRUCTURE

Satish C. Gupta, Padam P. Sharma, and Sergio A. DeFranchi I. 11. 111.

IV.

Introduction .................................................... Soil Structural Parameters ....................................... Mechanisms of Soil Structure Changes during Compaction ......... Guidelines on Water Contents and Mechanical Stresses Conducive to Lrreversible Changes in Soil Structure. ...............

V. VI . References . . . .

........................ ........................ ........................

31 I 312 328 33 I 335 337 337

TISSUE CULTURE IN RICE IMPROVEMENT:STATUS AND POTENTIAL

Satish K . Raina Introduction .................................................... Embryo Culture. ................................................ Anther, Pollen, and Ovary Culture ............................... IV . Somatic Cell Culture ............................................ I.

11. 111.

339 34 1 34 1 366

...

Vlll

CONTENTS

V . Protoplasts ..................................................... VI . Overview and Strategies for the Future ........................... References .....................................................

378 385 389

BREEDING ANNUAL Medicago SPECIES FOR SEMIARID CONDITIONS IN SOUTHERN AUSTRALIA

E . J . Crawford. A . W . H . Lake. and K . G . Boyce I . Introduction .................................................... Plant Introduction: The Basis for Development .................... 111. Plant Breeding: The Creation of New Genetic Combinations . . . . . . . IV . Preservation and Commercialization .............................. V . Scope of the Future ............................................. References ..................................................... 11.

INDEX ................................................................

399 402 416 431 433 434

439

CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors' contributions begin.

K. G. BOYCE (399), Department of Agriculture, Adelaide, South Australia, Australia E. J . CRAWFORD (399), Department of Agriculture, Adelaide, South Australia, Australia SERGIO A. DEFRANCHI (3 1 l), Instituto di Agronomia, Universita Degli Studi, Della Basilicata, Italy M. DHGAN-BULARDA (229), Department of Plant Physiology, Babes-Bolyai University, 3400 Cluj-Napocu, Romania CHARLES A. FRANCIS (l), Department of Agronomy, University of Nebraska, Lincoln, Nebraska 68583 SATISH C. GUPTA (31 l), Department of Soil Science, University of Minnesota, St. Paul, Minnesota 55108 H. H . JANZEN (199), Agriculture Canada, Lethbridge Research Station, Lethbridge, Alberta TIJ 4B1, Canada S . K I S S (229), Department of Plant Physiology, Babes-Bolyai University, 3400 Cluj-Napoca, Romania R. M . N. KUCEY (199), Agriculture Canada, Lethbridge Research Station, Lethbridge, Alberta T1J 4B1, Canada A. W. H. LAKE (399), Department of Agriculture, Adelaide, South Australia, Australia RATTAN LAL (85), Department of Agronomy, Ohio State University, Columbus, Ohio 43210 M. E. LEGGETT (199), Philom Bios, Inc., Saskatoon, Saskatchewan S7N 2x8, Canada D. PASCA (229), Department of Plant Physiology, Babej-Bolyai University, 3400 Cluj-Napoca, Romania SATISH K. RAINA (339), Biotechnology Centre, Indian Agricultural Research Institute, New Delhi 110012, India K. L. SAHRAWAT (279), International Crops Research Institute for the SemiArid Tropics, ICRISAT Patancheru P. 0.. Andhra Pradesh 502 324, India JAMES M. SCOTT (43), Department of Agronomy and Soil Science, University of New England, Armidale, New South Wales 2351, Australia PADAM P. SHARMA (31 I), Department of Soil Science, University of Minnesota, St. Paul, Minnesota 55108

ix


PREFACE

This volume continues the broad subject focus that is typical of Advances in Agronomy. One review covers the use of a series of cell and tissue culture techniques to improve rice characteristics such as insect and disease resistance, cold tolerance, and lodging. The potential of annual Medicagos for semiarid areas such as those in West Asia and North Africa is reviewed, with special emphasis given to tolerance of low pH and low nutrient contents, especially phosphorus. The influence of soil microorganisms on the soil-plant cycling of phosphorus is the subject of one chapter. Mechanisms for phosphate solubilization by fungi and bacteria receive the most attention. The effect of nitrification inhibitors on nonnitrification activities such as movement and loss of nitrates from soils, ammonia fixation and volatilization, and denitrification is also reviewed. An excellent and comprehensive review of conservation tillage for tropical and temperate climates is the subject of another contribution. It should be of special interest to scientists in the developing countries. The subject of sustainable agriculture also receives attention in a fine review of multiple cropping opportunities in temperate regions. The effect of soil compaction on the relationship between macroscopic and microscopic soil structure parameters is reviewed, along with mechanisms by which microscopic soil structure changes with compaction. The potential of enzymatic activity to indicate the evolution of mine spoils and plant wastes into agricultural and forest soils is covered, and refers to work in countries with developed market economies and those with centrally planned economies. A final contribution provides an excellent review of the influence of coating seeds with various chemicals and adhesives, and other coating materials, on factors such as seedling establishment, legume inoculation, and protection from disease, insects, pests, and weeds. We are indebted to scientists from six countries for these reviews, a fact that again emphasizes the international significance of agronomy. N. C . BRADY

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ADVANCES IN AGRONOMY, VOL. 42

BIOLOGICAL EFFICIENCIES IN MULTIPLE-CROPPING SYSTEMS' Charles A. Francis Department of Agronomy University of Nebraska Lincoln, Nebraska 68583

1. Introduction 11. Importance of Multiple-Species Systems A. Historical Background B. Pasture Systems C. Grain and Root Crop Systems 111. Efficiency of Resource Use by Multiple Species A. Efficiencies in Time and Space B. Light Use C. Water Use D. Nutrient Use IV. Pest Management in Multiple-Cropping Systems A. Weed Management B. Insect Management C. Plant Pathogen Management V. Biological and Economic Stability of Cropping Systems A. Variations in Biological Output B. Income Stability C . Buffering and Compensation in Systems D. Statistical Analysis of Multiple-Species Systems VI. Future Applications for Multiple-Cropping Systems References

I. INTRODUCTION Biological complexities and interactions in multiple-species cropping systems present an interesting challenge to scientists who work to improve system productivity. A number of efficiencies in resource use become operative when two or more crops are present in the same field during 'This article is a contribution from the Department of Agronomy at the University of Nebraska, Lincoln, Nebraska 68583.

I Copyright 0 1YR9 by Academic Press. Inc. All rights of reproduction in any form re5erved.

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CHARLES A. FRANCIS

the same year, and these can be most complex when crops are grown simultaneously. Such interactions have been called the “integration efficiencies” of cropping systems (Hanvood, 1984). Different types of weed, insect, and plant pathogen relationships with crops may occur when the cropped field is not planted to one homogeneous species (Altieri and Liebman, 1986). There is both biological and economic buffering in systems in which there is production of more than one crop in the field (Lynam et al., 1986). Information about these biological efficiencies can lead to management options that differ from those in monoculture agriculture, and the increasing literature on multiple cropping is worthy of review (Francis, 1986). An early review by Aiyer (1949) described the principal mixed cropping systems in India. Key papers on the nature of competition and resource use were published by de Wit (1960) and Donald (1963). Another major review of cropping patterns appeared in the book “Multiple Cropping,” which included the 1975 Symposium papers from the American Society of Agronomy (Papendick e f al., 1976). Since then several symposia have been sponsored by international centers or bilateral aid organizations, for example, by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) (1981) in India and by the United States Agency for International Development (USAID) in Tanzania (Keswani and Ndunguru, 1982). Most of these papers and reviews are descriptive in nature, based on empirical observations from existing cropping systems or data from relatively simple experiments on component technology. The reviews by Willey (1979a,b) presented excellent descriptions of previous research and provided some of the first insight on how the information then available could be used to explain biological interactions in complex cropping systems. Since then several books have brought more of the sometimes obscure data for several regions of the developing world to light. These include the publications of Beets (19821, Steiner (1982), Gomez and Gomez (1983). and Francis (1986). There now exists a body of information on which to base some generalizations about multiple-species systems and to use for developing recommendations and management tools. Some confusion exists in the terminology used by authors to describe multiple-cropping systems. An attempt to standardize the terms and their usage was made at the 1975 Symposium, and the definitions published in the proceedings of the conference (Andrews and Kassam, 1976) are helpful. With much of the field research published in languages other than English (notably Spanish and French), there is more confusion brought into discussions of these complex systems. Table I presents a series of definitions that were published recently (Francis, 1986) and that form the common language for the discussion that follows.

3

MULTIPLE-CROPPING SYSTEMS Table I Definitions and Terminology in Common Usage with Multiple-Cropping Systems"

Miiltiple cropping: the intensification of cropping in temporal and spatial dimensions; growing two or more crops on the same field in one year Seqiienticil cropping: growing two or more crops in sequence on the same field per year; the succeeding crop is planted after the preceding crop has been harvested; crop intensification is only in the time dimension; there is no intercrop competition Double cropping: growing two crops per year in sequence Triple cropping: growing three crops per year in sequence Quadruple cropping: growing four crops per year in sequence Ratoon cropping: cultivation of crop regrowth after harvest, although not necessarily for grain Intercropping: growing two or more crops simultaneously on the same field; crop intensification is in both the temporal and spatial dimensions; there is intercrop competition during all or part of crop growth Mixed intercropping: growing two or more crops simultaneously with no distinct row arrangement Row intercropping: growing two or more crops simultaneously; one or more crops are planted in rows Strip intercropping: growing two or more crops simultaneously in different strips wide enough to permit independent cultivation but narrow enough for the crops to interact agronomically Relay intercropping: growing two or more crops simultaneously during part of the life cycle of each; a second crop is planted after the first crop has reached its reproductive stage of growth but before it is ready for harvest Cropping index: number of crops grown per year on a given area of land x 100 Cropping pcrttern: yearly sequence and spatial arrangement of crops or crops and fallow on a given area Cropping system: cropping patterns used on a farm and their interactions with farm resources. other farm enterprises. and available technology that determine their makeup Lnnd eqrrii~ulentrrrtio (LER): ratio of the area needed under sole cropping to that under intercropping at the same management level to give an equal amount of yield; LER is the sum of the fractions of the yields of the intercrops relative to their sole crop yields Monocriltitre: repetitive growing of the same sole crop on the same land Rottition: repetitive cultivation of an ordered succession of crops (or crops and fallow) on the same land; one cycle often takes several seasons or years to complete Sole cropping: one crop variety grown alone in pure stand at normal density; synonymous with solid planting; opposite of intercropping Agrisili~ic.rrltrire:growing of trees for timber but with cultivated crops grown beneath Competition effect: competition of intercropped species for light, nutrients, water, CO,. and other growth factors Complementarv effrct: effects of one component on another that enhance growth and productivity. as compared to competition Componcnt crops: individual crop species that are a part of the multiple-crop system Component technology: procedure for growing each component crop fnterplanting: all types of seeding or planting a crop into a growing stand; used especially for annual crops grown under stands of perennial crops Overyielling: production of component crops in an intercrop that is higher than the sum of appropriate monoculture crops; this is indicated by an LER greater than unity (continued)

4

CHARLES A. FRANCIS Table I (Continued)

simultaneous growth of two or more useful plants on the same plot: this includes mixed cropping, intercropping, interculture, interplanting, and relay planting Spufial urrungement: physical or spatial organization of component crops in a multiplecropping system

Simultaneous polyculture:

“From Tables 1.2, 1.3, and 1.4 in Francis (1986).

II. IMPORTANCE OF MULTIPLE-SPECIES SYSTEMS In order to understand how multiple-species systems function in both crop and pasture mixtures, it is valuable to briefly examine the history of multiple cropping. A wide range of species mixtures are currently used in both temperate and tropical regions, and they have evolved through conscious intervention of farmers to meet their goals of producing food and income. Much of this development was due to trial and error, but careful observation of what was successful undoubtedly led to many of the patterns that are seen today in mixed cropping systems (Plucknett and Smith, 1986). Both the history and prevalence of multiple-cropping systems are explored here.

A. HISTORICAL BACKGROUND Early cropping systems were certainly mixtures of desirable species used for food, fiber, and other needs in the community. Plucknett and Smith (1986) describe six stages in the evolution of crop domestication over the past 10,000 years, a process that moved at different rates and reached different stages around the globe (Table 11). Monocropping is a relatively recent innovation in agriculture. From gathering and later protection of preferred plants to gardening and subsistence farming, mixtures of crops have emulated to some degree the natural ecosystem and its diversity. Several reviews have described the evidence for plant diversity in these early systems (Baker, 1970; Hawkes, 1970; Sauer, 1947). Authors have also described the relative stability that this diversity brings to the natural ecosystem, although there is not total agreement in this area (Goodman, 1975; Hall, cited in Smith and Francis, 1986). Multispecies cropping systems probably began in the tropics and today are generally more diverse in the lowland cultivated areas in these regions.

MULTIPLE-CROPPING SYSTEMS

5

Table 11 Crop Domestication Stages from Gathering to Commercial Farming“ Stage

Characteristic

Gathering Protection of preferred plants

Wild plants in native stands Wild plants in native stands; volunteer plants around camps and along trails Transplanted seedlings, roots, cuttings of wild plants, planting of seed crops Trees, shrubs, herbs, and grasses, usually grown in polycultural assemblages under shifting agriculture conditions Polyculture common in tropics, less so in temperate areas; cash crops often grown in separate fields With tropical tree crops, polyculture still common, but trend is toward monocropping

Gardening Subsistence farming

Subsistence and cash cropping

Commercial farming “From Plucknett and Smith (1986).

Diversity of cultivated systems declines with higher altitudes in the tropics and with increasing latitude (Harris, 1976). Mixtures of species were chosen by farmers over the centuries to make use of rainfall and native soil fertility, and empirical choices of patterns were made among the bestperforming combinations observed. Plucknett and Smith (1986) describe the components of these systems in Africa, Asia, and the Americas. Many multiple-cropping systems persist today on farms on which resources are limited and the level of new technology is low. Intensive cropping systems, often with mixtures of species, have reached high yield levels through the use of pesticides, improved cultivars, and other high-input technology in countries such as China, Taiwan, the Philippines, and Thailand.

B. PASTURESYSTEMS The vast areas grazed by indigenous animals over millenia and the large expanses of rangeland used for domesticated cattle, sheep, and goats are actually important “intercropping” systems (Gomm el al., 1976). Native grasslands and mixed timber-grass ranges in the western United States are typical ecosystems that have been managed for grazing ruminant animals €or more than a century. These lands have been exploited through ruminants because they cannot be economically cropped in other ways due to low rainfall, topography, or fertility limitations. There has been serious depletion of the grazing capacity of some areas due to lack of

6

CHARLES A. FRANCIS

understanding of the biology of the ecosystem and how management of animal numbers should fit the productivity of the range (USDA, 1936, 1941). Some range lands have been renovated by controlling unwanted, nonedible plant species (Herbel et al., 1973; Clary, 1974) and thus increasing the carrying capacity of the land (Cook, 1966). Judicious application of fertilizers can shift the species balance to one that is more desirable for grazing animals (Rogler and Lorenz, 1957). Interseeding cool season grasses into the range can increase productivity with good precipitation (Wight and White, 1974), and planting forbs and browse species can also improve the carrying capacity of high mountain valleys (Plummet-, 1968). These are adaptations of the natural, diverse ecosystem, which has a capacity to buffer dry matter production through a series of dissimilar seasons during which rainfall and temperature are unpredictable. Yet relatively little is known about how to fine-tune these systems due to limited knowledge about their biological interactions. Seeded pasture mixtures of grasses and legumes provide the basis for sustainable forage production for ruminants in temperate regions. Research on these mixtures has provided the majority of information on interspecific competition and complementarity (de Wit, 1960; Donald, 1%3). Both intraand interspecific competition are intense in high densities of this mixed intercropping situation. One of the best known systems combines white clover (Trifolium repens L.) with various grass species (Chestnutt and Lowe, 1970). Although initial planting densities of components influence their populations, later management including height of cutting, frequency of cutting or grazing, and amount of fertilizer applied have a greater influence on the eventual relative proportions of these perennials. Mixtures of legumes and cereals were also important for forage production in the temperate zone before the use of high levels of chemical fertilizers. Both the advantages and drawbacks of these systems have been reviewed (Brown, 1935; Haynes, 1980; Nicol, 1934, 1936; Wilson, 1940). Results from maizedat and maize-soybean mixtures for fodder showed the former to be highly productive but low in protein, and the latter to have potential for both high tonnage and protein content (Mason and Pritchard, 1987). More details on competition for resources are given in later sections.

c. GRAINANDROOTCROP SYSTEMS Multiple cropping of cereals, grain legumes, and root crops forms the basis of farming systems for many subsistence farmers in the developing world. Annual crops planted with perennials are another type of intercropping that is common in some lowland and medium-elevation farming regions. It has been estimated that high proportions of basic cereals are


7

produced in multiple-cropping systems in many parts of the world (Francis, 1986). This includes 76% of maize, 90% of millet, 95% of peanut, and 99% of cowpea in Nigeria (Okigbo and Greenland, 1976); 84% of maize, 56% of peanut, 81% of beans, and 76% of pigeon peas in Uganda (Jameson, 1970); and 90% of beans in Colombia, 80% of beans in Brazil, and 60% of maize in all of the Latin American tropics (Francis et al., 1976). Reviews have summarized the agronomic literature on cereals (Rao, 1986) and legumes and starchy roots (Davis et al., 1986) in multiple-cropping systems. Most of the results found in published form are based on empirical field studies, and there are limited definitive results on competition and resource use. The recent books on multiple cropping focus primarily on cereals, grain legumes, and root crops in complex cropping systems (Beets, 1982; Francis, 1986; Gomez and Gomez, 1983; Steiner, 1982).

Ill. EFFICIENCY OF RESOURCE USE BY MULTIPLE SPECIES Agronomic studies over the past three decades have led to descriptions of a number of distinct relationships between and among crop species grown in mixture. Results have most often been expressed in final grain yield and on occasion total dry matter; much less often have measurements been made of light, water, or nutrient use. Yet final component crop yield is one valid indicator of the integrative success through the entire growing season of that component crop in competing for scarce growth resources in the specific environment or cropping pattern. There is a general agreement that when interspecific competition for a given limiting factor is less than intraspecific competition among plants for that same factor, there is a potential for “overyielding,” or higher total production in the intercrop pattern (Andrews, 1972; Willey, 1979a). A number of models and terms have been developed to describe this partitioning of inter- and intraspecific competition (for example, Hart, 1974; Hill and Shimamoto, 1973; Trenbath, 1975). Willey (1979a) attempted to simplify the types and final results of competition in two-crop systems by defining three broad categories of interactions using expected yields as those which would occur if inter- and intraspecific competition were equal: 1. Mutual inhibition, in which the actual yield of both species is less than expected. This is a rare occurrence in the field and only a few cases have been reported (Ahlgren and Aamodt, 1939; Donald, 1946; Harper, 1961). 2. Mutual cooperation, in which each species produces more when two crops are planted together than when planted alone; this can occur more

8

CHARLES A. FRANCIS

frequently at low levels of technology and when crop densities are relatively low and suboptimal. 3. Compensation, in which one crop produces more and the other produces less than expected; the most common situation, this relationship involves a more competitive (dominant) crop and a less competitive (dominated) crop in the mixture (Huxley and Maingu, 1978). A number of different models for compensation were presented and described by Willey (1979a), most based on the replacement series methodology, which compares a series of density combinations ranging from a pure stand of species A through various mixtures to a pure stand of species B. These models help to describe the gross effects of competition and compensation but do not shed much light on the internal interactions in the system that lead to final yields. IN TIMEAND A. EFFICIENCIES

SPACE

Conceptually, there is agreement that some complementarity between species in mixed cropping must occur if there is to be an advantage in yield, or overyielding, from the intercrop pattern. This complementarity could be in the temporal dimension or in the temporal and spatial dimensions (Francis, 1986; Trenbath, 1976, 1986; Willey, 1979a). Efficiency in the temporal dimension only is illustrated by double-cropping systems used in the Southeast United States or in tropical regions with a bimodal rainfall pattern. Lewis and Phillips (1976) reviewed the literature on double cropping in the United States, especially systems involving summer soybeans and winter small grains. China has used double cropping for centuries to increase food production from a scarce land resource (Xian and Lin, 1985). An intensive double-cropping pattern is favored in areas with a long growing season, sufficient rainfall for the two crops, favorable markets for both crops, and potential to use minimum or zero tillage to establish the second crop into the residue of the first. These practices save both time and soil moisture (Wicks, 1976). Planting maize with a subsequent forage crop has potential for the Southeast (Widstrom and Young, 1980). Growing maize as a double crop with legumes for green manure is another valuable option (Duncan, 1980; Smith and Prine, 1982),especially if there is a potential for seeding maize directly into legumes or their residues (Robertson et al., 1976). These systems are similar to both relay and ratoon cropping, two patterns which combine time- and space-related efficiencies. Relay cropping means that a significant part of the life cycle of the second crop overlaps with the cropping cycle of the first crop. Specific and intensive modifications of this scheme include seeding small grains


9

into soybeans before the harvest of the latter (Clapp, 1974);relay planting peanuts, sweet potatos, or soybeans into corn (Akhanda er al., 1977);and the intensive three-crop relay intercropping of potatoes, maize, and climbing beans in the Central Highlands of Colombia (Francis, 1988).Another variation is ratoon cropping, in which a crop is allowed to regrow from the crown after one grain or forage harvest. Common examples include alfalfa, sorghum-Sudan hybrids for forage, and even ratoon cropping of grain sorghum (Duncan, 1983). Additional examples of overlapping crop growth cycles come from tropical areas, where the potential growing season may be the entire year, provided water is available. Norman (1974) describes a number of systems in northern Nigeria in which crops of different growth duration are planted together to take advantage of intermittent and unpredictable rainfall. On the south coast of Guatemala, farmers plant sesame, soybeans, or a second maize crop into the main crop of maize at about flowering time; this second crop takes advantage of residual moisture and late rains to produce a cash crop. Sorghum and maize are planted together in parts of Central America, and maize is harvested in 3-4 months, whereas the tall sorghum grows slowly at first and then stretches upward after the maize harvest, using residual moisture and fertility. When there is enough overlap in time to cause at least some competition in resource use or cause modifications in management procedures, these are called relay systems. The most intensive use of time and space occurs with the simultaneous or near-simultaneous plantings of two or more crops. Detailed growth analysis and measurement of resource use are being used to broaden the knowledge base on competition and productivity (for example, Clark and Francis, 1985a, b). This type of intercropping has received increased attention during the past two decades, and much of the work of Willey, Rao, Baker, Okigbo, Andrews, Gomez, Harwood, Trenbath, Francis, and others has been summarized in the recent book “Multiple Cropping Systems” (Francis, 1986). Most of the subsequent discussion centers on this form of intensive intercropping, and many examples are used to describe its efficiency and biological interactions.

B. LIGHTUSE The reviews of light use efficiency by Willey (1979a) and Trenbath (1976) are especially useful. Unlike rainfall and nutrients, solar energy cannot be captured and stored for later use in the way that other natural resources are managed; light is “instantaneously available” and needs to be “instantaneously intercepted and used” if this resource is going to be useful to produce photosynthate and plant dry matter (Donald, 1961). Competition

10

CHARLES A. FRANCIS

for light is really between leaves rather than between plants, and a leaf that receives light below the compensation point (level needed for photosynthesis) will soon perish (Etherington, 1976). Plants that are favored in the mixture are not necessarily those with the most leaves and foliage, but those with leaves in the best position to intercept solar radiation. There are both temporal and spatial ways in which multiple-cropping systems use light more efficiently than single crops. If water and nutrient requirements of crops are met, then light is most frequently the limiting resource. Both photosynthesis and plant growth of each component crop will be proportional to the amount of radiation that component intercepts (Trenbath, 1976). Double cropping, which includes two crops in the field in a sequential pattern, provides opportunity for much greater temporal interception of total radiation through the year compared to any single crop, unless that one crop has an extremely long growth cycle. Even long-term crops such as cassava or photoperiod-sensitive sorghum may initially grow slowly and establish full canopy only after several months, opening an opportunity for a short-cycle crop to be grown between rows of the longer-cycle component. Sugar cane also grows slowly in its first several weeks, and some intensive cultivation schemes include beans, cowpeas, soybeans, or maize between the rows of the plant crop (first planting of cane) or ratoon crop. One study from South Africa showed reduced tiller emergence and lower leaf area of cane grown with intercropped beans, but after two seasons there was no difference in final yield of cane or sucrose (Leclezio et a/., 1985). It is obvious that crops that cover the ground over more of the year are going to intercept more light and thus produce more total dry matter than a single crop of shorter duration. Willey and Roberts (1976) concluded that light energy was often the most important factor in overyielding by crop mixtures that exhibited temporal complementarity. How much light is intercepted over the entire growing season is primarily a function of leaf area duration (LAD) of one or more crops developing in the canopy. Willey et a/. (1983) illustrated with a sorghum-pigeonpea intercrop the relative efficiency of light interception and dry matter production compared to monocrops. With two rows of sorghum alternating with one row of pigeonpeas, all at full population in the row, sole-cropped sorghum and sorghum-pigeonpea intercepted about the same total light and produced the same dry matter (Fig. 1). At sorghum harvest, intercrop grain yield was only about 5% below that of sole crop, probably due to the high density of sorghum (Natarajan and Willey, 1981). After sorghum harvest, continued pigeonpea growth led to eventual dry matter production that was 53% of sole-crop pigeonpea, and a higher harvest index resulted in a grain yield that was 72% that of sole-crop pigeonpea. Because more of the total growing season was used and more total radiation was inter-


1000 *

9 800' E

-

m

5 600-

!i>

400.

b

200

lo

'

,.**' ..--

20

40

60

80

100 120 140 160

Days from sowing

FIG. 1. Dry matter accumulation and light interception in sorghum and pigeonpea sown as sole crops and as a two-row sorghum: one-row pigeonpea intercrop. Values are means of 3 years. (From Willey er al., 1983.)

cepted, only 5% was lost from the sole-crop sorghum yield, and 72% of the potential sole-crop pigeonpea grain yield was obtained from the same field. Intercrops have potential to intercept light in different ways than sole crops. Changing the spatial organization of plants to achieve greater efficiency of light interception and conversion to dry matter is more complex. To achieve optimal (high) levels of light interception, it is possible to increase plant density of a monoculture crop. There is little potential for further increasing interception by mixing crop species (Willey and Roberts, 1976). Although intercrops often intercept a higher proportion of total radiation than low densities of sole crops frequently found on subsistence farms, this total interception for at least part of the season could be

12

CHARLES A. FRANCIS

achieved by seeking a more nearly optimum density of the sole crop(s) (Willey, 1979a). Willey (1979a) concluded that “better spatial use of light . . . (will have) to be achieved through more efficient use of light rather than greater light interception” and will hold the most promise for further increasing yield potential of crop mixtures, Advantages of different leaf inclination or modified leaf dispersion in the canopy have been minimal in the field studies reported to date (Willey, 1979a; Rhodes, 1970) and in simulation models (Trenbath, 1974). It would appear that fine-tuning an intercrop system with plants of similar height and growth habit will have little effect on light interception and dry matter production, since most of the light is being intercepted when densities of the component crops are adequate. This was confirmed by studies of mixtures of different-height maize hybrids, which show no advantage in yield (Pendleton and Seif, 1962), and slight advantages in mixtures of sorghum genotypes with different heights (Osiru, 1974). Most of the traditional mixtures of food crop species-maize-bean, sorghum-pigeonpea, banana-coffee, maize-cassava-involve intercrops of plants with dissimilar size and growth cycle in the field. This type of intercrop gives a better vertical distribution of leaves in the total canopy. Willey (1979a) and Trenbath (1976) described the potential advantages of modified light distribution in a canopy of distinct species, while building on the theoretical work of Kasanaga and Monsi (1954). If a tall crop, especially a cereal with C , light response, were combined with a shorter dense crop with C, response, the total use of light could be enhanced in the mixture (Crookston and Hill, 1979; Willey, 1979a). Practical examples of this reaction include the maize-bean intercrop combination, with apparent differences in total yield depending on the bean plant type (Clark and Francis, 1985b). In this study in Colombia, climbing beans outperformed bush beans by 28% due to a longer development cycle and leaf area duration; maize yield was unaffected by bean plant type. In a related study, intercropped maize-beans achieved full cover (leaf area index of 4.0) 3 weeks before monocrop maize and 1 week before monocrop beans, further explaining the potential for overyielding in an intercrop mixture of the two species (Clark and Francis, 1985b). In another crop pattern, Fawusi (1985) found greater light interception (lower transmission) in a maize-okra intercrop when the two components were planted in alternate hills, and Fawusi et al. (1982) found greater light interception due to larger leaf area index (LAI) in a more leafy cowpea cultivar in maize-cowpea intercrops. Willey (1979a) also suggests the potential for choosing crop components adapted to differences in light quality that occur due to vertical distribution of leaf area (Allen er al., 1976; Szeicz, 1975). One notable example of great vertical distribution of components and leaf area is the multistoried tropical rain forest and the analogous cropping systems such


13

as coconut palm-papaya-pineapple mixture grown in the Philippines and Indonesia. Difficulties of converting this rainforest ecosystem to a cropping system are reviewed by La1 (1986). There can also be annual crops such as maize or rice grown under the taller crops (Nelliat et al., 1974). These systems provide the principal efficiencies that can be achieved in light interception and dry matter production through use of appropriate intercrops. A method used to quantify these concepts in light use, light use efficiency (LUE), was defined and explored by Trenbath (1986). He also presented computer simulations of light interception by different intercrop canopies. These could be useful in planning alternative cropping patterns and testing them in an efficient way before planting extensive trials in the field.

C. WATERUSE Intercropping studies in which water use was measured are limited. Water is often the most limiting factor in crop growth, and thus the ability of roots to explore a large soil volume and extract water is critical (Etherington, 1976). Trenbath (1976) describes the highly interrelated water and nutrient relationship with regard to root growth and interactions with the soil solution. Water may be depleted as far as 25 cm from a single root under experimental conditions (Klute and Peters, 1969), and in the field mobility of water to roots may be even greater (Stone et al., 1973). Intercrops may be more efficient in exploring a larger total soil volume if component crops have different rooting habits, especially depth of rooting (Willey, 1979a). For example, a deep-rooted component crop may be forced to develop even deeper roots if grown together with a shallowrooted crop (Fisher, 1976; Whittington and O’Brien, 1968). It would appear that roots of some intercropped species grow in the same general region (Lai and Lawton, 1962). This was the case in maize-bean intercrops in Colombia where lodging in maize was significantly reduced compared to sole-cropped maize; although evidence is circumstantial, the authors attribute the reduced maize root lodging to the intermingled roots of beans, which helped anchor the maize (Francis et al., 1978a). Where moisture is the most limiting resource, intercrops may offer both a temporal and spatial advantage in water use (Baker and Norman, 1975). An early component such as millet in an intercrop with maize could more efficiently use water that would be in excess for either sole crop (Kassam, 1973). Water use efficiency may be confounded by nutrient availability, as shown by the differences in nitrate leaching between sole cropping and parallel multiple cropping (alternate rows or strips) of pigeonpea-maize and sugarcane-black gram (Yadav, 1982). In this study, more nitrate was

14

CHARLES A. FRANCIS

lost below the root zone in the sole crop patterns. No differences were shown in total quantities of water taken up nor in patterns of water depletion when sole crops of cowpea and sorghum were compared with their intercrop (Shackel and Hall, 1984). Although water extraction patterns were different in an intercrop of sorghum-peanut, greater water use efficiency was found in two patterns of sole-cropped sorghum (Shinde and Umrani, 1985). In a comparison of sole crops and several intercrops in India, the wheat-mustard intercrop had greater water use efficiency than any of the other options including other intercrop combinations with wheat (Sinha et al., 1985). These results illustrate some of the complexity of water use by crops and the current lack of in-depth understanding of what to expect when crops are planted together. D. NUTRIENTUSE An excellent review of multiple-cropping fertility relationships was that of Sanchez (1976). As noted above, mobility of soluble ions such as nitrate is the same as that of water in the soil, and roots may attract nitrates from as much as 25 cm away in the soil solution (Barber, 1962; Barley, 1970; Klute and Peters, 1969; Trenbath, 1976). Nutrients such as ammonium, calcium, phosphorus, and potassium are strongly held on surfaces of soil particles and are present in low concentration in the soil solution; they move almost entirely by diffusion (Brewster and Tinker, 1970; Olsen and Kemper, 1968). The distance moved as measured by the depletion of phosphate, for example, may be as little as 0.7 cm (Bhat and Nye, 1973). The ability of an intercrop to make more efficient use than sole crops of soluble and nonsoluble nutrients will depend on the extent of root growth of component species, soil water levels, and how completely the intercrop mixture explores the entire soil mass in the rooting zone. Biological efficiency is likely to result when the intercrop either explores a larger soil mass or explores the same soil mass more completely, compared to sole plantings of the same species. There is also a possibility of differences in time of peak demand for different nutrient elements by components in the mixture (Willey, 1979a). There is much debate about the release of nutrients from one crop in a mixture for use in the same season by another. Willey (1975) cites evidence that shade trees in coffee, tea, and cacao plantations shed their leaves, and the decomposing material makes nutrients available for the lower-story crop. Sanchez et al. (1985) described the soil physical properties and fertility consequences of tree crop culture and fallow periods between tree and crop cycles. Some approaches to the study of nitrogen relations and root interactions have employed nodulating and nonnodu-


15

lating legume variants (Nambiar et al., 1986) and plastic barriers in the ground to separate the roots of intercropped species (Willey and Reddy 1981). Root densities were affected by pearl millet-peanut intercrop as compared to sole crops (Vorasoot, 1983). Below-ground effects are less easy to visualize and to study than those of the growing crop canopy. Higher total nutrient uptake by intercrops than by sole crops has been reported by several authors: for example, nitrogen (N) (Dalal, 1974), potassium (K) (Hall, 1974a,b), and magnesium and calcium (Dalal, 1974) all show this effect. Merences in total yield by intercrops has been explained by this greater uptake, although it is difficult to know if this is the cause or the effect of greater dry matter production (Willey, 1979a). Contrasting results were reported by Baker and Blamey (1985), who found less N uptake by a sorghum-soybean intercrop compared to sole-cropped sorghum; intercropping still produced significantly higher yields than sole cropping. The greatest advantage for intercrops was found at low phosphorus (P) levels in cowpea-maize intercrops in Costa Rica (Chang and Shibles, 198513) and at low N levels in soybean-maize intercrops in Iowa (Chui and Shibles, 1984), compared to sole crops of the components. Another advantage of intercrops is the conservation of fertilizer nitrogen for a subsequent crop or mixture: pigeonpeas planted with maize in India were found to capture nitrogen and make this more available for the subsequent sugarcane crop, compared to maize alone before the cane (Yadav, 1981). Wahua (1983) presented the concept of a nutrient supplementation index (NSI) to describe the fertilizer needs of an intercrop mixture and illustrated the concept with an example from the maize-cowpea alternate row system. Competition for nutrients, especially in pasture mixtures, was reviewed by Haynes (1980). He concluded that legumes in general are poor competitors with grass species for nitrogen. Poor competitive ability of white clover for phosphorus (Jackman and Mouat, 1972a,b), for potassium (McNaught, 1958; Mouat and Walker, 1959), and for sulfur (Neller, 1960; Walker and Adams, 1958)was ascribed to different root morphology compared to the associated grasses. When pastures are fertilized with phosphorus, the response is generally an increase in total dry matter production and an increase in the proportion of legumes in the mixture (Baylor, 1974; Rabotnov, 1977). This is similar to the competition for nitrogen reported for the cowpea-maize intercrop, a relationship which changed with N application and stage of cowpea growth (Chang and Shibles, 1985a).Efficiency of nitrogen use by maize changed with intercroppinglegumes, both with legume density and nitrogen rate (Ofori and Stern, 1987). There may be short-term yield reductions in cereals due to intercropped ground cover legumes, but long-term benefits of nitrogen and reduced soil erosion make them advantageous (Leach et al., 1986). Additional complexities of nutrient use in pastures can result from other fertilizer element applications,

16

CHARLES A. FRANCIS

clipping or grazing patterns, rainfall, species and varieties included in the mixture, and method of fertilizer application (Haynes, 1980). Also important are the effects of these several treatments on symbiotic Rhizobium spp. and the resulting nodulation patterns (Bergersen, 1971). Nutrient requirements for legume nodulation and symbiosis have been reviewed by Munns (1977). Intercropping cowpeas, maize, and melons increased the rhizosphere counts of bacteria for the first two crops but not for the melons (Wahua, 1984). He also found more bacteria in the intrarow than in the interrow intercropping of these species. Yadav and Prasad (1986) observed changes in phosphorus use efficiency by sugarcane as a result of intercropping the cane with mung beans. Differences among legume species (soybean, black gram, peanut) were observed in their bacterial activity and effect on intercropped maize yields in India (Singh et al., 1986). Duncan (1980) found differences among hybrids of grain sorghum planted after a crimson clover green manure crop, but no hybrid-byplanting date interaction. Ten soybean varieties were planted in all combinations with three leguminous cover crops in Malaysia, and a significant variety-by-intercropping system interaction was observed (Mak and Pillai. 1982). Early results of studies in controlled environments seemed to prove that excreted nitrogen from legumes could be taken up by nonlegumes (early literature reviewed by Willey, 1979a). It was suggested that legumes in a lower story that were shaded would fix less nitrogen and thus provide less to grasses in the field. Since the effects of nitrogen are often confounded with water and light competition between two component crops, it is difficult in the field to sort out these interaction effects. Important factors that may influence the potential of a legume to provide nitrogen to an intercropped cereal include densities of the two crops, light intercepted by the legume and thus its ability to fix nitrogen, species of legume, and limitations of other nutrients, especially phosphorus. Willey (1979a) described the importance of both direct transfer during a given cropping season and the availability of residual nitrogen for a subsequent cereal crop. For example, Agboola and Fayemi (1972) showed that mung beans gave higher transfer to maize in the same season, but cowpeas gave a greater contribution of residual nitrogen to the next crop of maize The fertility relationships in an intercropping system are raised to a more complex level when animals are introduced into the system. Most data comes from studies of manure applications to plots, but some work has documented the relationships between rice and fish, for example, Manjappa et al. (1987). Not only is the crop-fish mixture of interest, but the authors also mixed four species of fish in careful proportions for the experiment in raising fingerlings. This is but one example of the biological efficiencies that can be used to advantage in crop-animal systems, and


17

that could be used by farmers to increase diversity in food production and income. Much more research is needed in this complex field.

IV. PEST MANAGEMENT IN MULTIPLE-CROPPING SYSTEMS Much less information is available from the literature on weed, insect, and pathogen relationships in multiple-cropping systems, compared to data on agronomy and physiology of mixtures. One review (Litsinger and Moody, 1976) highlighted the importance of integrated pest management in these complex systems. Another review stressed the importance of using recently acquired biological information about pest species to reduce the need for active control measures in cropping systems (Altieri and Liebman, 1986). Reviews listed by Altieri and Liebman that cover literature on cover crops, agroforestry, strip cropping, and living mulches include those of Altieri and D. K. Letourneau (1982), Altieri and D. L. Letourneau (19841, and Cromartie (1981). There is general agreement that species diversity in multiple cropping reduces most insect pest problems, and the cropping intensity of carefully designed multiple-species mixtures can successfully outcompete weeds. This review presents relevant biological information on weed, insect, and disease control management, and how this can bring another dimension of biological efficiency to complex systems. A summary of how pest problems are influenced by crop and variety choice as well as cultural practices or degrees of intensity in cropping systems is presented in Fig. 2 (Litsinger and Moody, 1976). Although there are exceptions to these general relationships, Fig. 2 is useful to show how the crop itself and the temporal and spatial arrangement of crops can influence pest severity. It is important to keep in mind these complex and simultaneously occurring relationships, since change in any component of the system is likely to influence others.

A. WEEDMANAGEMENT

Interpretation of studies of weeds in traditional intercropping systems is complicated by the multiple uses of weeds by farmers: weeds are not always considered pests (Bye, 1981; Chacon and Gliessman, 1982; Kapoor and Ramakrishnan, 1975; Mishra, 1%9; Weil, 1982). In fact, weed pressure may be the most serious factor limiting food production in developing countries (Holm, 1971; Muzik, 1970); control of weeds may present the highest labor demand of the entire year and may even limit the area planted

18

-

CHARLES A. FRANCIS High P a t Potential

Low Pest Pdential

- CROP ITSELF

Large Pest Complex N d Competitiw with W W S Susceptible Variely

Crop Species

Tolerant Variety

Small Pest Complex Highly Competitive with Weeds

Resistant Pure Line

Annual

Perennial

Long-Maturins

Short-Maturing

Resistant Multigenic

--CROP ARRANGEMENT IN TIME

Monoculture

Crop Species Rotation

Continuous Planting

Discontinuous Planting

Asynchronous Planting

Synchronous Planting

Season favorable to Pest

Season UnfavorWe to Pest

CROP ARRANGEMENT IN SPACE

Sole Cropping

Low Planting Density Large Field

large Host Crop Area

Host Fields Aggregated

Rowor Strip Intercrwping

-

Mixed Intercrooping High Planting Density Small Field Small Host Crop Area

Host flelds Scattered

RG.2. Kinds of crops and their arrangement in time and space evaluated as to the potential development of pest problems. Some effects are seen to be high in pest potential, some intermediate, and some low. (From Litsinger and Moody, 1976.)

(Moody, 1977). Many factors influence weed incidence in the field, and this provides a wide range of options for management. These factors include crop species and varieties, crop densities of sole species or intercrop components, spatial organization of crops, fertility levels, cropping and weed history of the field, and integration of animals in the system. Perhaps this complexity has discouraged some researchers from working on weed control for intercrop systems; it certainly has forced attention to narrower questions that can be answered through a reductionist research approach. Although there is no other logical way to begin to study weed management, the reductionist approach may ignore important interactions that are critical in the field situation. These interactions form the basis for use of biological efficiency in controlling weeds in complex systems. There is an increasing concern about complete reliance on chemical


19

herbicides for weed control, both in temperate and tropical countries (Akobundu, 1980; Walker and Buchanan, 1982). Some of the problems include lack of flexibility in choosing cropping options due to herbicide residues (Bender, 1987), herbicide resistance in weed species (Lebaron and Gressel, 1982), personal safety and effects on the environment (Pimentel et al., 1980), and lack of purchasing power by some farmers to gain access to this technology (Akobundu, 1980). Because many of the crops planted in developing regions are intercropped, there are far fewer options in use of chemicals (Moody and Shetty, 1981). Given the alternatives of cultivation, intensive densities to compete with weeds, allelopathy in some combinations, and other biological methods of control, these need to be emphasized in a discussion of biological efficiency of intercrops (Plucknett et al., 1977). More practical experiments today are including intercrop combinations and testing both the yield and economic consequences of alternative weed control strategies (for example, Zaffaroni et al., 1982). There are large differences both among species and among varieties within species in competitive ability with weeds (Litsinger and Moody, 1976). These differences are due to variations in plant growth habit, time of planting relative to rainfall and temperature cycles in the field, and combinations of species in an intercrop pattern. Association of different weeds with different crops is a common occurrence (Plucknett et al., 1977; Muenscher, 1980). In the temperate zone, rotations of summer crops such as maize, sorghum, or soybeans with winter cereals such as wheat or barley or with a perennial such as alfalfa will help to break the reproductive cycles of weeds. Use of different herbicides in these dissimilar crops also promotes better long-term weed control compared to continuous monoculture of one summer crop such as maize. Francis et al. (1986) stressed both the linear and cyclical nature of biological processes in the field, which can be influenced by management, including choice of crops and rotations. There are specific references to crops that have different competitive ability with weeds. In maize intercrop systems, mung bean was more competitive than peanut with weeds due to its rapid early growth, and there were differences between two mung bean cultivars in ability to suppress weeds (Bantilan er al., 1974). Other differences have been observed among crop cultivars in their ability to compete with weeds, for example, in rice (Kawano et al., 1974), squash (Stilwell and Sweet, 19741, potatoes (Yip et al., 1974), maize (Moody and Shetty, 1981), and soybeans (McWhorter and Hartwig, 1972). Study of the particular traits of those varieties or selections that show competitive ability versus weeds may reveal what traits are useful in promoting this biological type of control. One of the most easily applied management methods to reduce weed

20

CHARLES A. FRANCIS

problems is increasing density of sole crops or intercrop components. Shading of the soil and competition for water and nutrients will certainly suppress weed germination and growth (Altieri and Liebman, 1986; Staniforth and Weber, 1956). Highest crop yields and greatest weed suppression often are found with the highest densities of components in an intercrop trial, for example, in pigeonpea-sorghum combinations in India (Shetty and Rao, 1981). Use of cover crops can also help to control weeds by competing for growth factors (Altieri and Liebman, 1986). The live mulch can produce a low-growing, high-density cover that suppresses weeds between rows of taller, desirable crop species such as maize or sorghum. Legumes are often used for cover because they are less competitive with cereals and have the capacity to produce nitrogen. Melon and sweet potato were shown to replace hand weedings in yam or yam-maize-cassava systems in Nigeria (Akobundu, 1980). Control of weeds in perennial crops or perennial-annual mixtures may be a longer and more costly activity (Wycherley, 1970). Yet there is less cultivation to bring up weed seeds, greater suppression of weeds by shading, and thus more stability in a well-designed perennial system once the pattern is established (Litsinger and Moody, 1976; Rao, 1970). Use of a perennial ground cover under a perennial tree crop can provide excellent weed control over time, as shown by the kudzu-oil palm intercrop, which is practiced on the coastal plain in Ecuador. The kudzu also provides nitrogen for the associated tree crop. Another common intercrop in the medium elevations of the Andean Zone is banana-coffee, in which case judicious hand cultivation and selection toward low-growing, noncompetitive weed species provides relatively inexpensive weed control based on knowledge of the biology and competitive nature of the tree crop species. Allelopathy is an active weed control mechanism in some monocrop systems with crop species as unrelated as oats (Fay and Duke, 1977), squash (Chacon and Gliessman, 1982), and cucumbers (Putnam and Duke, 1974; Lockerman and Putnam, 1979). Since intercropping involves planting of two or more dissimilar species, it is important that allelopathy not provide interference between or among crop components. This may limit the options for designing new patterns but will provide a useful and economic mechanism for weed control. There needs to be selectivity of the allelopathic effect toward the unwanted weed species without affecting the desired crops in the mixture (Altieri and Liebman, 1986). Using extracts of squash leaf applied to different crop species under controlled conditions, Gliessman (1983) demonstrated this differential selectivity. He had observed earlier that squash was consciously planted into maize-bean intercrops in the lowlands of Central America, primarily to control weeds in the system. Lack of allelopathy or interference provides an opportunity


21

for intercropping and control of weeds in an economical way, for example, the use of soybeans as a short-term cash crop in new plantings of eucalypts in Brazil (Couto et al., 1982). Each of these interactions adds complexity to the pattern of crops and weeds and makes it more difficult to understand, yet knowledge of the interactions can lead to economical and sustainable control methods. Altieri (1983) presented an intriguing look at the role of weeds in the total ecosystem, citing the potential consequences of complete elimination of current weeds from the farming environment (adapted from Tripathi, 1977): 1 . Herbicide-susceptible weeds are replaced with more resistant selections. 2. Decrease in overall dry matter production per unit area results from eliminating weeds. 3. Drastic reduction in total genetic resources in ecosystem occurs. 4. Insects that have attacked weeds now attack crop plants. 5. Reduction in beneficial insects that use weeds as alternate food, shelter, or breeding sites results. 6. Soil erosion increases due to lack of weeds in field after harvest. 7. Nutrients previously mined, taken up, and stored by weeds are lost. Looking at weeds as “ecological components” of the total system (Altieri, 1983) may lead to new perspectives on management of weeds as compared to their total control. New methods for simulation analysis of crop-weed competition have been proposed by Spitters (1984).

B. INSECT MANAGEMENT The agroecology approach to understanding insect population dynamics and pest management is proposed by Altieri (1983) as an alternative to current practices directed at control. This includes considering a larger biogeographic region rather than a single field, studying natural ecosystems to find sustainable models that can be emulated in cropping systems, and looking at the interactions among crops, weeds, insect pests, and their natural enemies in building biological control systems. Multiplecropping systems provide one option in the array of possible methods that help to implement this approach. In a unique review of literature on 198 insect species that attack crop plants, Risch et al. (1983) found that 53% showed lower abundance in multiple-species mixtures than in sole crops, 18% were more abundant in mixtures, 9% showed no difference, and 20% were variable in their response. Altieri and Liebman (1986) cited a number of multiple-cropping systems in which insects were less prevalent than in sole crops.

22

CHARLES A. FRANCIS

Several specific examples of cropping system effects on insect incidence and damage illustrate the concepts above. A maize-peanut intercrop had a reduced incidence of maize borer compared to sole-cropped maize in the Philippines (Raros, 1973). In Nigeria, cowpeas planted into a sorghummillet intercrop about one month after the cereals are planted have less insect damage on the grain legume (Baker and Norman, 1975), extending the range of cowpea cultivation into a region where it would otherwise not be planted. Monoculture cucumber had much higher levels of striped cucumber beetle than mixtures of cucumber with two other crop species (Bach, 1980). Altieri and Liebman (1986) presented evidence that cabbage aphids and flea beetles were less prevalent on cauliflower planted with vetch or with weeds compared to sole-cropped cauliflower. Several possible mechanisms were presented by Hasse and Litsinger (1981) that could explain why insects are less prevalent in multiple crop systems. They are grouped into mechanisms that interfere with insects finding their hosts and mechanisms that influence reproduction of the insect population and its survival. Their data are summarized in Table 111. Insects may have more difficulty finding host plants in an intercrop due to presence of other nonhost plants, to camouflage of the preferred host, to changes in the texture or color of the total background, to masking of a chemical attractant, or to presence of a repellent from a nonhost plant. Even if insects successfully find the host, there may be interference with reproduction and survival. Mechanical barriers may be present in the form of nonhost crop plants, insects may leave the field more quickly if it is not a pure crop stand, or there may be differences in either the microclimate or the natural enemy population in the intercrop compared to a sole-crop environment. Tahvanainen and Root (1972) described the complex interaction of biological, physical, and climatic conditions of the intercrop system to provide an “associational resistance” to insects, compared to sole crops of component species. Root (1973) suggested that this resistance could function as a result of two major characteristics of the intercrop environment: he proposed a “natural enemy” hypothesis and a “resource concentration” hypothesis. The first would be explained by a higher population of natural enemies of insect pests due to the diversity of the intercrop pattern in the field. Predators tend to have broad habitat adaptation and could adapt to and persist in the intercrop environment better than in a sole crop (Altieri and Liebman, 1986). Resource concentration in the form of a single type of host plant (uniform food source) would favor buildup of a pest species more than would occur in a diverse intercrop combination. Both visual and chemical stimuli from an intercrop would be less than from a sole crop, and an individual insect might have greater difficulty locating the desired host in this situation (Altieri and Liebman, 1986).


23

Table Ill Possible Effects of Intercropping on Insect Pest Populations"

Factor

Explanation

Example

Interference with host-seeking behavior Camouflage

A host plant may be

protected from insect pests by the physical presence of other overlapping plants Certain pests prefer a crop background of a particular color and/or texture

Crop background

Masking or dilution of attractant stimuli

Repellent chemical stimuli

Presence of nonhost plants can mask or dilute the attractant stimuli of host plants leading to a breakdown of orientation, feeding, and reproduction processes Aromatic odors of certain plants can disrupt host finding behavior

Camouflage of bean seedlings by standing rice stubble for beanfly Aphids, flea beetle, and Pieris rapae are more attracted to Cole crops with a background of bare soil than to ones with a weedy background Phyllotrera cruc$erue in collards

Grass borders repel leafhoppers in beans, populations of P1u:ella xylosrella are repelled from cabbagehomato intercrops).

Interference with population development and survival Mechanical barriers

Lack of arrestant stimuli Microclimatic influences

Biotic influences

All companion crops may block the dispersal of herbivores across the polyculture; restricted dispersal may also result from mixing resistant and susceptible cultivars of one crop by settling on nonhost components The presence of different host and nonhost plants in a field may affect colonization of herbivores; if a herbivore descends on a nonhost, it may leave the plot more quickly than if it descends on a host plant In an intercropping system favorable aspects of microclimate conditions are highly fractioned, therefore insects may experience difficulty in locating and remaining in suitable microhabitats; shade derived from denser canopies may affect feeding of certain insects and/or increase relative humidity, which may favor entomophagous fungi Crop mixtures may enhance natural enemy complexes (See natural enemy hypothesis in text)

"Data from Hasse and Litsinger, 1981.

24

CHARLES

A. FRANCIS

These authors presented a series of examples to substantiate the two hypotheses in sole crop-intercrop comparisons in the field. Willey et al. (1983) presented additional examples from studies of intercrops in India. Although insect species were similar in plant crop and ratoon crop of sorghum, the severity of damage was greater on the ratoon crop in Georgia (Duncan and Gardner, 1984). More information is needed on these insect relationships to be able to generalize about systems and to predict the potential success of new combinations. Integrated pest management (IPM) is a logical approach to be applied in multiple-cropping systems. The need for basic information on the biology of insects and their hosts and natural enemies is obvious. Careful study of these interactions can lead to new management options that involve minimum cost and maximum use of cultural approaches to control. The concepts were presented by Litsinger and Moody (1976) and by Altieri and Liebman (1986). Since multiple-crop systems often extend the cropping season, there is a need to know what happens to pest populations during the entire year and how they are affected by having a crop mixture available for a longer period. Are there changes in weed species and densities in the more complex systems? The single principle that emerges is the need to consider a range in pest management strategies and specific control measures. These include cultural control, rotations, resistant varieties, biological control agents, and judicious applications of pesticides. The concept extends beyond insects to plant pathogens and weeds as well, and complex interactions in the total system need to be considered.

C. PLANTPATHOGEN MANAGEMENT Less is known about disease dynamics and plant pathogens in multiplecrop systems, compared to weeds and insects. The species diversity of natural ecosystems and thus the dispersion of individual host species apparently restricts the spread of plant pathogens (Browning, 1975). In an intercrop combination, there is a mixture of susceptible and resistant (nonhost) plants, and thus greater distance from one host plant to another (Altieri and Liebman, 1986). The more the intercrop system resembles the diversity of the natural (“resistant” or “tolerant”) ecosystem, the more success there will be in avoiding destructive levels of plant diseases (Larios and Moreno, 1977). In contrast, there may be multiple-species combinations that change the microclimate, e.g., cause higher humidity, and thus favor greater disease incidence. However, it is difficult to generalize. The cassava-maize intercrop has less incidence of cassava scab than sole-cropped cassava, but doubling over the maize to allow light to the lower crop causes an increase in the disease (Larios and Moreno,


25

1977). To illustrate the complexity of these relations, angular leaf spot on dry beans was highest in a bean-maize intercrop and lowest in beansweet potato and bean-cassava intercrops. Altieri and Liebman (1986) presented several examples of how cropping system influences nematode populations and severity of problems. More research is needed on potential intercrop combinations before they are widely promoted for farmer acceptance.

V. BIOLOGICAL AND ECONOMIC STABILITY OF CROPPING SYSTEMS Sustaining yield and income from the total farming system may be a more important objective for farmers with limited resources than maximizing either yield or income (Francis, 1985). In addition to harvested yield and immediate income from crop sales, the family objectives include maintaining food supply and income through the year, minimizing risk of failure in every season, keeping cash costs at a minimum, and meeting other social obligations in the community. These are factors not often considered by the crop scientist involved in developing new technology. From these concerns of farmers comes the notion of yield and income stability, a new dimension or yardstick by which to measure success in a plant breeding or agronomy research and extension program. The criterion of yield stability has been used by plant breeders to evaluate small grains, maize, and other crops during the steps of genetic improvement. Regression methods of Finlay and Wilkinson (1963) and Eberhart and Russell (1966) are those most frequently used for analysis. The criteria for favorable o r stable varieties o r hybrids usually include a mean yield above the mean of all genotypes, a response to improving environments (b) that is not significantly different from unity ( b = 1 .O), and minimal deviations of yield from the regression line. This concept has been extended to evalution of bean variety yields in contrasting cropping systems (sole-cropped versus intercropped with maize) and consecutive seasons (Francis et al., 1978b,c) and t o analysis of yield components of sorghum hybrids (Heinrich et af., 1985). Although this analytical approach to stability has not been used for total system yields, income, o r risk, it would appear to be a useful methodology to quantify the results of whole farm systems. More important than the specific method of analysis is choosing an appropriate criterion by which to measure stability. As suggested above, this may be more complex than a single number such as grain yield or net income, and some of the criteria listed may be dificult to quantify.

26

CHARLES A. FRANCIS

Measures of biological stability have been reviewed by Trenbath (1974) and by Willey (1979a). New analytical tools were described by Mead (1986). There have been some reports of specific comparisons of stability in the intercrop systems as compared to sole cropping (Francis and Sanders, 1978; Rao and Willey, 1980), and these are discussed in the following section. These two articles also present an analysis of economic stability, although it is only a comparison of the three contrasting crop patterns and not extended to a whole-farm system. Biological diversity is important in yield stability, according to the majority of authors, although there is limited experimental evidence of this relationship (Willey, 1979a) and some conflicting reports. Perhaps the most interesting biological and economic aspect of intercropping is the potential for compensation among components of the system. This could be called the biological or economic “buffering” in the system (Francis, 1986) that leads to greater stability of total yield or income of intercrops. Willey (1979a) concludes that much more research is needed to assess the stability of cropping systems, especially to assure farmers that a new system will not be less stable than a traditional form of intercropping. A. VARIATIONS IN BIOLOGICAL OUTPUT

Conventional wisdom about intercropping in traditional agriculture is that relatively low-yield systems are more stable over a range of conditions and seasons (Willey, 1979a). This is attributed to greater diversity within each intercropped field and the ability of a mixture of crop species to react differently to a given set of climatic constraints in a given season, and as a total system to produce a more consistent yield. There is sustained biological production in natural ecosystems, but generally there is no harvest and most of the dry matter produced is recycled within the system. Multispecies cropping systems approximate the natural plant mixture to some degree, and there is a range of diversity within both natural ecosystems and cropping systems (see Fig. 3). Genetically more diverse systems such as the 13-crop mixture in Nigeria (Fig. 4) traditionally have been stable but lower-yielding due to the low level of added inputs and lack of available improved technology for this type of system. In contrast, single-cross maize hybrids have extremely high grain yield potential, but this can be realized and sustained only through systematic and frequent additions of fertilizer, irrigation in some regions, and pesticides to control unwanted weed and insect species. The long-term sustainability of the latter type of system and its applicability for many subsistance farmers has been challenged (Francis et al., 1986). There is little quantitative evidence to

MULTIPLE-CROPPING SYSTEMS Natural Ecosystems Tropical rain forests

Maximum Genetic Diversity

27

Cropping Systems Shifting cultivation in humid forests 12-crop mixtures in Africa

Temperate zone forests

Maize-cassava-bean Maize-bean

Natural grasslands

Bean cultivar mixture Boreal forests Multiline cereals Wheat varieties Spartina marshes

Geothermal pools

V

Double -cross maize hybrids Single-cross maize hybrids

Minimum Genetic Diversity FIG. 3. Spectrum of genetic diversity in natural ecosystems and in cropping systems. (From Smith and Francis, 1986.)

support this contention, but today there is a growing concern among development experts about the need to search for alternatives that depend more heavily on internal, renewable resources available on the farm (Francis and King, 1988). Variance in yields is one possible measure of stability. Francis and Sanders (1978) showed greater variation in sole-crop maize and sole-crop bean yields compared to intercrop maize-bean in an analysis of 20 experiments in Colombia. In a similar analysis from more than 90 trials in India, Rao and Willey (1980) found greater variation in sole-crop sorghum and sole-crop pigeonpea than in the sorghum-pigeonpea intercrop. However, Mead (1986) presented an example to show that these measures ignore some information about the structure of the data set. Pairs of observations of sole-crop and intercrop yield from a series of locations or environments need to be analyzed with that pairing in mind in order not to lose information and be misled by the results. Mead suggests some variant of the Finlay and Wilkinson (1963) method as a better alternative, depending on a rational choice of the environmental index. This index generally is the mean of all genotypes in a breeding trial, and in the case of cropping systems would logically be the mean of all systems in a given

28

CHARLES A. FRANCIS

Distance, rn

Ca - Cassava

Cu - Melon

- Peenut L - Laganaria

M - Maize Pk

D3 - 0. bulbMera

Pp . Pigeonpea

A

04 - D. cayenensis

P

- Pumpkin - Rice

D1 - Dioscorea rotundata D2 - 0. alaia

V . Voandzeie

FIG.4. Spatial distribution of 13 crop species on and between raised mounds in Nigeria. (From Okigbo and Greenland, 1976.)

site, although there are still problems with this approach. An alternative for analyzing risk is presented in the next section. Using the summary by Willey (1979a), the review of Trenbath (1974), and the data above, a list of stability comparisons of intercrops with sole crops can be constructed (if only a single crop is listed, the intercrop is two or more dissimilar components of the same species).


29

1. There is a substantial improvement of stability in intercrop: barleyoat (Daniel (1955); maize-bean (Francis and Sanders, 1978); and sorghumpigeonpea (Rao and Willey, 1980). 2. There is a marginal improvement of stability in intercrop: soybean (Byth and Weber, 1968; Schutz and Brim, 1971); oats (Frey and Maldonado, 1967; Qualset and Granger, 1970); sorghum (Ross, 1965); and maize (Funk and Anderson, 1964). 3. There is no improvement of stability in intercrop: barley (Clay and Allard, 1969) and oats-rye (Pfahler, 1965).

Although there is little confounding of crops across groups, it would be ill-advised to conclude from these few reports that a barley-oat intercrop will always show increased stability and an oat-rye intercrop will not, for example. These are only indications of what is happening biologically in the mixtures, and no doubt the most dissimilar components in the above intercrops had the best chance of demonstrating increased stability. Differences among components could be in plant height, maturity, timing of resource use, root structure, or carbon metabolism; the greater the differences, the more likely there would be overyielding (Andrews, 1972), and perhaps greater stability. B. INCOMESTABILITY Variation in gross or net income from crop production systems is a function of yields, prices, and costs, and the information presented above provides a general guideline on the stability of income as well as biological productivity. What distinguishes intercropping systems from monoculture is the introduction of several new factors into the income equation: relative prices of two or more commodities; efficiencies of production, which result in lower production costs; and compensation between the two or more crop components. The last topic is described in another section. Some of the stability related to differences in component crop prices can be achieved by diversification on the farm and does not necessarily require intercropping. The discussion focuses on how biological efficiencies affect income stability. Lower variation among locations and seasons in intercropping systems compared to sole cropping can lead to apparent improvements in both yield and income stability. A maize-bean study in Colombia (Francis and Sanders, 1978) showed a probability of at least breaking even of 0.65 with a maize monocrop, 0.80 with a bean monocrop, and 0.92 with the maizebean intercrop. Rao and Willey (1980) reported the probability of at least breaking even as 0.91 with a pigeonpea monocrop, 0.95 with a sorghum monocrop, and 1 .OO with the sorghum-pigeonpea intercrop. The latter data set was reported in a different form by Willey et al. (1983), in which

30

CHARLES A. FRANCIS

the percentage risk of failure (probability of failure x 100) was plotted against the “disaster level income” in rupees per hectare for three cropping patterns of sorghum and pigeonpea (Fig. 5). The advantage of intercropping is clear from this presentation, which is based on 94 trialS in India. Relative prices received for component crops can also influence the economic success of an intercropping pattern. This same relationship is important in diversifcation of cropping and would cause different decisions in allocation of land to different crops if prices could be anticipated before planting. Francis and Sanders (1978) reported net returns from the intercrop and the two sole crop patterns at a range of bean:maize price ratios from I :1 to 8: 1, a range which included all the known actual price ratios in Central and South America at that time. Two additional variables were introduced into the analyses: differences in bean and maize yields and differences in production costs. The decision to plant sole-crop beans versus intercropped maize and beans depended on both bean yields and costs of materials and labor in the production of the sole-crop climbing beans, as well as on the price ratio. There was no unique decision at all levels of input and yield. At the prevailing price ratio, yield level on the farm, and production cost of subsistence farmers there was a clear advantage to intercropping, an indication of why this system persists in the middle elevations of Colombia, where the experiments were conducted. Mead (1986) described a method of evaluating stability based on risk analysis. This assumes that (1) risk is the best way to measure stability; (2) the bivariate distribution of yields can be described by a simple model; 80

1

70 60,

f .--

2

s

Sole pigeonpea

50. 40.

30 ’

10 i

250

1000

1750

2500

3250

Disaster level of income (Rslha)

FIG.5. Yield stability of sorghum and pigeonpea in sole cropping and intercropping: the probability of crop failure. (From Willey et al., 1983; data from Rao and Willey, 1980.)


31

(3) yields for two species can be quantified on a single scale such as economic value; and (4) variation among years and among locations is similar, so that data sets can be combined for analysis. With data from sole-crop sorghum and a sorghum-pigeonpea intercrop converted to net income from the systems using a price ratio of I .8:1 (pigeonpea:sorghum), a comparison of incomes between the two systems can be plotted; this is shown in Fig. 6a. Yields (incomes) falling on the line b = 1.0 would be expected from a completely additive (nonoveryielding) situation in the intercrop, and would be the same as planting two adjacent fields in the two crops to diversify income. It is apparent in Fig. 6a that intercropping often produces higher income. If the probability of getting an intercrop income greater a lOOr

0

50 Sole-crop yield

100

b

Monocrop risk

80

FIG. 6. Bivariate plot of intercrop yield (return) against sole crop sorghum for 51 experiments in India: (a) intercrop and sole crop yields (returns); (b) relative risk graph for the fitted model. (From Mead, 1986.)

32

CHARLES A. FRANCIS

than some fixed level is plotted against the probability of a sole-crop sorghum income greater than that same fixed level, the results from these trials appear as in Fig. 6b (Mead, 1986). As shown by the figure lines, a monocrop risk of 0.5 of not reaching a fixed level of income is reduced to about 0.26 with the intercrop. Conversely, an intercrop risk of 0.5 would correspond to a risk of about 0.73 in sole-cropped sorghum. The graph clearly indicates why farmers plant a diverse mixture of species rather than sole crops, and one of the reasons is the compensation or buffering that can occur in the intercrop.

C. BUFFERING AND COMPENSATION I N SYSTEMS Compensation among yield components in agronomic crops has long been recognized and described (Grafius, 1957). Since yield is the product of heads or pods or ears per unit area, number of seeds per head or pod or ear, and weight of individual seeds or kernels, reduction of any one of these may result in an increase in another. What other component changes depends on temporal development of the plant (Castleberry, 1973: Grafus, 1957; Heinrich et al., 1983). The relationships among these yield components and their capacity to change results in “buffering” or a compensation among them and a certain stability of biological production (Heinrich et al., 1983). Compensation could be viewed at a number of different levels. Removal of some seeds from a sorghum head causes the remaining seeds on that head to increase in weight; removal of the main head causes greater tiller devolopment, depending on the stage of removal. Removal of the top ear in maize causes development of one or more lower ears. This could be called intraplant compensation. At the plant level, increasing density of seeding reduces dry matter produced per plant; some spatial organizational effects also are well known, such as row spacing changes or geometric distribution of individual plants. This could be called interplant compensation within a species. In an intercrop pattern in which two or more species are involved, higher yields of one component due to favoring that component with higher density, earlier planting, meeting specific fertility needs, or spatial organization to promote its growth often will be accompanied by lower yields of other crops in the mixture (Donald, 1963; Davis and Garcia, 1983, 1987; Davis ef al., 1987; Gliessman, 1986). That the reduction of yield in one component is not proportional to the increase in yield of another is due to complementarity in resource use, as described in previous sections. Willey and Reddy (198 1) were able to partition the aboveground and belowground interactions in a way that demonstrated the importance of intimate inter-


33

actions in the root zone between pearl millet and peanut. Thus it is a combination of complementarity in resource use and of the ability of one component to use resources not needed by another component that lead to compensation in systems and overyielding in the right combinations of crops. Economic buffering in a production system is more difficult to document. Part of the advantage of having diversity in the system is being able to provide a range of products to the market. Before planting, the strategy could be to produce a range of crops to (1) take advantage of those with highest market value and (2) spread the risk of drastic change in price of one or more commodities. It is assumed to be less probable that prices of several crops will go down between planting and harvest than might occur with a single crop, thus the diverse mix of enterprises would provide a more stable income than any single crop. This argument relates to diversity, with or without intercropping. The other dimension of economic buffering is related to the nature of intercropping or mixtures of species in each field. The first factor is overyielding; if higher total yields result from intercropping, there will be an economic advantage if the most valuable species are well represented in the harvest. The second factor relates to the biological buffering described above. If two crops are planted throughout a field, rather than in half of a field, and if a devastating insect, disease, or drought situation selectively eliminates or strongly suppresses one component, the remaining crop is likely to take advantage of the available resources and produce a relatively high yield over the entire area. This would not occur in a diverse system in which the farm was divided into a series of sole-crop enterprises. From this biological and economic review, it could be concluded that appropriate intercropping combinations are more financially stable than sole crops. Introducing other measures of stability could expand this argument, namely, diversity and stability of food supply, distribution of food and income, and risk associated with alternative cropping strategies.

D. STATISTICAL ANALYSIS OF MULTIPLE-SPECIES SYSTEMS Statistical comparisons among intercrop alternatives and between intercrop and monocrop systems have been cited throughout the review. The simplest and most frequent type of statistical evaluation is the analysis of variance. This was discussed in detail by Mead (1986) and will appear in a forthcoming book by W. T. Federer (personal communication). The limitations of analysis of variance methods relate to assumptions about uniformity of variance among treatments and normal distribution of observations on those treatments. The researcher’s judgement is critical in

34

CHARLES A. FRANCIS

deciding whether these assumptions are met in a given trial (Mead, 1986). Mead (1986) presented valuable guidelines on design of experiments and treatments and how to present results on main effects and interactions from these trials. Researchers find it necessary to evaluate complete systems on some quantitative basis, and this is difficult when more than a single crop is involved and there are farmer criteria other than biological yield and net income by which success is measured. One simple index used in descriptive reports is the cropping index, or cropping intensity index, which is a measure of crops per year on a given field. Double-cropping or relaycropping wheat and soybeans would give an index of 2.0, while monoculture maize would give an index of 1.0. Averages may be reported for a specific region, for example, “Valley X has a cropping intensity of 1.8,” indicating that, on the average, 1.8 crops per year are harvested from each field in the area. This index tells nothing about the productivity of those crops nor about efficiency of resource use. A frequently used index is the land equivalent ratio (LER), which in fact evolved from the relative yield total (RYT) used by de Wit and van den Burgh (1965). This is the area needed under monoculture to produce the same yield as that same area would produce with intercropping; the concept has been reviewed by Willey (1979a,b). Distribution of the LER has been studied by Oyejola and Mead (1981) and reviewed by Mead (1986), with the conclusion that standard analyses can be conducted with this index. The most difficult step is choosing a valid denominator for the index, since this choice can drastically affect the results of the analysis and the conclusions for a trial. Any index or reduction of data into simpler form results in some loss of information. Mead and Willey (1980) and Mead (1986) describe an “effective LER,” which can guide experimental changes in proportions of the component crops and lead to eventual choices of the best combinations and desired products from the system. Another modification of this index is the “area-time equivalency ratio” (Harwood, 1979), which takes into account the time the component crops are in the field, an evaluation of temporal as well as spatial efficiency of intercropping. Other indexes that have been used to describe competition and resource use were reviewed by Willey (1979a). One of these is the relative crowding coefficient, which describes whether each species in a mixture produces more or less yield than expected in a replacement series (de Wit, 1960; Hall, 1974a,b). Another is the aggmsivity index or the measure of relative increase in one component compared to increase in another component (McGilchrist, 1965); if the index is equal to zero, both species are equally competitive. The competition index (Donald, 1%3) requires the calculation of equivalence factors for each species; this factor is the number of plants


35

of one species that are equally competitive as one plant of another species, and the product of the factors gives the index. Willey (1979a) described a number of trials in which this index has been applied with some success. In spite of the number of index approaches that have been proposed, the LER remains the index most frequently used by active researchers in the area of multiple cropping. It does give a measure of biological efficiency and can be used for standard statistical analysis, subject to the limitations outlined by Mead (1986).

VI. FUTURE APPLICATIONS FOR MULTIPLE-CROPPING SYSTEMS Continued observations of natural ecosystems, traditional, and improved farming systems in the field and experiments with closely planted species will build the knowledge base of how plants influence each other. It is this information base that will provide the foundation for improvements in multiple-cropping systems and from which technicians and farmers can design new mixtures and ways to organize compatible crops spatially in the field. Understanding of critical interactions between species and among crops, pests, and environmental conditions is still in its early stages, and far less is known about intercropping than about high-technology monoculture. The future importance of multiple cropping is difficult to predict: mechanization, increases in farm size, and greater specialization all work to reduce the diversity and complexity of agroecosystems. In spite of these trends, there are some areas in which double cropping has become standard practice, such as the winter wheat-soybean pattern in the southeast United States. Short-cycle, photoperiod-insensitiverice varieties have made possible two and three crops per year in South China and the Philippines when fertilizer and other inputs are available and the price justifies intensification of production. Although it is unlikely that intensive cerealgrain legume intercrops will become commercially viable on a broad scale, they will continue to be important for subsistence farmers. Other embodiments of the multispecies concept that are likely to become increasingly important are grass-legume mixtures for pastures, overseeding legumes into cereals during or after the crop cycle in temperate zone, and more intensive rotations including relay overlapping of some species. Multiple-cropping systems observed around the world appear to be a response by farmers to scarcity of production resources and a desire to make the most efficient possible use of what is available (Francis, 1986). The many variants of these systems, based on biological efficiencies

36

CHARLES A. FRANCIS

outlined above, may lead to the lowest possible production costs and the most stable, low-risk strategies to provide food and income for the family. How much research and development occurs in the future with these complex systems depends in large part on how national research decision makers view the importance of the small farm sector of agriculture. Field research is expensive. Simulation analysis of alternative cropping patterns may provide an efficient tool for future research on complex systems (Barker and Francis, 1986; Spitters, 1983; Trenbath, 1986; Whisler et al., 1986). A number of potentials have been discovered by observations of current farmer practices and especially by controlled experiments at research stations. There are several factors that should be considered as research planning proceeds for the next century, when food needs will grow with the burgeoning population: 1. More efficient use of sunlight, water, and nutrients can result from an appropriate intercropping or double-cropping pattern. 2. Increased biological diversity can lead to more production stability and reduced risk of failure in these complex systems. 3. Improved economic stability can result from diversification of crops, especially intercropping, which leads to buffering and compensation in each field. 4. Finite supplies of fossil fuels force the consideration of alternative production systems based on biological efficiencies such as fixation of nitrogen, reduced pesticide use due to lower pest incidence in diverse systems, and nutrient cycling in rotations and mixtures of diverse species.

Further research and development of improved multiple-cropping systems could be both timely and appropriate as a viable strategy to help solve the world food challenge through improved biological efficiency.

REFERENCES Agboola, A. A., and Fayemi, A. A. 1972. Agron. J . 64, 409-412. Ahlgren, H. L., and Aarnodt, 0. S. 1939. J . Am. Soc. Agron. 31, 982-985. Aiyer, A. K. Y. N. 1949. Indian J . Agric. Sci. 19, 439-543. Akhanda, A. M., Mauco, J. T., Green, V. E., and k i n e , G. M. 1977. Soil Crop Sci. Soc. Flu. Proc. 37, 95-101. Akobundu, 1 . 0 . 1980. In “Weeds and Their Control in the Humid and Subhurnid Tropics” (I. 0. Akobundu, ed.), pp. 80-100. Intl. Inst. Tropical Agric., Ibadan, Nigeria. Allen, L. H . , Sinclair, T. R., and Lemon, E. R. 1976. I n “Multiple Cropping” (R. I. Papendick, P. A. Sanchez, and G. B. Tnplett, eds.), pp. 171-200. Amer. SOC.Agron. Spec. Publ. 27, Madison, Wisconsin. Altieri, M. A. 1983. “Agroecology: The Scientific Basis of Alternative Agriculture.” Div. Biol. Control, Univ. California, Berkeley.


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Altieri. M. A., and Letourneau, D. K. 1982. Crop Prof. 1, 405430. Altieri, M. A., and Letourneau, D. L. 1984. C R C Crit. Rev. Plunr Sci. 2, 131-169. Altieri, M. A., and Liebman, M. 1986. In “Multiple Cropping Systems’’ (C. A. Francis, ed.), pp. 183-218. Macmillan, New York. Andrews, D. J. 1972. Exp. Agric. 8, 139-150. Andrews, D. J., and Kassam, A. H. 1976. I n “Multiple Cropping” (R. 1. Papendick, P. A. Sanchez, and G. B. Triplett, eds.), pp. 1-10. Amer. SOC.Agron. Spec. Publ. 27, Madison, Wisconsin. Bach. C. E. 1980. Ecology 61, 1515-1530. Baker, C. M.. and Blarney, F. P. C. 1985. Field Crops Res. 12, 233-240. Baker. E. F. I.. and Norman, D. W. 1975. Proc. Crop. Sysr. Workshop I . R . R . I . , Los Bano.s, Philipp. pp. 334-361. Baker, H . G . 1970. “Plants and Civilization.” Wadsworth. Belmont. California. Bantilan, R. T.. Palada. M. C., and Harwood, R. R. 1974. Philipp. Weed Sci. Bull. I, 1436. Barber. S. A. 1962. Soil Sci. 93, 3 9 4 9 . Barker. T. C., and Francis, C. A. 1986. I n ”Multiple Cropping Systems” (C. A. Francis, ed.), pp. 161-182. Macmillan, New York. Barley. K. P. 1970. A h . Agron. 22, 159-201. Baylor. J . E. 1974. In “Forage Fertilization” (D. A. Mays, ed.), pp. 171-188. Amer. SOC. Agron.. Madison, Wisconsin. Beets, W. C. 1982. “Multiple Cropping and Tropical Farming Systems.” Westview, Boulder, Colorado. Bender. J. 1987. Crop Prod. News Univ. Nehr.. Lincoln. Coop. E x / . Serv. 7, Oct. 23. Bergersen, F. J. 1971. Annrr. Rev. Plonr Phvsiol. 22, 121-140. Bhat, K. K . S.. and Nye. P. H. 1973. Plont Soil 38, 161-175. Brewster. J. L., and Tinker. P. B. 1970. Soil Sci. Soc. A m . Proc. 34, 421426. Brown, H. B. 1935. Effect of soybeans on corn yields. Buton Rorrge, L o . Agric. Exp. Sfu. Bull. 265. Browning. J. A. 1975. Proc. A m . Phvroputhol. SOC. 1, 191-194. Bye. R. A,. Jr. 1981. J. Ethnohiol. 1, 109-123. Byth. D. E. and Weber. C. R. 1968. Crop Sci. 8, 44-47. Castleberry, R. M. 1973. Effects of thinning at different growth stages on morphology and yield of grain sorghum ( S o r g h m hicolor (L.) Moench). Ph.D. thesis, Univ. of Nebraska, Lincoln. Chacon. J. C.. and Gliessman, S. R. 1982. Agro-Ecosyslems 8, 1-11. Chang, J . F., and Shibles. R. M. 1985a. Field Crops Res. 12, 133-143. Chang. J. F.. and Shibles. R. M. 1985b. Field Crops Res. 12, 14.5-152. Chestnutt. D. M. B.. and Lowe. J. 1970. Occus. Symp. 6th. Br. Gross/. Soc. pp. 191-213. Chui. J. A. N., and Shibles, R. 1984. Field Crops Res. 8, 187-198. Clapp. J . G.. Jr. 1974. Agron. J . 66, 463465. Clark. E. A,. and Francis. C. A. 1985a. FirldCrups Rrs. 11, 151-166. Clark. E. A,. and Francis. C. A. 1985b. Field Crops Res. 11, 37-53. Clary. W. P. 1974. J. Ronge Munoge. 27, 387-389. Clay. R. E.. and Allard. R. W. 1969. Crop Sci. 9, 407-412. Cook, C. W. 1966. Development and use of foothill ranges in Utah. P r o w Utuh Agric. Exp. Sto. Bull. 461. Couto. L.. de Barros, N. F.. and Rezende, G. C. 1982. Aust. For. Res. 12, 329-332. Cromartie. W. J. 1981. In “CRC Handbook of Pest Management in Agriculture” (D. Pimentel, ed.). Vol. I. pp. 223-251. CRC Press, Boca Raton. Florida. Crookston. R. K., and Hill, D. S. 1979. Agron. J. 71, 4 1 4 4 .

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SEED COATINGS AND TREATMENTS AND THEIR EFFECTS ON PLANT ESTABLISHMENT James M. Scott Department of Agronomy and Soil Science University of New England Armidale, New South Wales 2351, Australia

I.

11.

111.

IV.

V.

VI.

VIl. VIII.

1x. X.

Introduction A. Why Seeds Are Coated B. Evolution of Seed Coatings C. Definitions The Seed-Coating Process A. Coating Equipment B. Adhesives C. Coating Materials Coatings to Facilitate Planting A. Precision Sowing B. Improved Ballistics lnoculant Coatings A. Rhizobia B. Vesicular-Arbuscular Mycorrhizal Fungi C. Other Organisms Protective Coatings A. Diseases B. Insects. Pests, and Other Fauna C. Protection against Herbicides Nutrient Coatings A. Need for Early Seedling Nutrition B. Macronutrients C. Micronutrients D. Efficacy of Nutrient Seed Coatings E. Injury Caused by Fertilizers Herbicide Coatings Other Coatings A. Hydrophilic Coatings B. Hydrophobic Coatings C. Oxygen Supply Treatment Processes Conclusions References

43 Copyright 0 1989 by Academic Press, inc. All nghfs of reproduction in any form reserved.

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I. INTRODUCTION A. WHY SEEDSARE COATED The successful establishment of crop and pasture species from seed depends on a broad array of factors including the species sown, the inherent vigor of the seeds, the soil type and its fertility, the climatic conditions, the time of year, sowing depth, soil tilth, method of soil cultivation and sowing, and the presence or absence of antagonistic or beneficial organisms such as weeds, insects, diseases, rhizobia, or mycorrhizas. Farmers have an opportunity to control only some of these factors; many factors remain uncontrolled and can, either singly or in combination, cause a delay or reduction in establishment. Commonly, farmers attempt to overcome some of these adverse conditions by applying materials such as herbicides and fertilizers to the whole area of land to be planted. Such broad-acre applications can be expensive and there is a risk of considerable financial loss if establishment is inadequate or fails altogether. An alternative approach is to apply materials either in “bands” adjacent to the seed or on the seeds themselves in seed “coatings” in an effort to increase the effectiveness of the treatments. Seed coating is a mechanism of applying needed materials in such a way that they affect the seed or soil at the seed-soil interface. Thus, seed coating provides an opportunity to package effective quantities of materials such that they can influence the microenvironment of each seed. By not having to treat the remaining bulk of their soil, farmers may be able to save on the inputs required and the associated costs of applying them. Because seed coatings offer such opportunities for cost saving and increasing effectiveness, they have been studied widely for many years and yet, with some exceptions (for example, precision coating of sugarbeet and some vegetable seeds, fungicide and insecticide seed treatment of grain crops, and inoculant coatings on legume seeds), much of the world’s crop and pasture seed is still sown without any coating. In this review, an attempt has been made to draw not only upon the scientific literature, but also the patent literature, which contains much of the current expertise associated with seed coatings. The review places particular emphasis on areas that have not been dealt with in detail before, namely, the seed coating process and nutrient and herbicide seed coatings.

B. EVOLUTION OF SEED COATINGS Seed coatings have evolved from those which protect the seed from fungal and insect attack to a diverse range of coatings, the objectives of

SEED COATINGS AND TREATMENTS

45

which include the protection of rhizobia, supply of micro- and macronutrients, protection from birds and rodents, supply of growth regulators, attraction of moisture, supply of oxygen, germination stimulation, germination delay, increase in seed weight or size, and the supply of selective herbicides or herbicide antidotes. In spite of a considerable amount of research, reliable and effective seed coatings are not currently available for many crop and pasture situations. Even in the case of legume inoculant coatings, which have been intensively studied, there is still no universally accepted practice of inoculation, particularly for coatings applied well in advance of planting (as in preinoculation). The treatment of seeds with fungicides and/or insecticides is a relatively common practice compared to other coatings and, provided the materials are not phytotoxic, few problems occur. Problems with seed coatings become much more apparent when relatively large quantities of coating materials are applied to the seeds. Reports of ineffectiveness of coatings or lower seedling establishment due to coatings are relatively common in the literature. Successful results with coatings are also reported, however and it is these success stories that indicate that seed coatings do have the potential to overcome some of the problems of plant establishment. Much of the literature related to seed coatings consists of reports of ad hoc testing of various chemicals, coating materials, additives, etc. applied to seeds in ways that are often ill-defined; these diverse reports indicate that, so far, there has been little concerted effort to view seed coating as a branch of science requiring many basic principles to be understood before substantial progress can be made. This view is supported by Heydecker and Coolbear (1977), who stated in their review of seed treatments that “progress in the technology of pelleting is not facilitated by the fact that manufacturers keep their materials and processes a closely guarded secret.” I n fact, a significant proportion of the literature consists of work validating or evaluating the efficacy of proprietary products that are applied to seeds. Factors that may be crucial to the success or failure of coated seeds (such as the particle size distribution of the coating material, the exact specifications of the adhesive used, or the porosity of the coating) have largely been ignored by researchers reporting on seed coatings. By not specifying precisely how coatings have been prepared, researchers have made it virtually impossible for their work to be validated by others, and hence, there are many reports of workers finding different results from supposedly similar coatings. Because of the limitations mentioned above, the potential of seed coatings has not yet been explored thoroughly. When coating materials and techniques are identified more precisely, producing repeatable coatings will become relatively straightforward and an understanding of how each coating affects seeds under various conditions can be developed.

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Figure I shows how some of the factors that affect the performance of seed coatings can be conveniently grouped into seed, coating, soil, and aerial environments and then further partitioned into physical, chemical, and biological sectors. Much of the research done to date has been conducted in such a way that general principles have been difficult to transfer from one area of seed coating research to another. Knott and Lorenz (1950) reviewed much of the early work on seed coatings, which largely concerned the development of relatively inert coatings that permitted the rounding and enlarging of small seeds (particularly vegetable and sugarbeet seeds) sufficiently to facilitate precision mechanical planting. More recently, reviews

I

AERlAL ENVIRONMENT

AERIAL ENVIRONMENT

BIOLOGICAL FIG. 1. Schematic diagram showing the relationships among factors affecting the performance of coated seeds. Factors that are italicized are those on which some research has been reported.


47

have covered seed treatments and coatings with an emphasis on osmoconditioning (Heydecker and Coolbear, 1977),seed treatments, especially those containing fungicides and insecticides (Jeffs, 1986), and pelleting and other pre-sowing seed treatments (Tonkin, 1979, 1984).

C. DEFINITIONS The literature contains many inconsistencies in its terminology regarding seed coating; for example, pelleting has been defined as a process of inoculation followed by lime or clay (Johnson, 1971), a process whereby the number of seeds per pellet is not accurately controlled (Roos and Moore, 1975), a process primarily aimed at creating single seeds to aid precision planting (Johnson, 19751, and a process of creating more or less spherical units for precision sowing, usually incorporating a single seed with the size and shape of the seed no longer readily evident (International Seed Testing Association rules, cited by Tonkin, 1984). In view of such inconsistencies, I would first like to define some of the terms commonly found in the literature before reviewing aspects of the production of coated seeds and of the many types of coated seeds which have been described. 1. Seed coating. A general term for the application of finely ground solids or liquids containing dissolved or suspended solids to form a more or less continuous layer covering the natural seed coat: includes pelleting and many other seed treatments. 2. Seed treatment. A broad term that does not specify the application method but merely indicates that seeds are subjected to a compound (chemical, nutrient, hormone, etc.), process (such as wetting and drying), or to various energy forms (e.g., radiation, heat, magnetism, electricity). This also includes the less commonly used term seed dressing which refers to the application of finely ground solids (usually a fungicide or insecticide) dusted onto the surface of seeds in small quantities to protect seeds from disease and/or pests. 3. Seed pelleting. The application of solid materials to seeds in sufficient quantity to make the pelleted seed substantially larger and/or heavier and approach a spherical or elliptical shape. 4. Seed soaking. A process by which seeds can be led to absorb nutrients, protectants, growth regulators, etc. by immersing them in appropriate solutions for extended periods. 5 . Seed tablet. A composite of seed and solid materials formed by compression in a tablet press such as is used in the pharmaceutical industry.

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JAMES M. SCOTT

6. Seed inoculation. The application of microorganisms (e.g., rhizobia, bacteria, mycorrhizas) to seeds.

II. THE SEED-COATING PROCESS Specific information on the chemical and mechanical engineering aspects of seed coating is rare in the scientific literature. Most information resides in the patent literature and as “art” or “skill” in commercial organizations involved in the production of coated seeds and hence is not widely available. Much can be learned, however, from the literature from other fields in which the binding of particles in granules or tablets is important, such as in the iron ore, fertilizer, and pharmaceutical industries. Rumpf (1962), for example, has reviewed the literature concerning the bonding mechanisms between particles with particular emphasis on the development of strong granules that are able to withstand mechanical handling. A broad review of the literature concerning the pelletizing of iron ore is provided by Goldstick (1%2), and Newitt and Conway-Jones (1958) describe in detail the processes of granule formation. Coating small quantities of seeds with a uniform and consistent quantity of material is a difficult task for research workers in agronomy. Gilbert and Shaw (l979), for example, noted the difficulty of preparing relatively large seed pellets containing sulfur and overcame the problem by placing the sulfur close to the seeds (within 2 mm) to simulate the effect of coating. Any physical effects of seed coatings are thus ignored by this simulation and the results cannot be interpreted as being the same as if the seeds were actually coated.

A. COATINGEQUIPMENT The equipment used in the granulation industries has been described by Lyne and Johnston (1981) and Kapur (1978);the methods and equipment used for the coating of tablets in the pharmaceutical industry have also been well described (Lachman et al., 1970). The most commonly applied seed coatings are those in which a trace quantity of fungicide and/or insecticide is applied to seeds in such a way that this small quantity is evenly distributed among the seeds. Excellent reviews of the many types of equipment used to apply such materials are provided by Purdy (1967), Harris (1975), and Jeffs and Tuppen (1986). When relatively large quantities of materials are applied to seeds, rapid continuous flow equipment (such as can be used for fungicide application)


49

is inappropriate, because larger coatings require equipment that allows some “residence” time so that the coating can accumulate on the seeds’ surfaces. This is most commonly achieved in equipment similar in principle to a cement mixer, such as an open, inclined drum or pan that rotates at constant speed (5-35 rpm, depending on diameter) while the load of seed, adhesive, and coating material tumble in the drum, with the adhesive and coating material usually being added sequentially. Such equipment has been used by commercial seed coating companies as well as by research workers (e.g., Fraser, 1966). The process involves the gradual accumulation of successive layers of adhesive and coating material on the seed and the operation of such equipment requires considerable skill (Kirk, 1972). Some of the best descriptions of the pan coating process for coating seeds are contained in patents (e.g., Funsten and Burgesser, 1951; Ostier, 1953). The quantities of seed which can be processed in coating pans are quite limited: pharmaceutical coating pans most commonly permit coating of up to 100 kg of tablets in a few hours. However, some continuous flow operations (in which only a relatively small quantity of coating material is being applied) can allow processing rates of up to 7 metric tons (Mg) of product per hour (F. E. Porter, personal communication). One of the problems of seed coating is that usually only singulated coated seeds are required and the production of any oversize material is quite undesirable. In contrast, most of the agglomerating equipment used for the production of granules in the mineral industries is designed for cpntinuous operation, relying on the screening out of agglomerates too large or small for the required product and their subsequent return to the processing chain. Such machines can generate four times as much return material as that being taken off as acceptable product; this type of agglomeration is usually carried out in an inclined horizontal drum or in a pelletizing pan (inclined disk). These machines can produce tens of metric tons per hour of operating time (Lyne and Johnston, 1981). The equipment’s diameter, rotational speed, angle of inclination, and the characteristics of the coating material (e.g., particle size distribution, moisture content) can all have a profound effect on the speed of agglomeration and the size and quality of the agglomerates produced. Among the more novel processes that have been used for coating seeds are extrusion, compression, and fluid-bed methods. The extrusion of large pellets containing several seeds per pellet has been described by Hall et al. (1974). A range of smaller pellets, containing from one to several seeds per pellet, have been described in patents of processes in which the extrusion process is followed by rolling of the pellets (Coated Seed Ltd., 1975a,b). The production of seed tablets by compression has been described by

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JAMES M. SCOTT

Hirota (1972b). Further descriptions are contained in the patents of Brink (1975) and Adams (1971). Other researchers have patented methods by which damage to the seed during compression can be avoided (Brink, 1971; Knapp, 1973). Clifford (1971) patented a method of producing a tablet which allows for adequate “ventilation” of the seed within the tablet and thus allows the free entry of air and water into the seed. Fluid-bed processing of seed has been utilized for the application of inoculants (Nack and Porter, 1965; Mullett et al., 1974), protectant coatings (Dannelly, 1981a), and germination delay coatings (Schreiber and La Croix, 1970). A further novel method of coating “seeds” is that reported by Rogers (1983) whereby somatic embryos produced by tissue culture can be encapsulated within gelatinous capsules, which are then coated with a biodegradable polymer to improve handling characteristics. B. ADHESIVES The process of seed coating usually involves the use of adhesives (also known as glues, binders, or stickers) to bind materials to the surface of seeds; coating without them using, for example, water alone will usually lead to fragile coats that are extremely prone to dusting and cracking and subsequently to the loss of the active ingredient. Such dusting and breakage can also lead to severe problems in handling the seed due to hazards to operators and to problems in mechanical planting. The physical integrity of coated seeds is of great importance in any handling, transport, and planting operations. In spite of this, no literature was found on methods available for evaluating the physical quality of seed coatings; in the pharmaceutical literature, however, there are published methods available for testing the strength and integrity of tablets and coatings (Lachman et al., 1970). When fine particles are agitated, as in a coating drum, they tend to aggregate naturally even without adhesives (due to cohesive mechanical, van der Waals, and electrostatic forces) and hence the need for an adhesive is for one that assists these natural forces of aggregation while allowing the packing of particles and “densification” to continue while the particles are tumbling. Thus, the adhesive required need not be extremely strong but must be an appropriate adhesive, one that has affinity for both the natural seed coat and for the coating material. Current adhesive technology permits adhesives to be selected with the appropriate affinity for selected substrates, the required degree of water solubility (or insolubility), the required strength and plasticity to prevent


51

breakage, dusting, etc., and the most appropriate viscosity for ease of application. However, studies of the binding qualities of adhesives used for seed coating are relatively rare. Again, this is in contrast to other industries (e.g., the pharmaceutical, building, and manufacturing industries) in which there has been intensive research carried out into bonding mechanisms, bond strengths, failure to bonds, etc. The tablet making and coating processes of the pharmaceutical industry are quite closely akin to those of seed coating, and the processes and adhesives used have been well reviewed by Lachman et al. (1970). Millier and Bensin (1974) have shown that the degree of attraction or repulsion of moisture by the seed coating can have large effects on the germination of coated seeds. In terms of their binding ability and their propensity to cause undesirable agglomeration of seeds, there are large differences between the performance of adhesives. Hirota (l972a) examined a range of adhesives for binding diatomaceous earth to seed of Vicia villosa and found that a mixture of methyl cellulose and gum arabic performed the best. The coating of seeds with activated carbon has been successfully achieved with gum arabic plus a plasticizer (Sharples, 198I), methyl cellulose (Vogelsang, 1954), or polyvinyl acetate (Nagju, 1973). Tests of the performance of adhesives for adhering lime to grass seeds were conducted by Hathcock et al. (1984a), who showed methyl cellulose to be the most effective; when the coated seeds were sieved for 1 min, methyl cellulose resulted in the best retention of lime, but this was still only 71% of the lime applied. Scott (1975b) also evaluated a range of adhesives and coating materials for seed coating, but the number of interactions involved make it difficult to draw general conclusions about the efficacy of particular adhesives. By far the most studied aspect of adhesives used in seed coating is not their binding ability but rather their effect on the survival of rhizobia following seed inoculation. Those recommended by various authors include gum arabic (Brockwell, 1962; Norris, 1972), methyl cellulose (Faizah et al., 1980; Norris, 1972), gelatin and casein (Thompson, 1961),and caseinate salts (Lloyd, 1979). In practice, however, methyl cellulose is most widely used due to its ease of use, availability, low cost, and low rate (3% w/v solutions) compared to gum arabic (up to 45% w/v). Many claims regarding the efficacy of adhesives are contained within the patent literature. There are patents covering the use of mineral oil (Rushing, 1982),plastic resins (Eversole and Roholt, 1963),polyvinyl acetate (Barke and Luebke, 1981), and insoluble polyelectrolyte complexes (Dannelly, 1981a)to bind pesticides to seeds, polyethylene oxides to prevent erosion of surface-sown seed (Porter and Kaenver, 1976), polyurethanes to bind lime in a way that resists coat abrasion (Porter and Kaerwer,

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1974), blends of polyvinyl alcohol and polyvinyl acetate to bind vermiculite (Kirk, 1972), and polyelectrolytes (Hedrick and Mowry, 1953) or dextran (Peake, 1956) to aggregate soil around the seed, thereby improving the aeration of the sown seeds. Kitamura et al. (1981) also patented a process for the surface coating of fine powders with a water-soluble binder to aid subsequent adhesion of the powder particles during the coating process. C. COATINGMATERIALS The literature concerning the various solid materials used in seed coatings is concentrated, as with adhesives, on the effects of the materials on the survival of rhizobia (e.g., Brockwell, 1962; Lowther, 1975). The most common materials used as protectants for rhizobia include lime, gypsum, dolomite, or rock phosphate. Other materials mentioned in the literature include clay minerals such as montmorillonite (Bergersen et al., 1958; Hirota, 1972a; Burba, 1981) and vermiculite (Sharples and Gentry, 1980), which have been used principally as carriers for chemicals or as pelleting materials. Several other mineral materials that have been found to be useful for different purposes (e.g., pelleting, inoculation, or simply as diluents) are bauxite (Norris, 1973), diatomaceous earth (Hirota, 1972a), pumice, sand (Burba, 19811, and talc (Brinkerhoff et al., 1954; Vartha and Clifford, 1973a). Many organic materials have been used in coatings, usually as protectants or sources of nutrition for rhizobia or the developing plant. These include blood (Brockwell, 1962), bonemeal (Faizah et al., 1980), peat, poultry manure, moss (Hirota, 1972a),and mucilage (Harper and Benton, 1966). There is little information in the literature regarding the specifications of coating materials used by workers studying seeds coatings. The chemicals analysis, pH, purity, and particle size distribution are rarely noted. Thus, the many reports of coatings made with lime, for example, have been produced using materials from different sources and perhaps with different physical and chemical attributes. The effect of particle size of solid materials, in particular, is crucial to any coating or agglomeration process; such effects have been well researched in the iron ore-pelletizing industry. Urich and Han (1962), for example, report that iron ore pellets made with smaller particles (73% < 15 pm) had a porosity one-third and a strength three times that of pellets made from larger particles (6% < I S pm). Not only is the mean particle size important, but the particle size distribution also affects the quality of granulation (Newitt and Conway-Jones, 1958). The addition of small


53

particles to a material containing relatively large particles, will result in more rapid granulation, increased granule strength, and decreased porosity. Thus, this is one of the dilemmas in seed coating; although high integrity and strength of the coating are desirable, low porosity is undesirable because it may restrict the movement of air to the seed. The question of how the particle size of seed coating materials can affect the performance of seed coatings has been addressed by few workers. Loperfido (1975) noted in a patent that many commercial coatings for precision sowing contain particles that are sufficiently fine that they can decrease germination due to the limitation of free gas and water exchange between the seed and the soil. In his patent, he claims the use of relatively large, hydrophobic, polypropylene beads (15G750 pm in diameter), which can be coated on lettuce seed to permit precision sowing without loss of germination ability. In this way, he suggests the coating can have a porosity of 15-25% made up of voids 2-100 pm in diameter. Sharples (1981) also investigated coatings to facilitate precision sowing of lettuce seed and suggested a model of seed coatings in which relatively large particles (minimum diameter of 180 pm are coated in a layer adjacent to the seed to facilitate oxygen diffision to the seed. Outside this inner layer, he claims that smaller particles (such as diatomaceous earth or clay), can be applied without detriment to the germination of the seed.

Ill. COATINGS TO FACILITATE PLANTING The mechanical planting of seeds is facilitated by having seeds that are of uniform size and shape, have sufficient size and weight to be easily separated mechanically, and flow readily without clumping together; seed coatings have been employed to achieve all of these features. A. PRECISION SOWING Increasing the size and weight of seeds is particularly useful for very small seeds (such as some vegetable and flower seeds), thus permitting precision planting, which results in uniform plant populations and can eliminate the need for crop thinning (Johnson, 1975; Robinson and Mayberry, 1976; Robinson et af., 1983). Such coatings can increase the weight of small seeds by 4-500 times the raw seed weight (Burgesser, 1951~). Coatings have also been used to change the shape of seed (e.g., from flat to round) and to add powdered lubricants to aid in the planting operation.

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Roos and Moore (1975) cite a survey of vegetable growers in the United States which found that 43% of growers used precision planting techniques for their crops and that coated seed played a significant role in such plantings. Roos and Moore also found that commercially coated lettuce seed produced by seven different companies did not differ in performance from raw seed except for causing a slight delay of 1-2 days in emergence. Seed coatings have also been widely used to aid the sowing of sugarbeet seeds, which have an irregular shape. Funsten and Burgesser (1951) describe a patent of a process in which the use of a nonswelling sub-bentonite (i.e., dominated by Ca and Mg ions) permits the coating of sugarbeet without using any binder. Another method of coating seeds for precision sowing, proposed in a patent by Hamrin (1973), suggests the encapsulation of carrot seeds within a gel coating. Several patents concerning coatings for precision sowing suggest that emergence can be a problem under high moisture conditions and claim that if this occurs, the aeration of the seed can be improved by blending of vermiculite with bentonite in the coating (Burgesser, 1951b), by using acid-activated bentonite (Burgesser, 195la), or by coating the seed with relatively large (150-750 pm in diameter) polypropylene beads (Loperfido, 1975). Sachs et al. (1981) demonstrated that delays in emergence observed with clay-coated sweet pepper seed were due to a reduction in oxygen supply; they reported that improved commercial formulations of coated sweet pepper seed have recently become available but, as with much of the information concerning commercial preparations, there was no disclosure of how these formulations achieved their results. B. IMPROVED BALLISTICS Increasing the weight of seeds to aid in aerial sowings has been described by several workers. For pasture seeds, Scott (1975a) found that doubling the weight of grass seeds through coating increased their terminal velocity in air, but similar coatings had little effect on the velocity of the more dense legume seeds. Coating grass seeds to a weight of 2-10 mgheed, he found that when aerially sown they behaved similarly to legume seeds and small fertilizer granules, thus reducing the separation of seed and fertilizer that can occur in such sowings. Hay (1973) found that seed coatings increased the capacity of aerially sown seed to penetrate into standing vegetation compared to raw seed but that this advantage was negated if any wind disturbed the vegetation. Similar coatings are also claimed to have improved the ballistics of rice seed aerially sown into flooded rice paddies (Mickus and Munson, 1978).


55

IV. INOCULANT COATINGS A. RHIZOBIA I . Inoculation Processes Methods of inoculating seed of temperate species have been reviewed by Burton (1967) and Brockwell(1977), and many aspects of the inoculation of tropical legumes have been summarized by Date (1976). The process of inoculation is principally concerned with choosing a culture of viable rhizobia of the appropriate strain contained in a suitable carrier or medium and applying it to the seed of the legume host. By far the most common method of inoculation is to apply the rhizobia within a coating on the surface of the seed, usually employing an adhesive that improves the binding of the inoculum to the seed and aids the survival of the rhizobia until planting (e.g., Burton, 1961). Other techniques have been employed, such as impregnating the seed using vacuum (Porter, 1960) or by pressure impregnation (Brockwell and Hely, 1962), but it is generally agreed that the seed coat itself is a relatively hostile environment for rhizobia. One of the adverse conditions faced by rhizobia following inoculation is rapid desiccation, the effect of which may be lessened by growing the organism in a complex organic substrate, such as peat, to which the organisms can become tightly adsorbed and thereby gain protection. Thus, peat is usually the preferred camer for rhizobia inoculated on seed. Adsorption onto other substrates such as montmorillonite clay can also aid in the protection of rhizobia from desiccation (Marshall and Roberts, 1%3). Rhizobia may also require protection from microbial antagonisms (Bergersen et al., 1958) and from toxic materials released from the natural seed coat (Thompson, 1961; Hale, 1976; Hale and Mathers, 1977). The work of Hale and Mathers has been further developed and included within a patent for a commercial process that assists the survival of large numbers of rhizobia on seed such as white clover (which otherwise would be toxic to the rhizobia) by the inclusion of an adsorbent in the inoculum (Coated Seed Ltd., 1983).

2 . Lime Coating Early work in Australia (Loneragan et al., 1955) and New Zealand (Lobb, 1958) established that on acid soils the inoculation of legume seeds

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could be made much more effective if the inoculated seed was enclosed within a lime coat. Today, lime coating of legumes following inoculation is widely practiced in both Australia and in New Zealand, particularly on acid soils. A practical guide to the operations involved in the lime coating of small batches of legume seeds was given by Plucknett (1971). In Canada, Kunelius and Gupta (1975) found lime coating increased alfalfa establishment on soils with a pH of 5.0-5.6. However, in the United States, some workers have shown that lime coating is not nearly as beneficial as has been shown in Australia and New Zealand (e.g., Olsen and Elkins, 1977). This experience may be associated with somewhat higher soil pH, as was found by Lowther (1974) in New Zealand; he demonstrated that lime coating could even depress plant growth at or above a pH of 6.2, supposedly due to a lime-induced nutrient deficiency. With tropical and subtropical legume species, the benefits of lime coating are less clear. Although positive responses to lime coating have been obtained by some workers (e.g., Cook, 1978), others have claimed that lime coating can be ineffective (e.g., Norris, 1967). A useful review of this controversial area is given by Snyder and Kretschmer (1981). B. VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI The effect of vesicular-arbuscular (VA) mycorrhizas on the growth of plants has received considerable attention from researchers. Seed inoculation of white clover with VA mycorrhizas was found to be ineffective by Boatman et al. (1980), due perhaps to loss of viability of the mycorrhizas or to the soil already being colonized by effective mycorrhizas. Powell (1979) found that the pelleting of either ryegrass or white clover with soil that was heavily infested with VA mycorrhizas increased yields by 4080%. Nevertheless, large-scale inoculation is not yet feasible because a means of culturing VA mycorrhizas in vitro has yet to be developed (Gianinazzi-Pearson and Diem, 1982).

C. OTHERORGANISMS Most work on the application of organisms other than rhizobia or VA mycorrhizas to seeds concerns disease-controlling organisms. The inoculation of cotton seeds with spores of Trichoderma spp. has been found to control Rhizoctonia solani (Elad et al., 1982) while the application of some biotypes of Trichoderma viride suppressed damping-off diseases of peas and beans (Papavizas and Lewis, 1983). Similarly, Rhizoctonia bataticola of gram was controlled with coatings containing certain Bacillus


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and Streptomyces species (Singh and Mehrotra, 1980). A reduction in the severity of take-all of wheat has also been shown to be possible using Pseudomonas spp., and methods of wheat seed inoculation are being investigated (Wong and Baker, 1984). Antibiotics have also been used successfully in seed treatments to reduce disease.

V. PROTECTIVE COATINGS A. DISEASES The research work cited above concerning the biological control of disease is dwarfed by the literature concerning the use of seed-applied fungicides to control plant diseases. Such seed coatings or treatments are commonly used to combat diseases that can cause enormous losses to plant populations and productivity. Today, fungicidal coatings are routinely applied to seeds of a wide range of crops to protect against fungal pathogens resident in soils and also carried in the seed coat itself. A useful historical account of the development of fungicide and insecticide seed treatments was given by Callan (1975) and a detailed text on all aspects of seed treatments has been compiled by Jeffs (1986). Some examples of the many successful fungicide seed treatments which have been developed include the control of Septoriu nodorum on wheat (Cunfer, 1978), bunt and smut diseases of cereals (Alcock, 19781, powdery mildew of barley, Rhizoctonia in cotton (Cole and Cavill, 1977), and sugarcane downy mildew of maize (La1 et a f . , 1979). Newer fungicides have been developed that not only provide some protective action but can also have a curative effect on some diseases (Nesmith, 1984). An example of such a fungicide is metalaxyl, which is more effective in controlling Pythium and Phytophthora diseases when applied to seeds than foliar sprays (La1 et a f . , 1979). Pasture species also benefit greatly from fungicide seed treatment. Damping-off diseases of alfalfa and ryegrass, for example, can be effectively controlled by fungicide seed treatments (Falloon, 1980). Seed-applied fungicides have been found to not only improve emergence but to increase yield, presumably due to reducing the level of subclinical disease (Falloon and Fletcher, 1983). The effects of fungicides are often more noticeable when conditions suit the development of fungi, such as when low temperatures prevail (Yen and Carter, 1972). As sowing conditions become less favorable, the benefits of seed treatment with fungicide become more apparent. The compatibility of fungicide seed treatments with other pesticides has

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been examined by several workers. For example, Chatrath et al. (1977) found that either of two fungicides, carboxin or benomyl, could be used on wheat seed together with either of two insecticides, BHC or malathion, without any deleterious effect on germination or effectiveness. Some fungicides have been found to be incompatible with some insecticide seed treatments, however, the combinations adversely affecting germination and plant populations. Interactions have also been found between fungicide-insecticide seed treatments and soil-applied herbicides. Improvements in fungicide formulations have resulted in much better adhesion of the fungicide to seeds and less dusting, with the result that hazards to operators are reduced and efficacy is increased. Other developments in fungicide seed treatments that may lead to more effective control of diseases include the use of solvent delivery systems, which can improve the control of fungi within the seed (Vidhyasekaran, 1980), the development of controlled release pesticides (Anderson and McGuffog, 1983), and the improvement of integrated control practices (Nesmith, 1984).

B.

INSECTS, PESTS, AND OTHER

FAUNA

Seed coatings containing insecticides or acaricides have been used widely on many plant species, often in combination with fungicides. Examples of some of the pests controlled with seed treatments include leafhoppers on rice, barley, and soybeans; onion fly on onions; soldier fly larvae in ryegrass pastures, and shootfly in maize. Examples of the use of acaricide seed treatments include the control of spider mites in cotton and red-legged earth mites in pastures. The use of insecticides applied to the seed is a practice which, for many pests, is more amendable to integrated pest control than is overall spraying, as less chemical is used and it is localized in the area where it is needed. As with fungicides, insecticides used on seeds need to be compatible with any other pesticides being used. The theft of pasture seeds by ants can be a serious problem, particularly for surface sowings. The degree of ant theft has been reduced by seed treatments with permethrin or bendiocarb (Campbell and Gilmour, 1979) and to some degree by commercial seed coatings without insecticide, which the ants tend not to recognize as food (Johns and Greenup, 1976). The control of molluscs using seed coatings has been attempted using rnethiocarb on clover. Although some increase in establishment was observed for surface sowings in spring, no such increase was observed for autumn sowings. There appears to be a need for either a better seed coating


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formulaton of methiocarb or, alternatively, for a more effective active ingredient (Charlton, 1978; Welty et al., 1981). Some coatings designed for other purposes may also require the inclusion of an effective arthropod repellent; Dunning and Baker (1977) found that coatings for the precision sowing of sugarbeet seeds resulted in increased damage caused by millipedes. A number of workers have studied ways of using protectant seed coatings to protect seeds from birds or rodents. In some cases, pelleting of seeds with nutrients, clays, lime, etc. is sufficient because the pests may not recognize the seed as food. However, this is often not the case with birds and rodents and chemical repellents can be required. Some of the coatings used to repel birds include those containing methiocarb, endrin, or thiram (Naquin, 1978). Rodents have been successfully repelled with coatings containing mestranol on seed of Douglas fir and resorcinol on wheat seed (Fuchsman, 1972). AGAINST HERBICIDES C. PROTECTION

Materials which can aid in protecting seeds or seedlings from herbicides can be divided into those that act either chemically (i.e., antidotes) or as adsorbents.

1 . Antidotes The development of herbicide antidotes is a relatively new branch of science: the first patent was taken out by Hoffman (1964), who described “antagonistic agents” such as oximes, which reduced plant injury due to thiocarbamate herbicides. Reviews of herbicide antidotes have been written by Blair e? al. (1976) and by Pallos and Casida (1978), the latter authors concentrating on the chemistry and mode of action of the antidotes. Subsequent reviews summarizing this active area of development over recent years have been published by Hatzios (1983) and Parker (1983). The first commercially produced herbicide antidote was 1,g-naphthalic anhydride (NA). When used as a seed treatment to protect maize from S-ethyl dipropylthiocarbonate (EPTC) damage, NA permits good weed control while maintaining selectivity (Burnside et al., 1971; Hoffman, 1971). The use of NA as a seed treatment to protect maize from EPTC damage has now largely been replaced by a more effective antidote, R-25788, which is marketed as a mixture with the herbicide EPTC and renders seed treatment unnecessary (Chang et al., 1973). R-25788 has also

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been found to be effective for use on other crops such as barley (against the herbicides vernolate and EPTC) and wheat (against the herbicide triallate) (Miller and Nalewaja, 1980). Some antidotes have been shown to cause injury to the sown species in the absence of herbicide [e.g., N A on sorghum (Eastin, 1972) and allidochlor on corn (Chang et al., 1973)l. An active area of antidote development has been the protection of sorghum from acetanilide herbicides (Brinker et al,, 1981;Chang and Merkle, 1982; Ellis et al., 1980; Moshier and Russ, 1980; Schafer et al., 1980). The most effective method of antidote application has been shown to be quite different for various antidotes: MON-4606 and CGA-43089 are more effective as seed treatments than as tank mixes with herbicides, while N A is effective only as a seed treatment. Although R-25788 is effective as a tank mix, it may be more effective as a seed treatment (Spotanski and Burnside, 1972; Miller et al., 1973).

2. Adsorbents

The use of adsorbents as a means of inactivating herbicides has been reviewed by Blair et al. (1976)and Gupta (1976). Reports of the successful inactivation of herbicides using seed coatings of activated carbon include those on maize against a range of herbicides, on cotton against alachlor, on pregerminated rice seed against several herbicides (Nagju, 1973), on kikuyu seed against atrazine (Cook and O’Grady, 1978), and on Australian native grasses against diuron and chlorthal dimethyl (Hagon, 1977). Several patents also claim that activated carbon seed coatings can provide good protection against herbicides such as (2,4-dichlorophenoxy)acetic acid (2,4-D) (Vogelsang, 1954) and methylurea and triazines (Johnson et al., 1972). Hahn and Merkle (1972) found that although an activated carbon seed coating provided some protection for sorghum against the herbicide alachor, it nevertheless was far less effective than NA. Activated carbon seed coatings were also found to be relatively ineffective in protecting grasses (Bertges, 1977) and lettuce (Richardson and Jones, 1983) from several herbicides. Gupta (1976) reports that activated carbon and NA seed coatings were equally effective in protecting maize from low rates of EPTC, but at higher herbicide rates NA was more effective. The application of activated carbon in bands along the row of seed can be as effective as antidotes in protecting crops from herbicide injury, but as noted by Gupta (1976), such band applications may require high rates (up to 336 kg/ha) and also may suffer the disadvantage of protecting weeds located within the band. Activated carbon has also been examined as a minor component of


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coatings for the precision sowing of vegetables; when it was included in a vermiculite coating (at 2.5% w/w), the germination of coated lettuce seed was vastly improved due to the adsorption of germination inhibitors released by the natural seed coat (Sharples and Gentry, 1980). The use of seed coatings containing another adsorbent, polyvinyl pyrrolidone, has been proposed as a means of enhancing the establishment of direct-drilled seeds by the adsorption of phytotoxic phenolic compounds released by herbicide-killed swards (Habeshaw, 1980).

VI. NUTRIENT COATINGS The incorporation of nutrients in seed coatings provides a unique opportunity to supply each sown seedling with an accurately controlled quantity of nutrient that may be preferentially available to the sown species and less available to any neighboring weed species. This area of seed coating development has received relatively little attention in previous reviews. In addition to nutrient coatings, the soaking of seeds in nutrient solutions will also be dealt with in this section. A. NEEDFOR EARLYSEEDLING NUTRITION The need for young seedlings to utilize sources of nutrients external to the seed early in life has been clearly shown by Krigel (1967) with subterranean clover (Trifolium subterraneum). Even though subterranean clover has a relatively large seed compared to many other pasture species, Krigel observed responses to external sources of nutrients as early as 7 days after sowing for calcium, 10 days for phosphorus, 14 days for nitrogen and magnesium, and 21 days for potassium. Responses of pasture species to the early supply of external nutrients (particularly of phosphorus and nitrogen) has also been shown by McWilliam et al. (1969), Lazenby and Schiller (1%9), and Blair et al. (1974). Some species, of course, have much smaller seeds than subterranean clover and the extent of their nutrient reserves is quite limited. Ozanne and Asher (1965) suggested that the low potassium content of some pasture seeds may limit their establishment, particularly when sown at depth, and proposed that a seed coating containing a form of potassium safe for the seed may overcome such limitations. In view of the large influence which available sources of phosphorus (P) can have on early seedling growth, more information is needed concerning the P nutrition of seedlings (Silcock, 1980); the critical P concentration in the soil solution required for maximum growth is far greater

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during seedling growth than later in a plant’s life (Fox et al., 1974). Crops such as wheat require much higher rates of P up to the end of tillering than during the grain filling period (Sutton et al., 1983). With wheat growing in solution culture experiments, Ningping and Barber (1985)found a maximum concentration of P in the shoots at 25 days after sowing.

B. MACRONUTRIENTS 1 . Coatings

Although many workers have reported that nutrient seed coatings can cause damage during germination or that they supply little nutrition to seedlings, the literature nevertheless contains numerous reports of cases in which the supply of macronutrients by seed coatings has been substantial. Reports of P coatings that have been used successfully include a positive response of ryegrass to a P coating 2 years after sowing (Vartha and Clifford, 1973a), a two- to fourfold increase in the establishment of ryegrass coated with P compared to ryegrass with no coating (Vartha and Clifford, 1973b),an increase in the survival of direct-seeded lettuce grown from tablets incorporating a small quantity of P (Sharples and Gentry, 1980), and an increase in corn yield brought about by a seed coating containing as little as 0.2 kg P/ha (Guttay et al., 1957). Sulfur ( S ) has also been incorporated successfully in a number of seed coatings: gypsum-coated clover seed showed twice the establishment of lime-coated clover in a sulfur-responsive site (Lowther and Johnstone, 1979); Stylosanthes guianensis coated with elemental S or gypsum, produced significantly more yield than with the equivalent rate of broadcast S (Gilbert and Shaw, 1979);the addition of S and molybdenum to P coatings increased the establishment of clovers (Scott and Hay, 1974);and a coating containing elemental S approximately doubled the establishment of oversown legumes (Scott and Archie, 1978). Successful coatings containing nitrogen (N) appear to be those in which the N is present only in small quantities. When small amounts of an NP-K fertilizer have been applied to corn seed, for example, early growth and nutrient uptake have been enhanced (Miller et al., 1971), and the efficiency of uptake of P in particular was increased three to four times (Smid and Bates, 1971) compared to the effect of band application of the fertilizer. Other workers have found some low rates of macronutrient additions to be safe for germinating seeds, such as a coating on red clover seed containing 5% of either superphophate or an N-P-K fertilizer (Hirota,


63

1972a) or a seed tablet including a small proportion of an N-P-K fertilizer and I% algin (Hirota, 1972b). There are few reports of the benefits of coatings containing two or more nutrients. One example is the increased dry matter production and nutrient uptake which resulted from the inclusion of P and zinc (Zn) in seed coatings for wheat (Gawade and Somawanshi, 1979). N and P have been applied singly and together in various combinations with lime on seeds of tall fescue and Kentucky bluegrass with the result that some N and P formulations produced better growth than coatings containing either nutrient alone (Hathcock et al., 1984b). Combinations of N and P compounds can produce large responses in the early growth of forages and crops, but the chemical form of the N and P compounds must be chosen carefully to ensure maximum effectiveness (Sheard, 1980; Costigan, 1984).

2. Soaking The soaking of seeds in macronutrient solutions, as opposed to coating seeds, has received relatively little attention. Soaking seeds in such solutions has been claimed to increase the seedling growth of sugarbeets (Miyamoto and Dexter, 1960) and grain yield and nutrient uptake of cereals. However, the mechanisms responsible forthese claimed increases in growth and yield have not been adequately explained. In contrast, negative results were obtained by Guttay et al. (1957) from corn seed soaked in a phosphate solution. Of course, compared to nutrient seed coatings, the soaking of seed can only supplement the seed’s mineral reserves to a small extent. C. MICRONUTRIENTS

1 . Coatings A range of micronutrients has been successfully coated on seeds of various species. Molybdenum has been used in many seed coatings, particularly those applied to legumes, where molybdenum is an essential element for nitrogen fixation. Such coatings have increased clover establishment (Scott and Hay, 1974) and increased the growth and yield of cowpeas equivalent to liming the soil with basic slag at 3 Mg/ha (Rhodes and Nangju, 1979). Kerridge et al. (1973) found that the inclusion of molybdenum trioxide in rock phosphate coatings was as effective as a soil application in alleviating molybdenum deficiencies of a number of tropical legume species. Gault and Brockwell (1980) studied the various forms of

64

JAMES M. SCOTT

molybdenum and their compatibility with rhizobia and found sodium molybdate to be the only one which depressed nodulation. The application of zinc in seed coatings on rice has brought about yield increases (Thompson and Kasireddy, 1975) and generally been more effective than foliar sprays. Manganese applied to sugarbeet seed partially alleviated a manganese deficiency but required an additional foliar spray for complete correction of the deficiency (Farley, 1980). However, other micronutrients that have been coated on seeds have not been so successful, including magnesium (as dolomite) on legume seeds, in which case the beneficial effects were more likely due to the increased survival of rhizobia rather than to any direct nutritional effect on the plants (Brockwell, 1962).

2. Soaking The soaking of seeds in micronutrient solutions has been claimed to produce large responses in some situations, although the reasons for these responses are often not clear. Soaking seeds in solutions of magnesium salts increased the germination and fodder yield of pearl millet (Chhipa and Lal, 1976), whereas soaking oat seeds in manganous chloride alleviated a manganese deficiency (Drennan et al., 1961). Wilson and Notley (1959) overcame a molybdenum deficiency of tomatoes by soaking seed in a molybdenum solution, although the results of soaking lettuce were more variable. In other cases, negative effects of seed soaking in micronutrient solutions have been reported (Kereszteny, 1973). D. EFFICACY OF NUTRIENT SEEDCOATINGS The relative efficiency of fertilizer placement has been studied widely, particularly for comparisons between drilled and broadcast applications (e.g., Bates, 1971). Comparisons between nutrient seed coatings and alternative application methods have also been conducted. Smid and Bates (1971), for example, found that small additions of fertilizer in seed coatings were three or four times as effective in providing an early supply of P to corn seedlings as was band placement. The relative efficiency of fertilizer placed near the seed compared to broadcast applications has been shown to be higher under conditions where the available soil P is low (Fiedler et al., 1983). The early growth of buffel grass (Cenchrus ciliaris) seedlings was maximized by placing P such that it was available within 4 days of sowing (Silcock and Smith, 1982). Close placement, and therefore the availability, of nutrients to establishing seedlings appears to be most im-


65

portant for immobile elements such as P, especially under cool conditions, which restrict P uptake (Klepper et al., 1983). Scott and Blair (1988a) showed that the efficacy of different calcium phosphate seed coatings [mono- (MCP), di- DCP), and tricalcium phosphate (TCP)]in promoting seedling growth of phalaris (Phalaris aquatica) and alfalfa (Medicago saliva) increased with the solubility of the P source (i.e., from TCP to DCP to MCP). Each of the P sources studied markedly increased leaf number and increased yield (MCP had a greater effect than DCP, which in turn had a greater effect than TCP), but the biggest effect was on the P content of the seedlings, which increased greatly with increases in P rate and the solubility of the P source. In a related experiment, P seed coatings applied to phalaris seed containing the equivalent of 5 kg P/ha produced plants (at 27 days after sowing) as tall as those supplied with 20 kg P/ha by drill or broadcast methods (Scott and Blair, 1988b). The heights of individual plants were also more uniform in the coated treatment condition, suggesting that the seedlings obtained more uniform access to P from the coatings. At harvest, the differences between the coated and drilled application methods were greater for the dry matter yield and P content of roots than for shoots, suggesting that the coated seeds may have an increased chance of survival compared to drilled seeds if competition from weeds were for the belowground resources of either soil moisture or nutrients. Phalaris seed coatings with DCP containing 10 kg P/ha produced significantly more growth at 27 days than those supplied with 40 kg P/ha as drill or broadcast applications; nevertheless, DCP was less effective per unit of P than was the more water-soluble MCP. When MCP seed coatings were applied to phalaris seed oversown with rattail fescue (Vulpia myuros), the coat P treatment increased the P content of phalaris more than drill P; they were not, however, different without rattail fescue, thus suggesting that the P fertilizer in the seed coating was preferentially available to the phalaris and less available to the rattail fescue. Conversely, the yield of the oversown rattail fescue was significantly increased by the drill P treatment, but not by the coat P treatment, indicating the greater availability of the drilled P granules to the weeds, with a corresponding decrease in its availability to the phalaris seedlings (Scott, 1986). No other evidence for the supposed preferential supply of nutrients by seed coatings has been found in the literature to date. Other workers have reported that relatively insoluble nutrient coatings have had little or no effect on nutrient supply. These reports include studies of the surface sowing of pasture species in Australia, in which case Dowling (1978) observed no difference between the effect of inert- and nutrientcoated seed, and the surface sowing of legumes in New Zealand, in which

66

JAMES M. SCOTT

case the principal requirement was for nodulation rather than for the supply of nutrients (Lowther and McDonald, 1973; Lowther, 1974). Lime-reverted superphosphate (which contains phosphorus in the form of dicalcium phosphate) was investigated by Hay (1973) as a coating material for large pasture pellets, each containing several seeds of ryegrass, but negligible P was supplied to the seedlings within 9 weeks of sowing. Also using ryegrass, Scott et af. (1985) found reverted superphosphate coatings to be relatively ineffective in supplying P; additionally, a slow-release form of N (isobutylidene diurea) included in the coating was found to be ineffective in supplying N. Silcock and Smith (1982) found that dicalcium phosphate provided little phosphorus to buffel grass seedlings, but they did observe an increase in early emergence due to the coating. These results contrast with those of Terman et af. (1958), who found that DCP was more available to ryegrass and sudan grass than MCP in acid soils. Their results, however, were found in trials that were conducted over several months and in which the fertilizers were finely ground (to 4 0 pm) and either mixed into the soil or spread evenly at a depth of 35 mm. The effectiveness of nutrient coatings is very much dependent on several factors including the species sown (Watkin and Winch, 1974; Dowling, 1978),the time of sowing (Vartha and Clifford, 1973b; Watkin and Winch, 1974; Dowling, 1978), the soil type (Watkin and Winch, 1974), the type of coating, and the soil fertility and texture (Scott, 1975b). Just as there is no universally applicable fertilizer recommendation for all soils, regions, etc., the interactions listed above indicate that there is little likelihood of the development of any one effective nutrient coating having universal application.

E. INJURY CAUSEDBY FERTILIZERS The use of concentrated soluble fertilizer salts as seed coating materials has generally been deleterious to germination and early growth (e.g., Scott, 1975b). The differences between the various fertilizers in potential damage to plants has been evaluated by Rader et al. (1943) using a “salt index” that compares the osmotic effect of fertilizers relative to a standard of sodium nitrite (salt index 100). Thus, Rader identifies “safe” fertilizers, or ones with little osmotic effect, as dolomite (salt index 0.8). dimagnesium phosphate (4.3), calcium carbonate (4.7), superphosphate (7.8),and gypsum (8.1). The “osmotic” effects of fertilizers on the germination of seeds have been observed by many researchers as early as late last century. Much of this early work has been reviewed by Uhvits (1946), who also reported the toxic as well as the osmotic effects of NaCl on germination. The strictly osmotic effects of fertilizers or the “physiological drought”


67

that they impose on seeds has been questioned by Philip (1958), who suggested that toxic effects may be the real cause of damage, particularly for those materials with diffusible solutes. This may help to explain why a fertilizer such as superphosphate, which according to Rader has a slightly lower osmotic effect than gypsum, has nevertheless been found to be quite damaging to the germination of seeds when applied as a seed coating, whereas no such reports exist for gypsum coatings. Guttay (1957) showed superphosphate to be one of the most damaging fertilizers in causing depressed emergence when sown with wheat at rates greater than 100 kg/ha. When the separate components of superphosphate (i.e., Ca(H,PO,),-H,O; CaHPO4.2H,O;Ca3(P0,),; CaF,; and CaSO,) were studied separately by Guttay, none could be identified as the primary source of injury to the germinating seedling. He suggested that the small amount of residual free acid present in superphosphate (0.6%) may result in an increase in the availability of HF, which is quite toxic to germinating seeds. 1 . Injury Caused by Nutrient Coatings When used in seed coatings, both single and double superphosphate have been found to be damaging to the emergence of buffel grass whereas other soluble sources of P (e.g., monosodium phosphate) caused little damage at moderate concentrations (Silcock and Smith, 1982). In these experiments, Silcock and Smith evaluated many phosphorus sources consisting of salts of calcium, sodium, nitrogen, and potassium. Hence, the effects of the phosphorus source alone, without the possible interference of other elements, are difficult to identify. Research work in our laboratory has shown that injury due to seed coatings containing soluble phosphorus sources such as monocalcium phosphate varies greatly between species, with legumes being much less tolerant than gramineous species (Scott, 1986; Ascher et a f . , 1987; Scott and Blair, 1988a). This is in agreement with the work of Carter (1967), who noted that species differ widely in their tolerance of fertilizers, with crucifers being more susceptible than legumes, which in turn are generally more susceptible than grasses. In an effort to prevent any reduction in germination, which can be caused by soluble nutrients, many workers have concluded that relatively insoluble materials are most appropriate for use in seed coatings. Some slowrelease nutrient materials are the subject of a patent (Coated Seed Ltd., 1975a) that claims that isobutylidene diurea (IBDU), fritted potash, dolomite, gypsum, and lime-reverted superphosphate, when used to coat turfgrass seed by extrusion, are safe forms for the supply of N , K, Mg

68

JAMES M.SCO’IT

plus Ca, Ca plus S, and P plus S, respectively. In evaluating slowrelease fertilizer coatings, Scott and Hay (1974) reported that the main effect of the coatings is physical rather than an effect on nutrient supply. In contrast, Scott et al. (1985) showed that with nutrient-coated ryegrass, the composition of the coatings can have marked effects, with the coating containing the most water-soluble nutrients causing the longest delay and greatest depression of emergence and yet the greatest improvement in seedling growth. Other researchers too (e.g., Younger and Gilmore, 1978; Boatman et al., 1980) have noted that, in spite of reduced germination, the surviving plants still produced increased yields compared to uncoated controls.

2 . Effect of Seed Structure on Tolerance to Fertilizer Injury The mechanisms of tolerance shown by various species to contact with fertilizers at sowing has been discussed by several authors. Hadas (1982) noted that most salts affect germination “not directly through their osmotic effects-but rather through ion toxicity which depends on species susceptibility.” Guttay (1957) observed that oats showed less susceptibility to fertilizer injury than wheat, which he attributed to the oats’ caryopsis being enclosed within a lemma and palea. The seed structure of buffel grass (which has its caryopsis enclosed within glumes, lemma, and palea in a fascicle) has also been suggested as the reason for its tolerance of soluble P sources (Silcock and Smith, 1982). In the case of phalaris, McWilliam and Phillips (1971) noted that the structure of the seed resulted in a significant barrier to liquid moisture movement from the outer lemma and palea to the caryopsis. Scott (1986) found the lemma and palea of phalaris to be responsible for the protection of the caryopsis from injury due to MCP and postulated that the air space between the lemma-palea and the caryopsis may act as a semipermeable membrane, as do the air spaces in moderately dry soil (Philip, 1958), thus restricting the movement of toxic quantities of solutes into the caryopsis. Garrote et al. (1987) confirmed that the presence of the lemma and palea around seeds of a range of grass species is responsible for the protection of caryopses from injury due to soluble P sources. 3. Effect of p H Scott and Hay (1974) noted that, in general, the germination of nutrientcoated seeds was positively correlated with the pH of the coating. Scott


69

and Blair (1988a) observed a similar relationship between injury and the pH of various calcium phosphate seed coatings, but when different combinations of P and N sources were compared, there was no consistent relationship with pH nor with the partial salt index of the fertilizers used (Scott et al., 1987), suggesting that neither osmotic effects nor pH alone is responsible for injury during germination.

4 . Effect of Soil Moisture

Fertilizer effects have generally been claimed to be more damaging at low soil moisture contents (Guttay, 1957; Carter, 1967). For soluble N coatings applied to cereal seeds, this same trend has been demonstrated (Scott et al., 1987). However, examining the effects of relatively insoluble nutrient seed coatings applied to seeds of ryegrass, Scott et al. (1985) observed that injury during emergence was greatest at high soil moisture contents; this suggested that at nonlimiting moisture tensions aeration of the seeds may have been inhibited.

5 . Protection of Seeds from Fertilizer tnjury Protecting seeds from damage by coatings may be possible, because certain species (e.g., buffel grass) emerge with little damage from soluble nutrient coatings because the caryopsis is enclosed within a fascicle (Silcock and Smith, 1982);similar protection may be possible for other seeds if a suitable barrier could be applied during the coating process between the natural seed coat and the nutrient coating. The protection of seeds from soluble nutrient sources has been attempted by Smid and Bates (1971), who found that coating corn with either sucrose or polyvinyl acetate resulted in a slight reduction in the toxic effects of fertilizers, but the reduction was insufficient to make nutrient coating practical. Scott et al. (1987) investigated the effect of various forms and combinations of N and P applied to cereal seed in seed coatings. They showed that, even with otherwise very damaging materials such as urea, the injury caused during the emergence of wheat can be lessened somewhat by reducing the pH of the urea seed coating and more substantially by including the urease inhibitor phenyl phosphorodiamidate in the urea seed coating. Far more work is needed to find ways of overcoming injury to seeds caused by soluble fertilizers.

70

JAMES M.SCOTT

VII. HERBICIDE COATINGS Seed coatings incorporating herbicides to control weeds are a recent innovation with relatively little research yet being reported in this area. The first mention of the possibility of using the seed as a carrier for herbicides was in a patent on the use of (S)-benzyl-N,N-disec-butylthiocarbamate as a means of stimulating the growth of rice and providing a measure of weed control (Pellegrini et d ,1976). The most complete work done so far is that of Dawson (1978), who applied liquid EPTC [(S)-ethyl dipropylthiocarbamate)] to alfalfa seed already coated with lime. This herbicide-coated seed has been shown to retain its germinability after 1 year’s storage (Dawson, 1981a) and when planted, produces a band of weed control approximately 5 cm wide, with the alfalfa seedlings emerging relatively unharmed in the center of the band. Even though EPTC controls a broad range of weeds, it is highly selective, causing little damage to legumes such as alfalfa and beans. According to Dawson (1980), EPTC is effective as a herbicide applied in seed coatings because of its volatility, which permits it to move away from the seed coating and control weeds in the zone of soil around each seed. Alfalfa has, however, been found to be slightly susceptible to EPTC damage if exposed to high doses following emergence (Dawson, 1983). Successful weed control with EPTC-coated alfalfa seed has also been observed by Kapusta and Strieker (1982), who obtained higher yields from herbicide-treated seed than from conventional applications of granular EPTC, and by Krecker and Foy (1982), who found fewer weeds after sowing herbicide-treated seed compared to tests in which EPTC was applied conventionally followed by soil incorporation. Dawson (1981b) suggested even greater scope for seed-applied herbicides in a patent in which he claimed that a range of thiocarbamates (vernolate, cycloate, pebulate, and EPTC) can provide good weed control not only in alfalfa, but also in crops such as beans, soybeans, turnips, sunflowers, flax, tomatoes, and sugarbeets. The only herbicides other than thiocarbamates which have been reported as successful in seed coatings are those described by Dale (1983), who found that fluazifop-butyl, when applied in tung oil as a coating on soybean seeds, provided good control of grass weeds while causing little damage to the soybeans. A similar coating applied to cotton seeds resulted in significant damage to the cotton, perhaps due to the greater absorption of the herbicide by the cotton seeds compared to soybean seeds. In a greenhouse trial, Scott and Blair (1987) established the effectiveness of EPTC applied to alfalfa seeds in controlling germinating rattail fescue seed and markedly increasing alfalfa yield. None of eight other herbicides


71

tested (representing a wide range of herbicide classes) caused both significant control of the weed and increased growth of the alfalfa. In this experiment, significant weed control was obtained with seed-applied EPTC at rates as low as 0.1 kg/ha (soil-incorporated applications of EPTC are commonly 4 kgha), indicating that using the seed as a carrier for herbicides can be an effective means for their distribution. The most effective application rates for EPTC applied to alfalfa seed were 0.4-1.2 kg/ha. The behavior of seed-applied herbicides in soil, like that of conventionally applied herbicides, can vary considerably depending on conditions. Factors such as the soil's organic matter, clay and moisture contents, its exchange capacity, pH, and microflora, as well as temperature, all affect the movement, effectiveness, and degradation of herbicides (Helling et al., 1971). Thus, future work may need to take into account some of these variables as they affect seed-applied herbicides. Confirmation of the effectiveness of seed-applied EPTC under directdrilled conditions is still needed to assess the practicability of incorporating a volatile herbicide into soil that is not substantially disturbed. Also, the compatibility of EPTC seed treatment with Rhizobium inoculation of the alfalfa still remains to be studied (Anonymous, 1981). Future experiments concerning the development of herbicide seed coatings may need to study not only the most appropriate herbicides and rates, but also the formulation of the coating. More fundamental work on the movement and adsorption of EPTC from seed coatings is needed, particularly as it is affected by sowing method, cultivation, sowing depth, soil compaction, and soil moisture content. Recent developments in controlled release technology (reviewed by Kydonieus, 1980) offer opportunities to tailor the release rate of herbicides to within desired limits. The porosity of seed coatings may also be an important factor influencing the release of herbicides, as has been found to be the case for granular formulations of thiocarbamate herbicides (Schreiber and White, 1980).

VIII. OTHER COATINGS A. HYDROPHILIC COATINGS

Hydrophilic seed coatings have been seen by many to offer exciting opportunities to promote the rapid and complete germination of seeds. The use of hydrophilic polymers as coating materials was claimed in a patent on the use of water-soluble polyelectrolytes (with molecular weights greater than l0,OOO) to enhance germination by aggregating soil clay

72

JAMES M. SCOTT

particles adjacent to the seed, thus improving the movement of air and water to the seeds (Hedrick and Mowry, 1953). The use of starch graft polymers (which can absorb up to 1000 times their own weight of water) has been patented (as a seed coating procedure) by Weaver et al. (1976), with further improvements being claimed by Hall (1979), who suggested mixing fatty alcohols with the starch graft polymer to reduce the evaporation of absorbed water. The use of such materials in seed tablets containing lettuce seeds or in seed coatings on pasture seeds has shown no effect on germination (Sharples and Gentry, 1980; Campbell, 1985), whereas when used as coatings on rangeland grasses, they resulted in delayed and reduced germination and emergence (Yarris, 1982). This negative effect could be related to the depression of emergence reported for some seeds with a mucilaginous natural seed coat, which can enhance water uptake but can also result in impaired oxygen diffusion to the seed if conditions are too wet (Hadas, 1982). With crops, the use of hydrophilic polymer seed coatings on cowpea seeds resulted in a depression of emergence, whereas similar coatings on corn seed produced better emergence in only one cultivar out of four (Baxter and Waters, 1986). Other patented materials claimed to enhance water uptake by seeds include hygroscopic materials such as magnesium carbonate (Reams, 1972) and small, insoluble polymer particles (lo0 pm in diameter) is the

t

50

1

1

I

I

1

1

Oct. Feb. Oct. Feb. Oct. Feb. I980 I981 1981 1982 I982 1983 Spring Summer Sprlng Surmer Spring Sumner DATE

FIG. 18. Improvements in aggregation of a cultivated red-brown earth in Australia following adoption of a no-till method. (From Adem and Tisdall, 1984.)

143

CONSERVATION TILLAGE Table XXXVI Number of Earthworm Channels per Mete$ in Plowed and No-Tillage Lwss Soil in Germanf Channel diameter (mm) Plowed

Depth (cm)

2-5

2 20 30

21 60 124 174

60

5-8 5 18 58

I65

No-till

8-11

Total

2-5

5-8

8-11

Total

1 1

29 79 187 348

75 99 209 183

40 41 91 I72

2

117 141 305 363

5 9

1

5 8

"From Ehlers (1975).

most important soil requirement for successful application of a no-till system in the United Kingdom. As reported in Sections IV and VII, the infiltration rate in an untilled soil can be lower than in plowed soil due to the traffic-caused soil compaction. In addition to the effects of vehicular traffic, however, the presence of residue cover may also in some soils decrease infiltration rate and moisture retention. In some cases, plant residue may cause hydrophobicity. In loess-derived soils in northern Nigeria, for example, Maurya (1986) observed a lower infiltration rate with mulch than without the crop residue mulch (Fig. 20).

B. SOILAND WATER CONSERVATION Soil erosion is effectively controlled by no-till and conservation tillage systems. The beneficial effects of the no- till system on soil and water

Table XXXVII Effects of Herbicides and Cultivation on Number and Total Weight of Earthworms in a Soil from New Zealand" Total weight

Mean weight

Treatment

Number (per mZ)

Wm2)

(g)

Grass Herbicide Cultivation

831 2 31 141 ? I I 6?4

341 k 14 36 f 4 221

0.41 0.26 0.33

"From Haynes (1981).

144

RATTAN LAL

CONTINUOUS

CONVENTIONAL TILLAGE

NO TILLAGE @

cecomposition and Mineralization of

MIXED SOIL

FIG.19. Structural profile of a no-till soil. (From House and Parmelee, 1985.)

conservation have been extensively documented for North America (Harrold et al., 1972; Harrold and Edwards, 1972; SCSA, 1973; McGregor et al., 1975; Phillips et al., 1980a; Griffith et al., 1986).The degree of erosion control achieved depends on the quantity of crop residue mulch (Loch et al., 1987).The conservation effectiveness of no-till systems has also been demonstrated for a wide range of soils in the tropics and subtropics (Lal, 1984a). In southwestern Nigera La1 (1976a) observed that a no-till system drastically reduces soil erosion on slopes of up to 15%. In Parana, Brazil, Sidiras and Roth (1985) observed, using a rainfall simulator test, greater infiltration in no-till and minimum till fields compared with plowed land.

* HTILLED

L

.s

RESIDUE

0

I

I

120

d

NO TILL TILLED

NO RESIDUE

I

180 0 60 120 180 240 TIME (min) Re. 20. Effects of crop residue mulch on the infiltration rate of a loess-derived soil in northern Nigeria. (From Maurya, 1986.) 0

60

I45

CONSERVATION TILLAGE

In northern Thailand Ryan (1986) observed that water runoff and soil erosion increased with increase in frequency and intensity of mechanical tillage (Table XXXVIII). In upland rice culture, the mean annual soil erosion decreased from 12.2 t/ha for conventional tillage to 0.8 and 1.0 t/ha for no-till and minimum tillage treatments, respectively. In Queensland, Australia, Mullins et af. (1984) observed that high soil erosion losses of 200400 t/ha/yr measured on lands with burnt trash are reduced to tolerable levels by adopting a no-till or a minimum till system. There are several mechanisms involved in erosion control by conservation tillage. Conservation tillage reduces both the sediment origin and its transport. Some conservation tillage effects are sediment-limited, whereas others are transport-limited. The principal mechanisms involved in erosion control are

(i) Reduced soil erosion due to improved aggregation and a high proportion of water-stable aggregates. Aggregation is improved by a high soil organic matter content, high biomass carbon, and high biotic activity of soil fauna. Soil detachability is also reduced by a high proportion of roots concentrated in the top soil horizons (Maurya and Lal, 1979a). (ii) Reduced inter-rill erosion and soil splash due to the prevention of raindrop impact on the soil surface. (iii) Reduced rill erosion due to decreases in runoff rate, amount, and velocity. The high infiltration rate in conservation tillage land is due to the lack of a surface crust and the presence of stable and continuous biochannels. Crop residue mulch reduces the velocity of water runoff. Table XXXVIII Effects of Method and Intensity of Tillage on Runoff and Erosion from Upland Soils in Northern Thailand“ Treatment

Runoff (m3/ha/yr)

Erosion (tlhalyr)

Effects of tillage intensity (average for the period 1983-1985) I763 3.0 Two cultivations Two cultivations plus mulch 1290 2. I I051 I .8 Scarifier 1.5 1021 N o tillage Effects of tillage methods (average for the period 1981-1985) Bare fallow 3788 49.1 Conventional tillage 1777 12.2 Minimum tillage 563 I .o N o tillage 457 0.8 “From Ryan (1986).

146

RATTAN LAL

-

MAIZE

5

No till

MPlowed

"1

* 0 to

O'I 01

1 APR

2 MAV

3

1

2 3 JUNE

1

2 3 JULY

1

T I ME

FIG. 21. Effects of no-till and conservation tillage on soil moisture reserve in (a) tropical and (b) temperate zones (9 = soil moisture content) (Maury soil, Kentucky). [(a) From Lal, 1976; (b) from Thomas er a / . , 1973.1


e

147

b IQ

FIG.21. (continued).

c. FAVORABLE SOIL MOISTUREAND SOIL TEMPERATURE REGIMES Through effects on soil structure, aggregation, total porosity, and poresize distribution, tillage methods influence wettability, moisture retention characteristics, water transmission, the depth of the wetting or drying front, water extraction patterns, and transport of water and solutes through the profile (Ehlers, 1976a; Allmaras er af., 1982; Ghuman and Lal, 1984). Conservation tillage has a moderating effect on soil temperature and moisture regimes, prevents extremes, and regulates the rate of evaporation. Tillage methods also affect soil moisture through altering root distribution and morphology (Barber, 1971). Consequently, all other factors being the same, plant-available water reserves in a soil managed by conservation tillage are likely to be greater than in plowed soil. This is especially true when the soil moisture content corresponds to the first and second stages of evaporation. High moisture reserves of no-till and conservation tillage soils have been demonstrated by data from both tropics and the temperate regions (Lal, 1982; Ojeniyi, 1986; Blevins et al., 1971; Van Doren and Allmaras, 1978; Grifith et al., 1986). Examples of higher soil moisture reserve in no-till soil than in plowed soil are shown in Fig. 21a and b, summarizing data from tropical and temperate regions, respectively. The high soil moisture content is due both to improved soil structure and to the decrease in evaporation due to the crop residue mulch. The improvements in soil structure, however, take a long time. Furthermore, structural improvement is minor, it occurs, in single-grained or sandy soils. In these

148

RATTAN LAL

cases, the high soil moisture content is mostly due to the presence of a crop residue mulch. Gupta and Gupta (1986) reported that for a sandy soil from Rajasthan, in western India, moisture conservation in the coarsetextured soil was mostly due to the reduction in evaporation by crop residue mulch (Fig. 22). Soil temperature is influenced by the quantity and properties of crop residue mulch (color, durability, reflectance, composition, contact, etc.), and by soil properties. The latter include soil texture, clay mineralogy, bulk density, and moisture content. These soil properties influence a soil's thermal conductivity and heat capacity (Allmaras et al., 1977).The effects of conservation tillage on soil temperature are also confounded by the proportion of surface area covered by crop residue mulch and by the prevalent climate (winter versus summer). Consequently, trends in soil temperature for conservation versus conventional tillage are different in summer or tropical regions than in winter or temperate climates. In the tropics, the maximum soil temperature is generally less and the minimum more in no-till than in plowed soil (see Fig. 9). In the temperate zone or in winter, although the maximum temperature is lower the minimum is not drastically higher in no-till than in plowed soils. The lower maximum soil temperature and slow warming in the spring are responsible for poor seedling growth in no-till plots in temperate climates (Moody et al., 1963; Van Wijk et al., 1959) (Fig. 23). The undesirable microenvironments in seed zone (Chaudhary et al., 1985) are a major obstacle to successful adaptation of the no-till form of conservation tillage in temperate climates. In summer, however, as in tropical climates, the decrease in maximum

-I JULY-AUGUST FIG.22. Moisture conservation in a sandy soil in Rajasthan, India, by the crop residue mulch. (From Gupta and Gupta, 1986.)

149

CONSERVATION TILLAGE 25

$

21

3

s

a W

a 5

17

13 2

0

4

6

10

8

WEEK

FIG.23. Soil temperature regime in no-till and conventionally plowed plots in the temperate United States. (From Cruz. 1982.)

soil temperature in no-till soil is proportional to the amount of surface area covered by mulch (Van Doren and Allmaras, 1978) (Fig. 24). Similar effects are observed in the tropical climates (Lal, 1975). In arid regions of northwestern India the use of crop residue mulch lowered the maximum soil temperature by 5-10°C regardless of the intensity of mechanical tillage (Fig. 25a) (Gupta and Gupta, 1986).

AT = -.W + 2106 (0.01060,0010) a MFRAC I (.30-.13);EON 15

W

a

3 I-

a

II:

w

2.0 AT 10 cm I h

b I

0

I

.2

I

I

.4

I

I

.6

I

I

.8

I

I 10 .

FRACTIONAL RESIDUE COVER ( MFRAG 1 FIG. 24. The effects of surface area covered by mulch on soil temperature in the Corn Belt of the United States. (Van Doren and Allmaras, 1978.)

RATTAN LAL

150

NO DlSKlNG

NO MULCH

40

MULCH

4eL

-

44 -

- -No

40 -

-

b

I

6.' w

IT 3

w

4 c

i

32-

-

28 -

-

24 -

-

20 -

plant cover

Tlllage H Chisel plough 0 4 Conventional

-

36-

w

-Dense

3-cm soil depth

-

6-cm soil depth

FIG.25. Effects of residue mulch and of tillage intensity on the temperature regime of (a) sandy soil in western India and (b) an Oxisol in central Brazil. [(a) From Gupta and Gupta, 1986; (b) from Derpsch er al., 1985.1


151

Plastic sheets used as mulch influence soil temperature differently than does straw mulch. The transparent plastic mulch may create a greenhouse effect at the soil-atmosphere interphase. In northern India, for example, Tripathi and Katiyar (1984) observed a paddy straw mulch lowered the maximum soil temperature by 6-73°C and raised the minimum by about 3°C. In contrast, a polythene asphalt emulsion raised the maximum soil temperature by 4 4 ° C . Similar results were obtained for Alfisols in the subhumid regions of western Nigeria (Harrison-Murray and Lal, 1979; Maurya and Lal, 1981) (Fig. 26).

1F’lc. 26. Soil temperature regime of an Alfisol as influenced by the nature and properties of mulch material used. (From Maurya and Lal, 1981.)

152

RATTAN LAL

D. SOIL CHEMICAL AND NUTRITIONAL PROPERTIES AND FERTILIZER RESPONSE Effects of tillage methods on soil chemical properties differ among soils, climatic regimes, crop rotations, and the period of time for which the tillage systems have been in operation. Soil chemical properties are predictably different due to tillage-induced alterations in soil temperature and moisture regimes, biotic activity of soil fauna, and the return of crop residue to the soil surface in conservation tillage and its mixing and incorporation in the plowed layer in conventional tillage. In general, the surface layer of a soil managed with conservation tillage contains more organic matter and possesses a relatively higher fertility status than soil managed by conventional tillage. The chemical properties of the subsoil, however, may be more favorable in soil managed by conventional than conservation tillage. The differences in organic matter content among tillage treatments are also influenced by the climate. The data from soils of the tropics show that continuous use of conservation tillage for 5-10 years causes the top soil layer to have a higher organic matter content, more cation exchange capacity, and more basic cations than the plowed soil (Aina, 1979; Lal, 1986a). These conclusions are supported by the data on an Alfisol from western Nigeria (Fig. 27) (Aina, 1979). La1 and De Vleerchauwer (1982) reported that favorable chemical properties of the surface layer of no-till soil in Nigeria may partly be due to a high proportion of earthworm casts. Machado (1976) reported more available phosphorus and higher levels of exchangeable cations in the 0-15-cm layer of no-till soil than of plowed soil in Brazil.

w

ORGANIC MATTER (70)

LL 0

FIG. 27. Organic matter content of a tropical Alfisol as influenced by tillage methods. (From A h a , 1979.)


I53

Favorable soil chemical properties are also observed in conservation tillage systems in temperate zone soils. In Kentucky Blevins et al. (1977, 1983) observed that in the 0-2-cm layer, organic carbon and nitrogen were approximately twice as high in surface soil of the no-tillage soil as of the plowed soil. Similar results, indicating an increase in the concentration of plant-available nutrients in the surface layer of no-till soil, have been reported by others (Coutts et al., 1977; Ellis and Howse, 1980; Stinner et al., 1983). Modifications in physical, chemical, and nutritional properties by conservation tillage alter soil’s response to fertilizers and chemical amendments. The differential fertilizer response may also be due to the mode of fertilizer application. Whereas fertilizers are usually broadcast on the surface layer in no-till and conservation tillage, they are incorporated into the plow- layer in conventional tillage. In addition, important determinants of fertilizer response are root growth, soil fauna, and mulch. The latter influences fertilizer response both directly and indirectly. Indirectly, crop residues influence nutrient availability through altering temperature and moisture regimes and influencing losses in seepage and surface runoff. The effects of residue mulch in preventing nutrient losses due to runoff and eroded soil are important factors in improved fertilizer use efficiency on steep lands (Lal, 1976a; McDowell and McGregor, 1984). Directly, crop residue may contribute or immobilize plant-available nutrients. Crop residues with a low C:N ratio (from leguminous plant materials) contribute readily available nitrogen, whereas those with a high C:N ratio (cereals) may immobilize available soil nitrogen into an unavailable form through microbial activity. The fertilizer response, therefore, depends on the antecedent soil properties, drainage conditions, the quality and quantity of crop residue mulch, and the prevalent climatic conditions, The response to applied nitrogen may follow either of the two patterns shown in Fig. 28, depending on internal drainage, the antecedent soil conditions, and the quality and quantity of crop residue mulch. Nitrogen is a major nutrient whose uptake is influenced by tillage methods. No-till or conservation tillage may require, under some soils and moisture regimes, additional nitrogen to produce yields equivalent to those produced by the conventional tillage. In Nigeria, for example, Kang et al. (1980) observed that yields of no-till maize were less than yields of plowed maize with no or low rates of N application, but equal with greater rates. White et al. (1985) also reported, using data from studies in Queensland, Australia, that the slope of the curve relating wheat yield to N rate was maximal when the residue of the previous sorghum crop was incorporated into the soil. Sharma (1985) observed that production of irrigated forage in northern India was thwarted by the low availability of N in notill plots. The nitrogen requirements of conservation tillage, however, can

RATTAN LAL

154

T

PLOWED

-.-.-. NO TILL

n

A

ez K

0

0 W

fU A

W

K

N RATE-

FIG.28. Crop response to nitrogen with conservation and conventional tillage systems as affected by soil properties: (a) eroded soil or poorly drained soil or soil with residue mulch of high C:N ratio; (b) uneroded soil of good structure or with residue mulch of low C:N ratio.

be reduced by adopting soil and crop management systems that involve suitable rotations, growing leguminous cover crops, and growing nitrophilic crops in association with leguminous shrubs and woody perennials. The nitrogen requirement of conservation tillage eventually decreases in comparison with plowed soil because of the reduced losses due to erosion and the equilibrium attained between the amount of nitrogen immobilized and released. On soils similar to those studied by Kang et al. (1980), for example, La1 (1982) observed equivalent or better responses by maize to nitrogen on no-till compared to plowed soil about 10 years after these

155


treatments were imposed. Similar results are reported from Canada by Greaver et at. (1986). Soil drainage is an important determinant of the nitrogen response to tillage methods. Crop response to N on poorly drained soil is similar to that on eroded soils (see Fig. 28a). In Canada, Greaver and Bomke (1986) observed for a northern clay soil that the nitrogen response of barley varied among tillage methods depending on soil wetness. On a Typic Paleudult along the southeastern coastal plain in the United States, Campbell et al. (1984a) oberved that conservation tillage does not work well on poorly drained soils where fragipans exist. The nitrogen response was lower on conservation tillage soil than on conventional tillage plots. Response to P in relation to tillage methods depends more on soil chemical and mineralogical composition than on soil physical properties. For soils with a low capacity for P fixation, tillage methods have little effect on P uptake. For Alfisols in western Nigeria, for example, Kang and Yunusa (1977) observed that broadcast and hill methods of P application were equally effective in supplying adequate P to the maize crop at P application rates exceeding 20 kg/ha. Juo and La1 (1979) observed a satisfactory rate of P movement for Alfisols containing predominantly lowactivity clays (Fig. 29). For soils with a high capacity to fix P and with low plant-available reserves, however, incorporating fertilizer in the soil makes it more readily available to plants than when it is broadcast on the surface. The response to P is, therefore, lower with a no-till system, in which the fertilizer is broadcast, than with a plow system, in which it is incorporated into the soil.

70 T

-

NOTILLAGE CONVENTIONAL TILIAGE

10

n -. 2.5

7.5

12.5

17.5

22.5

27.5

32.5

37.5

42.5

47.5

DEPTH Rc. 29. P profile of no-till and conventionally plowed plots of a tropical Alfisol. (From Juo and Lal, 1979.)

156

RATTAN LAL

E. ROOT GROWTH Fertilizer use efficiency is also influenced by proliferation of the root system. Tillage methods influence the root-depth distribution and the relative proliferation in the surface versus subsoil horizons. Root growth in relation to tillage is influenced by factors that determine pore size distribution, stability and continuity of pores, and soil moisture and temperature regimes. In general, conservation and reduced tillage systems favor more root growth in the surface layer immediately beneath the residue mulch. The roots are'thickened, and the quantity (weight and number) or the total weight of root system is reduced in conservation tillage compared to that seen in a plow-based system. A much higher percentage of roots is found in the surface than in the subsoil horizons. Some isolated roots also grow actively in the deeper horizons of the conservation tillage soil compared with plowed land. Deep root penetration is facilitated by worm holes and biochannels. As a consequence of the differences in root system, water extraction patterns also differ among tillage systems. Generalized rooting depth distribution patterns for no-till and conventional tillage systems are shown in Fig. 30. The pattern is, however, modified by the type of conservation tillage used (minimum tillage, ridge tillage), land use history, antecedent soil properties, and crop and soil management practices adopted. In Nigeria Maurya and La1 (1979a) observed that in an uncompacted soil managed with manual farm operations, there was a greater root density in the surface layer of the no-till compared with the plowed plots. Furthermore, there were a few roots that penetrated beyond the I-m depth

ROOT DENSITY ---+

T

I I=

-NO

w

_ _ - _ _PLOWED

n

n

TILL

FIG.30. A generalized root profile in relation to tillage methods.


157

through biochannels that existed only in the undisturbed soil of the notill plot. Similar results have been reported for Nigeria by Osuji (1984). In Cameroon Ambassa-Kiki et al. (1984) observed that root distribution was restricted in the no-till soil (Fig. 31). Restricted root growth in an untilled soil was also observed in the West African Sahel by Chopart (1984) (Fig. 32). Similar trends in root developments were observed for a loessderived soil in Germany by Ehlers et al. (1980) (Fig. 33). Ehlers et a/. (1983) observed significant positive influence of worm channels on water conductance and on root growth of oats in a no-till soil.

F. ENERGY CONSERVATION Through its emphasis on reducing inputs, successful use of conservation tillage causes significant savings in energy costs without jeopardizing productivity. Tillage and petrobased chemicals such as fertilizers and pesticides are energy-intensive inputs. The data in Table XXXIX show that fuel costs represent as much as 13.6 and 20.0%of the total costs for maize and soybean production, respectively. Field machinery consumes as much as 469 trillion BTUs of energy used in agricultural production in the United States (Ritchie, 1983). The total energy use includes 764 trillion BTUs for fertilizers and pesticides, 370 for transportation, 263 for irrigation, 75 for crop drying, and 8 1 trillion BTUs for miscellaneous uses. Conservation tillage saves energy through reducing the frequency and intensity of tillage and decreasing fertilizer and irrigation needs by conserving soil and water.

ZERO MINIMUM CONVENTIONAL FIG.31. Effects of tillage methods on root system development of upland rice in Cameroon. (From Ambassa-Kiki er a / . , 1985.)

158

RATTAN LAL

50 I

(v

E

MNO TILL @=4 PLOWED

MYS AFTER SEEDING

FIG.32. Effects of no tillage and plowing on root system development in semiarid West Africa. (From Chopart, 1984.)

Energy conservation is an important reason for adopting conservation tillage. Whereas subsistence farmers of the tropics use few energy-based inputs, North American and western European agriculture is based on somewhat excessive use of commercial energy (Table XL). An important reason for adopting conservation tillage, therefore, is the conservation of nonrenewable energy.

SOIL DEPTH (cm)

FIG. 33. Rooting density in 10-cm soil layers and total root length on plowed and no-till loess soil in Germany. (From Ehlers et af., 1980.)

159

CONSERVATION TILLAGE Table XXXIX Cost of Production ($/acre) of Maize and Soybean in the United States” ~

~~~~

Variable

Maize

Soybean

Fuel Fertilizer Chemicals Drying Subtotal Other variable costs Total variable costs

16.00 40.50 15.00 4.40 75.90 41.60 117.50

13.60 7.50 13.50

-

34.60 33.50 68.10

“From Ritchie (1983).

G. PREVENTING SOILDEGRADATION AND MAINTAINING SOIL FERTILITY Ecologically compatible agriculture should be aimed at preventing soil degradation, maintaining soil’s productive potential, and reducing environmental pollution. Non-point source pollution is a major environmental hazard of modern agriculture. The problem is particularly severe in the United States and other economies geared to commercial surplus production. When soil degradation and desertification are severe problems, the question of crop yields is of secondary importance. In that event, the important question is not whether conservation tillage works but how to make it work. Table XL Commercial Energy (lO’*J) Used for Inputs to Agricultural

Fertilizer

Machinery

Irrigation

Pesticides

Percentage of world total

Region

A

B

A

B

A

B

A

B

A

B

North America Latin America Near East Africa

750 153 86 38

1429 468 351

1299 148 50 30

1427 349 167 73

36.6 6.1 30.8 1.2

42.0 13.7 54.7 3.1

55.3 5.3 1.4 1.2

64.5

28.2 4.1 2.2 0.9

22.0 6.3 4.3 1.4

111

“From Shahbazi and Goswami (1986). ’A, 1972-1973; B. 1985-1986.

13.8 8.3 8.3

160

RATTAN LAL

IX. ENVIRONMENTAL POLLUTION AND CONSERVATION TILLAGE The material presented in the previous sections indicates that low crop yields with conservation tillage are associated with the following soil and environmental factors: soil compaction, crusting and hard-setting soil, poor drainage, low soil temperatures, high P fixation capacity of the soil, use of crop residues with high C:N ratios, and damp and humid climates, which cause anaerobic decomposition of crop residue mulches. Low crop yields are caused by poor stand establishment, damage to seedlings by rodents and birds, and possibly by a high incidence of weeds, insects, and pathogens. A considerable literature exists relating the incidence of pests to the use of crop residue mulch. There are also research reports showing lesser pest incidence with conservation or mulch tillage than with conventional tillage (Shenk and Saunders, 1981). A serious environmental issue of modem times is the pollution of natural waters. This is particularly true in countries such as the United States, where the land area under conservation tillage has drastically increased over the two decades ending in 1985. For example, the land area in conservation tillage in the United States has been increasing at an average rate of 4.2% per year during the quarter of a century ending in 1985 (Mannering et al., 1987). Consequently, the use of pesticides, especially herbicides, has also increased. Herbicide use is especially heavy for maize and soybean production, because these crops are grown in the United States with heavy dependence on herbicides regardless of the tillage methods used. It is estimated that the annual discharge of pollutants to rivers in the United States amounts to more than a billion t of suspended solids, about one- half billion t of dissolved solids, one million t of P, and about 5 million t of N. Conservation tillage is also a concern in relation to the groundwater quality (Logan ef at., 1987). The use of no-till and conservation tillage systems is also expanding rapidly in some countries of the tropics and subtropics, such as South and Central America (especially Brazil), Australia, southern Africa, and southeastern Asia. In contrast to the United States and western Europe, however, little is known about the retention, biodegradation, and movement of these chemicals in surface and subsurface waters in tropical environments. The movement of agricultural chemicals is related to that of water in both surface and subsurface flows. Although conservation tillage decreases surface runoff and erosion, it is also known to increase the drainage and subsurface tile flow. High infiltration and subsurface drainage in conservation tillage are facilitated by the presence of large numbers of earthworm


161

channels. These channels serve as preferrential waterways that conduct water (Phillips, 1981; Gold and Loudon, 1982) and dissolved chemicals in the aqueous phase to the subsurface horizons. Because of a high infiltration rate, it is likely that the infiltrating water in no-till plots carries greater amounts of nitrate and the added nitrogeneous fertilizers than that in plowed plots (Tyler and Thomas, 1977). However, the leaching losses of total nitrogen are highly variable due to the many compensatory mechanisms involved (Wild, 1974; Kanwar et al., 1985). Similar to the loss of added fertilizers, infiltrating water in conservation tillage is also likely to transport more pesticides than water from plowed plots because the total amount of water infiltrating through the soil column in no-till plots is more than that from the plowed land. The effects of tillage systems on transport of fertilizers and pesticides are, however, confounded by other factors, including the time of application in relation to the time of plowing, the amount and distribution of rainfall, and the prevalent climate. The latter influences the amount and rate of volatilization and degradation. Conservation tillage, through its moderating effect on climate, alters the rate of uptake and degradation of pesticides (Glotfelty, 1987). A high proportion of pesticides is transported in surface runoff. Some pesticides (such as trifluralin, endrin, and toxaphone) are extremely insoluble and are transported along with solids only. Pesticides absorbed on clay and organic matter are washed away in surface runoff and eroded sediments. Because conservation tillage reduces the rate and amount of water runoff and sediment loss, it also decreases the losses of absorbed pesticides. It has now been proved that the quantity of pesticides that volatilizes is usually much larger than that which moves with runoff or leaching (Taylor, 1978; Glotfelty, 1987). Soil incorporation of pesticides and low soil moisture contents decrease volatility. This implies that volatilization losses of pesticides are likely to be greater in conservation than in conventional tillage systems. There are, however, other factors that decrease volatility of pesticides in conservation tillage soils. Important among these are diffusion and convective flow. The net effects of all these compensatory factors are, therefore, difficult to estimate. In addition to volatilization, major pesticides losses occur in water runoff and seepage flow. Most pesticides move in water runoff as soluble compounds (Fig. 34). Therefore, crop and soil management practices that reduce soil losses but not runoff volumes may have little effect on pesticide losses (Wagenet, 1987). Leaching loss is the third major source of pesticide loss into natural wzters. Some pesticides, such as paraquat, atrazin, and metolachlor, are relatively immobile in soils of average organic matter content. Because surface soil in conservation tillage contains relatively

162

RATTAN LAL W

-3

-2

-1

0

1

2

3

4

5

6

LOG (solubility, ppmw)

FIG. 34. Partitioning of pesticides between sediments and water in runoff samples, with the range of reported literature values indicated by the solid bars. (From Wanchope et a / . , 1985; Wagenet, 1987.)

more organic matter than that in conventional tillage, the retention of these pesticides in the top layer is likely to be greater with conservation than with conventional tillage. However, the presence of large and relatively continuous macropores in conservation tillage soil is another complicating factor. The presence of well-established macropores may influence pesticide leaching in two ways (Wagenet, 1987):

(i) If rainfall exceeds the infiltration rate and surface ponding occurs, the macropores serve as conduits to transmit water and dissolved pesticides to less biologically active subsoil horizons. The pesticides in this horizon of low organic matter are neither easily absorbed nor biodegraded and are transmitted en masse to the ground water. (ii) If heavy rains do not occur immediately after the pesticide application, then pesticides solubilize and diffuse into relatively small pores. Whenever saturated flow does occur through the large pores, it will transmit relatively clean water to the subsoil horizons. At present, there is little if any field data from the tropics or temperate zone to verify either of these scenarios and their relative importance in pesticide movement in relation to tillage methods.


163

X. THE SYSTEMS APPROACH TO CONSERVATION TILLAGE AND SUPPORTIVE CULTURAL PRACTICES Is sustainable agriculture necessarily based on low inputs? The word input is a relative term. At present, African agriculture is based on low or no commercial inputs. In contrast, agriculture in North America is heavily dependent on commercial inputs. Ecologically, no-input agriculture can be as harmful to the African environment as are excessive inputs and intensive agriculture in North America to that environment. Whereas farmers in North America must make an earnest effort to reduce inputs, subsistence farmers in Africa can achieve substantial yield improvements by even marginal increases in added inputs. Sustainable agriculture emphasizes reduction in chemical and energyintensive industrial inputs. The objective is to optimize the use of energyrelated inputs. High crop yields are, however, possible if other nonindustrial inputs are increased. These inputs may include improved cultivars, new crops, efficient cropping systems, improved tools, increased fertilizer use efficiency, and systems of integrated pest management. These inputs are in accord with principles of good farming and land stewardship. Good farming, by this definition, is that which is ecologically and environmentally compatible. The effectiveness of conservation tillage in soil and water conservation and resource management is greatly enhanced by adopting the systems approach. Conservation tillage requires a special set of cultural practices that may be different than those needed for conventional tillage. There may be some crops and varieties that are more suited to conservation tillage than others. The rate, time, mode, and type of application of fertilizers and other amendments are also likely to be different, as would be the measures for pest control. Conservation tillage also requires different types of seeding equipment and farm machinery to manage the uneven and trashy soil surface. Some crop rotations and farming systems are apparently better suited to conservation tillage than others. Mulch being an integral component of conservation tillage, cultural practices that ensure the production and availability of a large quantity of residue mulch are compatible with conservation tillage. Regulating vehicular traffic is an important consideration for reducing the risks of soil compaction. Method, time, and type of harvesting equipment used have important effects on soil compaction. Conservation tillage, therefore, is not just a single concept but a package of cultural practices that are specifically developed and adopted to conserve soil and water resources, sustain high and satisfactory returns, minimize degradation of soil and environments, and preserve the

164

RATTAN LAL

soil resource. The interrelationship between conservation tillage and supportive cultural practices is shown in Fig. 35. Some of the cultural practices specifically developed to enhance the effectiveness of conservation tillage are briefly described below.

A. AGROFORESTRY AND ALLEYCROPPING Agroforestry refers to a technique of growing food crop annuals in association with woody perennials to optimize the use of natural resources, minimize the need for inputs derived from nonrenewable resources, and reduce the risks of environmental degradation. The practice is also referred to as agrisilviculture, Taungya, and by many other names drawn from different cultures and languages (Roche, 1973). King (1968) lists 79 woody species and genera and 42 agricultural crops grown in one or another form of agroforestry used in the tropics. The most common tree species used in the tropics are Nauclea diderrichii, Lovoea trichilioides, Khaya ivorensis, and Tectona grandis. Lately, the emphasis has been shifted to some woody perennials (e.g., Leucaena, Gliricidia, and Flemingia).

FIG.35. The system approach to conservation tillage.


I65

Alley cropping is a form of agroforestry in which food crop annuals are grown between two adjacent hedgerows of leguminous shrubs and woody perennials (Kang et al., 1981, 1985). The woody perennials are regularly pruned to minimize shading and to procure nitrogen-rich mulch for food crop annuals. Satisfactory crop yields are obtained provided that compatible species are chosen and that the plant-available reserves of soil water are sufficient to meet the evapotranspiration needs of both species. The system is normally suited for humid and subhumid regions in which precipitation exceeds evapotranspiration during the crop-growing season. At present the system is labor-intensive and is suited more for resourcepoor farmers of the tropics than for large-scale commercial farming. MaizeLeucaena alley cropping can be economically promising if hired labor is available at low cost (Verinumbe et al., 1984). Field experiments conducted in the subhumid tropics have shown that when properly established maize grown in association with contour hedges of Leucaena leucocephala and Gliricidia sepium produces satisfactory yields (Fig. 36). The data in Table XLI show that despite the reduction in cropped area, maize grain yield with alley cropping was equivalent to that of no-till treatments. The yields of cowpeas, however, was drastically reduced by alley cropping. In the case of cowpeas, the yield reduction was due to poor stand establishment and reduced germination. An alleleopathic effect is a likely reason for the poor germination. In semiarid

FIG.36. Alley cropping of maize with Leucaena leucocephala.

166

RATTAN LAL Table XLI

Effects of Methods of Seedbed Preparation and of Hedgerow Spacing of Leucaena and Gliricidiu on Grain Yield of Maize and Cowpeas"

Grain yield (t/ha) Treatment

Maize

Cowpeas

Plowed No-ti11 Leucaena: 4 rn Leucaena: 2 m Gliricidia: 4 rn Gliricidia: 2 m

3.6 4.0 3.7 3.8 3.6 3.3

447 1193 58 I 503 670 678

"Unpublished data of La1 (1984).

and arid climates, however, growth suppression and yield reduction in food crop annuals are caused by excessive competition for soil moisture (Singh and Van Den Beldt, 1986; Nair, 1984). Contour hedges decrease runoff velocity and reduce its sediment transport capacity. Sediments trapped by the contour hedges facilitate the formation of natural terraces. Experiments conducted on relatively steep lands in the Philippines have shown that compared with croplands contour hedges of Leucaena reduce sediment transport by several orders of magnitude (Loch, 1985; Pacardo and Montecillo, 1983). The effectiveness of contour hedges in trapping sediments has also been demonstrated in Indonesia (Sukmana et al., 1985). Closely spaced narrow strips of shrubs or woody perennials are likely to be more effective in soil and water conservation than widely spaced single-row hedges. There is, however, an optimum spacing for erosion control and for satisfactory growth and yield of food crop annuals. The optimum spacing depends on slope gradient, soil type and its susceptibility to erosion, rainfall, crop species, and the soil and crop management system. Loch (1983) observed that erosion control by contour hedges depends on the sediment-carrying capacity of the water runoff. Hedges trap sediments as long as the sediment transport capacity of the overland flow is not yet fulfilled. The data in Table XLII show that 2-m-apart contour hedges of Leucaena are more effective in reducing runoff and soil erosion than are the 4-m- apart hedges. In comparison with conventional plowing, hedges of Leucaena and Gliricidia also reduced losses of cations and plant nutrients. The data in Fig. 37 show that growing contour hedges of perennial shrubs drastically influenced the accumulative infiltration. The ac-

167

CONSERVATION TILLAGE Table XLII Effects of Contour Hedges of Leucaem and Gliricidin on Runoff, Soil Erosion, and Total Nutrient Loss for Maize Grown in the First Season (April-August 1984) and Cowpeas Grown in the Second Growing Season (September-December 1984)

Treatment

Runoff (% of rainfall)

Erosion (t/ha)

Plowed No-till Leucaena: 4 m Leucaena: 2 m Gliricidiu: 4 rn Gliricidiu: 2 m

Maize (Rainfall 727 mm) 29.9 14. I6 0.8 0.026 I .2 0. I7 I .3 0.07 4.9 1.62 2.2 2.05

Plowed No-till Leucaena: 4 m Leucaena: 2 rn Gliricidia: 4 m Gliricidia: 2 m

Cowpeas (Rainfall 631 mrn) 2.4 0.18 0.08 0.57 0.32 0.71

0.74 0.006 0.02 0.04 0.05 0.27

Total nutrient loss (kglha)

101.2 12.9 4.8 2.1 12.3 2.2

1.13 0.29 0.10 0.61 I .69 0.53

cumulative infiltration was 83, 82, 70, 55, 54 and 40 cm per 2-hr period for Gliricidia at 4 m spacings, Leucaena at 2 m spacings, Leucaena at 4 m spacings, plowed soil, Gliricidia at 2 m spacings, and no-till treatments, respectively. There were also notable differences in runoff hydrographs among methods of seedbed preparation and hedge-row spacing treatments (Fig. 38). Despite its apparent advantages, a considerable amount of local-specific research is needed to develop appropriate alley cropping or agroforestry systems for different soils, crops, and ecological environments. Research is needed in choosing appropriate crop and tree species, suitable spacing, management of trees, and soil and crop management practices for food crop annuals. Trees are extensively grown as woodlots, on field boundaries, and along fence posts in temperate zone climates. An important research consideration would be to determine the proportion of land area needed to be allocated to trees so that ecological stability is maintained with intensive use of the remaining land as an arable area. A strong data base is needed to validate the nutrient recycling effects presumably attributed to growing deep-rooted perennials.

168

RATTAN LAL

-

-x

Leucaena - 4 m Gliricidia .4 m Notill

c-. Plowed 80

70

-E C

60

0 CI

E

=

c'

-

50

.-

m

5

8 a

40

30

20

10

0

20

40

60

80

100

120

4 4140 0

Time (min)

FIG.37. Accumulative infiltration as influenced by methods of seedbed preparation and spacing of perennial hedges. (Unpublished data of Lal, 1984.)

B. COVERCROPS Diversifying the cropping system is a necessary strategy to create ecological stability and reduce the incidence of disease and pests. Growing grass or leguminous cover crops at frequent intervals, once every 2-4 years in temperate zone and once every 1-2 years in the tropics, is necessary for successful adaptation of a conservation tillage system. Cover

I 69


-

Total RunoW Leucaena-4rn 176

Ghnc1dm-4m

0

10

148

205

41

53

19

Notill

Glmcidia - 2 rn

(mln) Eroslon (ke/ha)

12 0

Plowed

Rainfall = 67 3 mrn

20

30

40

50

60

70

80

90

Time attar Runoff initiation (min)

FIG.38. Effects of hedgerow spacing and methods of seedbed preparation on the hydrographs generated by a rainstorm with a total rainfall of 67.3 mm. (Unpublished data of Lal, 1986.)

crops have many advantages for conservation tillage systems (e.g., they restore fertility, control weeds, avoid repeated seeding and cultivation traffic, conserve rainwater, and reduce energy costs). In addition to controlling pests, cover crops improve soil physical properties and soil tilth and reduce soil erosion. Cover crops are beneficial regardless of the ecological region, although their advantages are relatively greater in the tropics than in the temperate zone. Adams rt al. (1970) successfully grew maize with conservation tillage in atrazin- treated sod species. Elkins et al. (1979) investigated the effects of growth retardants on many sod species and on the yield of maize grown as a succession crop. Satisfactory maize yields were obtained when sod species were adequately suppressed. Thomas et a1 (1973) and Ebelhar and Frye (198I ) reported that legumes boosted nitrogen for no-till maize production in Kentucky. They observed that growing a winter annual legume cover crop for no-till maize involves relay-sowing the cover crop in the fall before harvesting maize and killing it with herbicides in the spring just prior to seeding the next maize crop. The nitrogen contribution that could be produced by growing suitable cover crops in rotation with vegetables

100

170

RATTAN LAL

was shown by Mascianica et al. (1982). In the southeastern United States, Touchton et al. (1984) observed that winter legumes such as crimson clover (Trifolium incarnatum) and common vetch (Vicia saliva) are good nitrogen sources for succession-planted no-till cotton. The usefulness of different cover crops and their management for conservation tillage in the southern United States were discussed at length by Hargrove (1982), Unger et al. (1986), and Triplett (1986). Cover crops have long been used in the tropics for soil and water conservation in plantation crops (Okigbo and Lal, 1977). The importance of cover crops in conservation tillage for the management of some uplands in Ghana, West Africa, was demonstrated by the work of Kannegieter (1967a). Under average conditions, some grasses and legumes produce large quantities of biomass (Table XLIII). The biomass produced is useful as forage, mulch, and for other domestic uses. Kannegieter (1967b, 1969) developed a technique involving the combination of short-term pueraria fallowing and zero cultivation to reduce the requirement of fertilizer nitrogen of a maize succession crop and to abate soil erosion. Table XLIII Dry Matter Yield of Various Grasses and Legumes, Africa"

Dry matter yield (t/ha) Cover crop

West Africa

Cenchrus ciliaris (buffel grass) Seiaria sphacelaia Tripsacum laxum Stylosanrhes plus C . ciliaris Andropogan gayanus Botriochloa inscupia B. inscupf a plus Centrosema pubescens Panicum maximum plus Centrosema pubescens Pennisetum purpureum PIUS Centrosema pubescens S . sphacelaia plus Centrosema pubescens Psaphocarpus palustris Penniseium purpureum (elephant grass) Panicum maximum (guinea grass) Chlorea gayana (Rhodes grass) Pueraria phaseolodes (kudzu) Cenirosema pubescens Glycine wighiii

35.3 28.7 30.1 28.2 30.5 26.7 26.1 25.9

East Africa

23.0 25.2 11.0

-

10.0 13.0 6.5

11.3 10.0 8.3 3.7 2.3

"Based on data from Nateh and Anderson, 1962; Kannegreter. 1967a; Okigbo and Lal, 1977.


171

In addition to augmenting soil fertility, cover crops also improve soil structure and increase macroporosity . In northern Nigeria, Wilkinson (1975) observed significant benefits of grass fallow rotations on the infiltration of water into the savanna zone soil. The infiltration rate increased with increasing length of the fallow period. Similar observations were reported for soils of western Nigeria (La1 et al., 1979; Wilson et al., 1982) Mucuna utilis is now widely recommended as a cover crop in western Nigeria (see Fig. 4). The benefits of cover crops on soil structure and tilth improvement have also been demonstrated for East Africa (Pereira et al., 1954, 1958; Pereira, 1956; Wallis, 1960; Peers, 1%2; Stephens, 1%7). Cover crops are also found to be useful for erosion control with conservation tillage in soils of tropical America (Kemper and Derpsch, 1981). In Parana, Brazil Sidiras et al. (1985b) observed that cover crops (such as Avena strigosa, Raphanus sativus, and Lupinus albus) grown in winter have beneficial effects in controlling water erosion and on the yield of the following summer crops of beans, soybeans, and maize. Yield increases of 93% in beans, 73% in soybeans, and 81% in maize were observed when they were grown after an appropriate legume cover. The choice of an appropriate cover crop for different soils and ecological regions depends on many considerations. Some of the important considerations are (i) Ease and economics of establishment including availability of seed, (ii) Quick ground cover and growth rate during the off- season, (iii) N-fixing rather than N-consuming, (iv) Deep root system and consumptive water use, (v) Feed value for livestock, (vi) Alternate hosts for pests and cover for wildlife, (vii) Canopy height, (viii) Ability to suppress weeds, (ix) Growth duration (i.e., permanent versus annual), (x) shade tolerance, and (xi) Ease of management for growing a food crop with conservation tillage

There is considerable scope for selecting appropriate species and cultivars for suitable cover crops. Cover crops are an important tool in sustainable agriculture. C. LIVEMULCH Management of cover crops is a necessary step in conservation tillage. A difficult-to-suppresscover crop can be expensive and an energy-intensive activity. Consequently, the concept of live mulch or a green seedbed

172

RATTAN LAL

was proposed in the early 1940s (Spivack, 1942, 1984). A live mulch system is based on the principles of mixed cropping. A fast-growing perennial legume is established with the objectives of smothering weeds and growing a seasonal grain crop through it without severely suppressing the growth and yield of the food crop. A small strip is opened, with or without herbicides, to seed a seasonal food crop through an established live cover crop. The system works if the live mulch is a low-growing nonclimber and is not competitive for light, moisture, or nutrients. Like alley cropping, the live mulch system is also likely to be more successful in humid and subhumid regions with little or no water deficit than in semiarid or arid regions. The concept has been tried in West Africa with but modest success (Voelkner, 1979; Ogborn, 1980; Akobundu, 1980a; Wilson et al., 1982). Akobundu (1980a) reported satisfactory yields of maize using a live mulch system of Arachis, Centrosema, and Psophocarpus. Drastic yield suppression of food crops can occur, however, due to alleleopathic effects, smothering, and competition for moisture during periods of drought stress. D. ROTATIONS AND MULTIPLE CROPPING Crop rotations are an integral component of successful conservation tillage. Benefits of crop rotations in conservation tillage are widely recognized (Van Doren et al., 1976; Triplett, 1976; Triplett and Mannering, 1978). An ideal rotation should involve sequential cropping of a cereal followed by a legume, shallow-rooted by deep-rooted crops, fertility-depleting by fertility-conserving crops, soil-degrading by soil- regenerating crops, and crops demanding heavy inputs by those that can survive on low inputs. The objective is to create a desired level of crop diversification. Mixed and multiple cropping are the rule rather than the exception in the tropics. Although double cropping, growing two crops in a year, is practiced in the frost-free belt in the United States, mixed cropping is unusual in North America. The most commonly observed rotations in West Africa are maize-cowpea, millet-cowpea, sorghum-cowpea, and sorghum-yam as sequential crops; and cassava plus maize, cassava plus cowpeas, and maize plus yams as mixed crops (Okigbo, 1978). For tropical Alfisols, La1 (1976a,b) observed that maize-cowpea and maize-soybean rotations were compatible with no-till and conservation tillage systems. Aina et al. (1979) reported significant reduction in water runoff and soil erosion in maize plus cassava mixed cropping compared to either maize or cassava monocultures. Mixed cropping of compatible crops has been reported to maximize water use efficiency in Australia (Hulugalle and Willatt, 1985) and Nigeria (Hulugalle and Lal, 1986).

I73


Exploitative and intensive monocropping has more severe soil erosion risks than rotational farming. A relevant example depicting conservation effectiveness of different rotations in northern Thailand is shown by the data in Table XLIV. Runoff and soil erosion are more severe with intensive cultivation than when the land use intensity is low. Fallowing, natural or with planted cover crops, improves soil's physical conditions and reduces the risks of accelerated soil erosion.

E. SUMMER FALLOWING Fallowing, leaving the land uncropped and weed free, is commonly practiced in arid and semiarid regions to improve soil- water reserves for the succeeding crop. Increasing soil-water storage is the primary objective of the practice. It is defined as a cultural practice wherein no crop is grown and all plant growth is controlled by cultivation or chemicals during a season when a crop might normally be grown. Thus, production for one season is forfeited in anticipation that there will be at least partial compensation by increased crop production the next season. (Haas et al., 1974)

It is difficult to trace the origin of this practice, because it is widely used in many regions around the world characterized by low and erratic rainfall and marginal soil conditions. The practice has been widely used in the western United States since the latter part of the 19th century. The acreage managed under this practice increased from about 2 million hectares in 1900-1914 to about 15 million hectares in the 1960s and 1970s. The technique is considered useful in regions with uncertain rainfall, usually totalling less than 400 mdper year. The moisture-conservingefficiency of the practice has become a debatable issue, however. The net gains in soil-water conservation and in crop yields vary widely depending on soils,

Table XLIV

Effects of Crop Rotations on Runoff and Soil Erosion in Northern Thailand"

Treatment

Runoff (mm)

Erosion (t/ha)

Shifting cultivation Exploitative intensive cultivation Rice Peanut-mung bean

9 32 131 83

0.2 15.1 2.3 I .9

"From Ryan (1986).

174

RATTAN LAL

rainfall during the fallow and crop growth periods, and soil and crop management systems adopted. In the Great Plans of the United States, Greb et al. (1967) related net gains in soil-water storage during fallowing to the quantity of crop residue mulch. For the same ecological region, Smika and Wicks (1968) reported that soil-water storage was greater when herbicides rather than conventional tillage practices were used to control weeds. The storage efficiency was 35.4% for conventional tillage and 42.4% for herbicide treatment. Smika (1970) also reported some excellent yields with summer fallowing. In addition to conserving water, residue mulch also controls wind erosion, a serious problem in the Great Plains region (Black et al. 1974). Residue mulch is an important input for managing soil water in dryland crop production (Unger and Wiese, 1979). In contrast to the results reported above, however, some researchers have questioned the feasibility of bare (cropless) fallowing. It is argued thatplanted fallows, though they conserve less water, may be more suitable in terms of providing ground cover, creating residue, and improving the soil’s nutrient capital. Touchton et al. (1984) reported that water infiltration was more rapid in plots planted to legumes than in fallowed soil. In the fallowed soil, 34 kg/ha of additional N fertilizer was required to attain . near-maximum yields. In comparison, cotton yields in the plots planted to clover (Trifolium) were greater than those from the fallowed plot even without any application of nitrogenous fertilizer. Also in the southeastern coastal plains of the United States, Campbell et al. (1984b) found more satisfactory yields of soybeans seeded through a grazed cover crop of winter rye (Secale cereale) with a conservation tillage system than from fallowed land. The practice of fallowing has also been found useful elsewhere in semiarid and arid climates. On a Xeralfic AItisol in Western Australia, Hamblin (1984) conducted an experiment to compare soil properties and crop performance in plots cropped every year with those cropped every other year. Soil properties and yield progressively deteriorated in continuously cropped treatments. In arid regions of western India, fallowing was found to conserve more water in the soil profile than plowing to different depths and with different intensities (Table XLV). In Botswana, in southeastern Africa, Whiteman (1975) reported significant improvements in the grain yield of sorghum by fallowing. The data in Tables XLVI and XLVII show significant differences in grain yield when sorghum was seeded in fallowed land rather than in land previously growing a cover crop, maize, or weeds. The beneficial effects of fallowing were greater in dry rather than in normal rainfall years, and greater when weeds were effectively controlled than when plots were left weed-infested. Weed control during fallowing is also an important factor (Botswana, 1977). Experiments conducted in semiarid central Tanzania also proved that weed-free bare fallow land conserved

175

CONSERVATION TILLAGE Table XLV

Comparative Effects of Depth of Plowing and of Fallowing on Soil Moisture Storage in the Profile and on Maize Yield’ Soil moisture content (%) Depth (cm)

Plowing to 45-cm depth

Loosening to 20-cm depth

Plowing to 10-cm depth

Disking to 10-cm depth

Fallowing

6.7 9.5 10.4 11.2 11.5

5.1 7.6 8.8 8.8 8.9

3.4 7.7 7.6 8.0 8.9

2.2 4.5 6.8 7.3 8.0

9.0 9.7 9.3 11.2 13.2

2.5 5.3

-

0-7.5 7.5-15 15-22.5 22.5-30 30-37.5

Maize yield (t/ha) Grain Biomass

3.6 7.8

3.2 6.6

2.6 5.8

“Data from CAZRI (1975) and Kovda (1980).

Table XLVI Yield of Sorghum as Influenced by Previous Crop and the Weed Control System during Fall~wing‘~ Sorghum grain yield (kgha) Treatments

1970-197 I

Cropped Cover crop Maize Weeds

1103 1603 1375

Fallow Cultivated Ridged Herbicides

1907

Mean SE Rainfall (mm)

}

(1369)

1646 071

1972-1973

; :3!

I296

(1933)**

2 258

I97 1- 1972

1688 I700 1783 1540 %I17 614

353

I

}

(365)

Mean

1028

(1717)*** 2269 1368 2 194

289

“From Whiteman (1975). ?he figures in parenthesis are means; ** and *** indicate statistically signifcant differences at 1 and 0.1% probability levels.

176

RATTAN LAL Table XLVII Effects of Cultural Practices on Sorghum Production" Cultural practice Weeding Free of weeds throughout growth period Without weeding Plant population High plant population (80,OOO /ha) Low plant population (20,000 /ha) Water conservation Following a bare summer fallow Following a crop Time of planting Early-planted (early November) Late-planted (late December) Fertilizer application With fertilizer (100 N, 60 P, 40 K kg/ha) Without fertilizer

Number of years' data

Yield (kdha)

I

1802 487

4

2873 1836

3

2007 1028

3

1737 1029

3

3047 1647

"From Botswana Agricultural Research (1977).

more water and produced increased subsequent yields of peanuts, compared to continuously cropped land (Pereira et al., 1958). However, severe soil erosion occurred in the unprotected bare soil. Protection of the fallow by sowing shallow-rooting teff grass (Eragrostis abysinica) provided efficient soil conservation, controlled weeds, and enabled enough subsoil water to be stored to produce a satisfactory crop of peanuts. Erosion control, therefore, is a major consideration during fallowing. Similar observations were made in Senegal, West Africa, by Charreau (1970) (Table XLVIII). The conservation effectiveness of fallowing is drastically improved by the presence of crop residue mulch. Data from northern Kazakhstan, USSR, (Table XLIX) indicate importance of residue mulch in moisture conservation. Under the semiarid steppe conditions of northern Kazakhstan, mulching of the calcareous silty clay loam southern Chernozem soil conserved more plant- available moisture for the following wheat crop than did unmulched treatments. The beneficial effects of residue mulch in moisture conservation were observed for the entire growing season from sowing to harvest and were due to an increase in water infiltration rate. In comparison with the unmulched control, the infiltration rate increased by 78 and 85% for mulch rates of 2 and 4 t/ha, respectively. Mulching also increased the yield of spring wheat by 0.25-0.50 t/ha.

177

CONSERVATION TILLAGE Table XLVIII

Effects of Soil Surface Management during Fallowing on Runoff and Soil Erosion in Senegal"

Runoff Treatment

(mm)

(% of rainfall)

Erosion (t/ha)

Vegetation fallow Cultivated Bare soil

200 264 456

16.6 21.2 39.5

4.9 7.3 21.3

"From Charreau (1970).

Considering all pros and cons, the necessity of maintaining residue mulch at a rate of 2-4 t/ha, and the need to enhance the nitrogen status of the soil, it is logical that a suitable cover crop be sown during the fallow period. The cover crop must, however, be judiciously managed to conserve soil, water, and nutrient reserves for the grain crop to follow.

XI. SOIL GUIDE TO CONSERVATION TILLAGE The choice of the most appropriate type of conservation tillage depends on many factors. The most notable physical factors include soil properties,

Table XLIX Plant-Available Water (mm) in the 0-100-qn Soil Layer as Affected by Straw Mulch Rate during Fallowing in Northern Kazakhstan, USSR"

Growth stage Straw rate (t/ha)

0 1

2 4 LSDh

Sowing

Heading

Harvest

107.3 120.9 120.6 127.2 12.2

56.0 69.3 71.2 10.3

26.1 28.2 27.6 30.0 10.2

"From Bakaijev et a / . (1981). "LSD, least significant difference at the 5% level of significance.

178

RATTAN LAL

rainfall regime, climate, drainage conditions, rooting depth, soil compaction and erosion hazards, cropping systems, etc. In addition, there are socioeconomic considerations including farm size, availability of inputs, and marketing and credit facilities. The socioeconomic factors are as important as or more important than the biophysical factors. In addition to biophysical environments, high aspirations of members of modem society and increases in the cost of living are important factors that have altered farming systems and the type of tillage operations used. Impressive progress has been made in measuring the nutrient status of a soil using laboratory and field tests and in recommending precisely the fertilizer required to procure the desired yield levels. Soil scientists have developed reliable tests for diagnosing soil acidity, aluminum toxicity, and nutrient deficiencies and for prescribing corrective measures. However, soil scientists do not possess reliable and routinely measurable soil tests to determine the type of conservation tillage needed. We do not have a reliable and proven soil guide to assess specific tillage needs for alleviating soil- and environment-related constraints to crop production. Tillage operations are energy-intensive,form a major proportion of production costs, and have long-term effects on soil productivity and environmental quality. Developing diagnostic techniques to assess the curative tillage requirements deserves to be a high priority. Attempts have, therefore, been made to develop soil evaluation techniques to assess tillage needs. The most important factors considered in evaluating tillage needs are drainage, erosion, rooting depth, soil temperature regime, susceptibility to droughtiness, compactability, and susceptibility to crust formation. The applicability of conservation tillage can be drastically improved by developing cultural practices to alleviate production disadvantages and lower yields. Some relevant examples of soil guides to tillage needs are briefly described below. Triplett et al. (1973) developed a guide to assess application of a notill system to Ohio soils. They classified soils into 5 tillage groups primarily on the basis of surface and internal drainage. (i) Tillage group 1: These soils are perfectly suited to no-till conservation tillage systems. Soils in this category are well-drained and have a silt loam, loam, sandy loam, or loamy fine sand texture. These soils respond positively to mulch cover and require 70430% coverage of the soil surface. (ii) Tillage group 2: Soils in this group are somewhat poorly drained, have hydraulic conductivity of less than 0.5 cm/hr, and require additional management inputs to respond satisfactorily to conservation tillage. Additional inputs needed are generally in terms of providing surface and subsurface drainage. The predominant textural classes of this group are silt


179

loam, loam, sandy loam, or loamy fine sand. Mulch cover is also important for satisfactory crop performance. (iii) Tillage group 3: Soils in group 3 are very poorly drained and have extremely low hydraulic conductivity. These soils have loam, silt, loamy silty clay, or loam texture. In general, crop yields are better with conventional than with no-till or conservation tillage systems. (iv) Tillage group 4: Soils of this group are extremely poorly drained. Predominant textural classes in this group are silty clay loam, silty clay, or clay. These soils do not respond to mulch. (v) Tillage group 5: These soils do not respond to no-till and conservation tillage systems regardless of the management and additional inputs (e.g., peat soils, etc.). A similar guide was developed for soils of Indiana to assess the tillage needs for maize and soybean production (Cooperative Extension Service 1977, 1982). Galloway and Griffith (1978) described the most appropriate form of conservation tillage for each soil type. Within the Corn Belt of the United States, Galloway and Grifiith suggested that conservation tillage systems with proven erosion control potential can be widely adopted provided that weeds and other pests are controlled. Allmaras and Dowdy (1985) outlined nine tillage management regions (TMR) in the United States. These regions are based on climate, adapted crops, and cropping systems. Adoption of conservation tillage planting systems ranged from 2245% of the cropland in a TMR. Considerable progress has also been made in Europe towards relating the most suitable form of conservation tillage to soil properties and agronomic constraints. Two relatively independent research groups developed no-till or direct drilling systems for soils in the United Kingdom. Soane and Pidgeon ( 1975) related tillage requirements to soil physical properties in Scotland such as soil strength, aeration, water status, soil temperature, and the field situation. Pidgeon and Ragg (1979) observed that an important factor determining soil suitability for direct drilling is the inherent ability of some soils to resist or recover from compaction while maintaining a satisfactory pore-size distribution and drainage status. Similar to the soil groups proposed by Triplett and VanDoren for Ohio, Pidgeon and Ragg proposed the following groups for soils of the United Kingdom: (i) Soils suitable for no-till: These soils include well-drained, loamy soils and coarse sandy soils in which levels of organic matter are adequate (>2%). These soils have enough bearing strength to support the loads imposed without compacting to a damaging extent.

I80

RATTAN LAL

(ii) Less suitable soils: Soils relatively less suitable to no-till and conservation tillage comprise either better-structured, moderately well-drained or imperfectly drained clay soils and weakly structured, imperfectly drained loamy soils in which field drainage can control the water table. An important consideration is the interaction between climate and soil for impeded drainage. (iii) Least suitable soils: There occurs a drastic yield reduction when these soils are managed by no-till or a conservation tillage system. These soils include poorly drained and weakly structured clay loam and clay soils. In Britain Cannell et al. (1978) proposed a soil classification system based on the experimental results. There were four criteria considered in developing this classification system. These were (a) changes in soil conditions with repeated use of no-till; (b) limiting soil factors such as lack of tilth, topsoil compaction, drainage, texture, levels of organic matter, free lime, wetness caused by slow subsoil drainage, self-mulching, and presence of stone; (c) site factors such as slope, spring lines, and field variability; and (d) climate. These factors were further refined by Stengel et al. (1982, 1984) on the basis of crop performance. Three soil-related indices were prepared: structural stability, shrinkage, and compactability. A similar soil management guide was prepared for soils of New South Wales Australia, by Cooke (1982). He proposed a soil classification framework that groups together land of similar type for land management planning. A soil suitability guide for conservation tillage for soils of the tropics was proposed by La1 (1985a). A rating system was developed to assess the suitability of the type of conservation tillage for different soils. Soil and climatic properties included in developing the rating system are erosivity, erodibility, soil loss tolerance, compaction, soil temperature regime, available water-holding capacity, cation exchange capacity, and soil organic matter content. Also included is the quantity of crop residue mulch on the soil surface at seeding. Each of these factors were rated from 1 to 5 . The value of I corresponded with the characteristics desirable for notill and mulch farming and that of 5 for soil-inversion conventional tillage. The accumulative rating index in relation to the most desirable type of conservation tillage is shown in Table L. Based on the available information, general guidelines for the choice of conservation tillage in relation to soil type and climate are shown in Fig. 39. In the humid and subhumid tropics, with soils of coarse texture in the surface horizon no-tillage can be successfully applied for upland row crops. In the semiarid region, with fine textured soils some form of mechanical loosening of crusted and compacted soil is necessary. The

181

CONSERVATION TILLAGE Table L Accumulative Tillage Rating Index and the Appropriate Conservation Tillage System in the Tropics" Accumulative rating index of soil properties

Suitable conservation tillage system No-till farming, with cover crops and alley cropping Chiselling in the row zone Minimum tillage-permanent ridge furrow system Plowing at the end of the rainy season Both primary and secondary tillage

45

"From La1 (1985a).

Water Erosion

0 El

Water Erosion- Crusting Water Logging - Water Eroston Water and Wind Eroston Wind Erosion Drought SIress

-

X~'/RIDGE F U R R O ~ A IWATER w

a 2 c X

w

c

PER

HUMID

H~MID

SUBHUMID

MOISTURE

SiMIARID

+ARID

REGIME

FIG.39. Tillage systems (no-tillage and surface-tillage) and conservation objectives for the tropics depending on soil texture, climatic moisture regime, and major soil conservation problems. (From Lal, 1985a.)

I82

RATTAN LAL

frequency and type of mechanical operation desired depends on soil characteristics and the crops to be grown.

XII. RESEARCH AND DEVELOPMENT PRIORITIES A systems approach is essential for the wide adaptation of conservation tillage. For the conservation tillage system to be to be successfully adopted in a wide range of soils and environments, it must fit into the overall scheme of the present and future trends in the farming systems of the region and must meet the rising social and economic aspirations of the farming community. Conservation tillage cannot be adopted in isolation. It is a basic management tool for which the supporting packages of cultural practices must be developed and researched specifically for each benchmark soil and agroecological region. These cultural practices must be designed to render the system flexible for fine-tuning by the farmer concerned. Non-point source pollution is a major environmental hazard. Conservation tillage has become a source of controversy regarding its effects on transport of herbicides and nutrients in surface and subsurface waters (Crosson, 1981; Hinkle, 1983; Baker, 1985). Long-term and large-scale ecosystem studies are necessary to assess the effectiveness of conservation tillage systems in reducing the transport of sediments, nutrients, and pesticides to natural waters. Impressive progress has been made by soil scientists in assessing soil’s nutritional status and recommending the rates of fertilizers for meeting crop requirements. There are no such tests available for routinely measurable soil properties to diagnose soil tilth-related constraints to crop production and recommend the appropriate conservation tillage systems to achieve and maintain the desired seedbed. Soil structure and tilth continue to be elusive properties difficult to diagnose and assess. Minimizing energy-related inputs of tillage, fertilizers, and pesticides is a major objective that deserves to be a high priority. The objective is to sustain satisfactory and optimum levels of economic returns while minimizing dependence on those inputs that are either not available or based on nonrenewable resources. In the tropics and subtropics herbicides and farm chemicals are not easily and economically available and high rates of application are not ecologically and environmentally compatible. The dependence on herbicides as the preferred mode of weed control should be minimized. Alleleopathic effects of cover crops and biological methods of weed control should be evaluated. Regenerative cropping systems


183

must, therefore, be researched to reduce dependence on potentially hazardous chemicals. Seedling establishment is a major factor responsible for a low crop stand and low yields with conservation tillage. Poor seedling establishment may be due to an unfavorable microclimate in the seed zone or to high levels of pest incidence. There are many important factors that are need to be researched including time of sowing, seed rate, seed placement and seedsoil contact, row orientation, and integrated pest management. The role of appropriate cultivars and of seed characteristics (such as size, hardiness) cannot be overemphasized. Also important are alleleopathic effects related to anaerobic decomposition of crop residues in damp and cool environments. Suitable cultivars should be screened for the specific environments of the conservation tillage. Diversification is a key to ecological stability. Appropriate conservaton tillage should be developed for systems of row crop production integrated with those for raising livestock and growing perennial crops. Techniques of management of pastures or of woody plants and shrubs should be compatible with the specific requirements of the proposed conservation tillage systems for row crop production. The question of sustainability and of environmental quality remains a major challenge to agriculture for generations to come. What are the system’s performance indicators that assess sustainability and its ability to preserve the resource base? Should the agronomic returns be assessed in terms of production (i) per unit area; (ii) per unit time; (iii) per unit loss of an important soil property that plays a vital role in maintaining soil’s life-support processes such as pH or organic matter content; (iv) per unit loss of soil’s effective rooting depth (e.g, kilograms of grain produced per kilogram of soil eroded); (v) 0utput:input ratio evaluated in terms of calories; (vi) or per unit increment of major pollutants to first-order streams or groundwater, e.g., increase in N03-N, phosphorus, herbicides or insecticides in streams? Assessing suitability of conservation tillage in terms of the economics of crop production on a seasonal or annual basis alone is not enough. We do not possess appropriate system performance indicators.

XIII. CONCLUSIONS Conservation tillage is a systems approach to farming whereby longterm sustainability and preservation of environmental quality and resource base are given priority over the short-term economic returns. However,

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conservation tillage may not always yield the highest economic returns on a short-term basis. Conservation tillage is also soil- and site-specific and no single blueprint of cultural practices can be universally applicable. It includes no-till, minimum tillage, ridge tillage, chisel plowing, zonal tillage, and a range of other cultural practices developed to overcome specific constraints. The appropriate type of conservation tillage depends on both biophysical and socioeconomicfactors and on interactions between them. The most beneficial tillage practice is the one that creates or maintains a favorable porosity for water and air movement and root growth and development. Although basic principles governing sound management of soil and environments are the same for the tropics and temperate regions, there are subtle differences in the package of cultural practices needed to optimize the use of limited resources and to alleviate soil- and environment-related constraints to crop production. Variations in packages and cultural practices are due to differences in climate, soil, erosion risks, availability of commercial inputs, infrastructure, farm size, and socioeconomicfactors. The basic components of successful conservation tillage are based on use of crop residue mulch, reducing the intensity and/or frequency of mechanical tillage, and adoption of appropriate cropping sequences and combinations to provide needed diversity, minimize inputs, and preserve the soil resource. Major reasons for adopting conservation tillage are preventing soil erosion, providing favorable soil and microclimate environments, reducing risks of environmental pollution, minimizing the commercial inputs needed, and preserving the soil resource base. Conservation tillage is a risk-avoiding and a problem-solving approach, geared to providing satisfactory yield under the worst conditions rather than the highest yield under the best conditions. It is also aimed at alleviating specific constraints, e.g., accelerated erosion, drought stress, surface sealing and crusting, subsoil compaction, unfavorable soil temperature regimes, anaerobic conditions in the root zone, and other factors responsible for low soil fertility. Effectiveness of conservation tillage can be vastly improved by adopting other supportive practices based on principles of good farming. These include crop rotations, cover crops, mixed farming, agroforestry, and summer fallowing. The slow adoption of conservation tillage is due to the lack of suitable supporting practices that would enhance its effectiveness. Conservation tillage has become a controversial issue because its high dependence on herbicides. Herbicide transport from row crop agricultural lands is a major source of pollutants of natural waters. We do not understand the pathways, biodegradation, and transport processes in different systems of soil and crop management. Myths and suspicion should be replaced by facts through long-term and ecologically oriented field experiments.


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Conservation tillage is an important component of low-input sustainable agriculture systems. It is aimed at preserving the productive potential of soil and maintaining environmental quality. It is an approach that emphasizes the use and improvement of the natural resource rather than its exploitation and mining its productivity for quick economic gains. Conservation tillage is synonymous with good farming.

ACKNOWLEDGMENTS Help received from Karla Gutheil and Shirley Hall in typing this manuscript is gratefully acknowledged.

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Stengel, P., Douglas, J. T., Guerif, J., Goss, M. J., Monnier, G., and Cannell, R. Q. 1982. Factors influencing the variation of some properties of soils in relation to their suitability for direct drilling. Letcombe Laboratory, Wantage, Oxon, U.K. Stengel, P., Guerif, J., Monnier, G., Douglas, J. T., Goss, M. J., and Cannell, R. Q. 1984. Soil Till. Res. 4, 35-54. Stephens, D. 1967. J. Agric. Sci. 68, 391403. Stewart, B. A., and Musick, J. T. 1982. Conjunctive use of rainfall and irrigation in semiarid regions. Proc. Congr. Int. Soc Soil Sci., 12th, New Delhi. Stewart, B. A., Dusek, D. A., and Musick, J. T. 1981. Soil Sci. Soc. Am. J . 45, 419. Stibbe, E., and Arid, D. 1970. Neth. J . Agric. Sci. 18, 293-307. Stinner, B. R., Hoyt, G. D., and Todd, R. L. 1983. Soil Till. Res. 3, 277-290. Sukmana, S., Suwardjo, H., Abdurachman, A., and Dai, J. 1985. Prospect of Flemingia congesta for reclamation and conservation of volcanic skeleta. soils. Pemb. Pen. Tanah Dan Pupuk 3, 50-54. Suwardjo, Abdurachman, A., and Sutomo. 1984. Effect of mulch and tillage on soil productivity of a Lampung Red Yellow Podsolic, Indonesia. Pembr. Pen. Tunah Dan Pupul 3, 12-16. Swennan, R., and Wilson, G. F. 1983. Effects of mulching on plantains at Onne. IITA Res. Highlights, Ibadan, Nigeria. Tarawali, G., and Mohamed-Saleem, M. A. 1987. Effects of method of cultivation on root density and grain and crop residue yields of sorghum. ILCA Bulletin No, 27, ILCA, Addis Ababa, Ethiopia. Taylor, A. W. 1978. J. Air Pollut. Control Assoc. 28, 922-927. Thamburaj, S. 1980. National Seminar on Tuber Crops Production Technology, Nov. Nader Agric. Univ., India. Thomas, G. W., Blevins, R. L., Phillips, R. E., and McMahon, M. A. 1973. Agron. J . 65, 736-739. Tisdall, J. M., and Adem, H. H. Soil Till. Res. 6 , 365-376. Touchton, J. T., Rickerl, D. H., Walker, R. H., and Snipes, C. E. 1984. Soil Till. Res. 4, 391401. Tripathi, R . P., and Katiyar, T. P. S. 1984. Soil Till. Res. 4, 381-390. Tripplett, G. B., Jr. 1976. The pros and cons of minimum tillage in corn. Proc. Annu. Corn Sorghum Res. Conf, 3Ist. 144-158. Triplett, G . B. 1986. I n “NO-Tillage and Surface Tillage Agriculture” (M. A. Sprague and G. B. Triplett, eds.), pp. 149-182. Wiley, New York. Triplett, G. B., Jr., and Mannering, J. V. 1978. I n “Crop Residue Management Systems,” pp. 187-206. ASA, Madison, Wisconsin. Triplett. G. B., Jr., Van Doren, D. M., and Schmidt, B. L. 1968. Agron. J . 60, 236-239. Triplett, G. B., Jr., Van Doren, D. M., Jr., and Bone, S. W. 1973. An evaluation of Ohio soils in relation to no-tillage corn production. OARDC Res. Bull. No. 1068, Wooster, Ohio. UNEP. 1986. Farming systems principles for improved food production and the control of soil degradation in the arid, semi- and, and humid tropics. Expert meeting sponsored by UNEP, 20-30 June, 1973. ICRISAT, Hyderebad, India. Unger, P. W. 1978a. Soil Sci. Soc. Am . J . 42, 486491. Unger, P. W. 1978b. AgronJ. 70, 858-864. Unger, P. W . 1980. Trop. Agric. (Trin.) 59,220-122. Unger, P. W. 1984. Tillage systems for soil and water conservation. F A 0 Soils Bulletin No. 54. Unger, P. W., and McCalla, T. M. 1980. Adv. Agron. 33, 1-58. Unger, P. W., and Wiese, A. F. 1979. Soil Sci. Soc. Am. J . 43, 582-588.


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MICROBIALLY MEDIATED INCREASES IN PLANT-AVAILABLE PHOSPHORUS R. M. N. Kucey,’ H. H. Janzen,’ and M. E. Leggett2 ‘AgricultureCanada Lethbridge Research Station Lethbridge, Alberta T i J 481, Canada ‘Philorn Bios, Inc. Saskatoon. Saskatchewan S7N 2x8. Canada

1. Introduction 11. Sources of Plant-Available Phosphate in Soils A. Forms and Transformations of Soil P B. Added Phosphate Fertilizers C. Rock Phosphate as a Fertilizer Source 111. Mycorrhizal Effects on Plant Phosphate Availability A. Absorption of Unavailable Phosphate B. Alteration of Plant Growth or Enzyme Activity C. Extension of Phosphate Depletion Zone IV. Phosphobacterins and Organic Phosphate Mineralization V. Inorganic Phosphate-Solubilizing Microorganisms A. Solubilization of Phosphate in Pure Culture B. Occurrence and Numbers of PS Organisms in Soil C. Effect of Inoculation in Soils D. Mechanism of Action of PS Microorganisms VI. Sulfur Oxidation and Rock Phosphate-Sulfur Mixtures VII. Future of Technologies References

I. INTRODUCTION The importance of microorganisms in soil nutrient cycling and their role in plant nutrition has been realized for a long time. Their active part in the decomposition and mineralization of organic matter and release of nutrients is crucial to sustaining the plant productivity upon which we depend for survival. Only relatively recently have attempts been made to quantify other roles that specialized groups of soil microorganisms mediate. One group of microorganisms that is of importance to plant nutrition includes those organisms that allow plants to absorb phosphorus (P) from sources that are otherwise less available. Organisms that cause increases

Copyright Q 1989 by Academic Press. Inc. All rights of reproduction in any form FZSeNed.

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in plant-available P in the soil system belong to a diversified group including bacteria, actinomycetes, and several groups of fungi. Research has been conducted in many aspects of the study of microbially mediated increases in plant-available P. These have included research in the area of mycorrhizal fungi, P-mineralizing or -solubilizing microorganisms, and dissolution of P by the oxidation products of sulfur. The opinions regarding the feasibility of such systems for agricultural purposes depend greatly upon familiarity with the literature and the nature of related research results. The results produced in the study of these organisms are variable because of the differences in the organisms used as well as differences in soils and climatic parameters. Until now, no comprehensive review of this topic has been available. We have attempted to summarize the wide array of information on microbially induced release and increased availability of P in soils.

II. SOURCES OF PLANT-AVAILABLE PHOSPHATE IN SOILS A. FORMSAND TRANSFORMATIONS OF SOIL P

The concentration of total P in soils ranges from 0.02 to 0.5% and averages approximately 0.05% (Barber, 1984), the variation being largely due to differences in weathering intensity and parent material composition (Stevenson, 1986). Relatively high concentrations are often observed in calcareous soils of arid regions, whereas relatively low P concentrations are often observed in soils subjected to high weathering intensity. Vertical distribution of total P within the soil profile is usually quite uniform (Stevenson, 1986), although plant residue deposition may result in some accumulation of total P in the surface horizon (Barber, 1984). Soil P exists chiefly as orthophosphate, although phosphine and phosphonates have been detected under some conditions (Stewart and McKercher, 1982). The diverse soil P forms can be generally categorized as soil solution P, insoluble inorganic P, or insoluble organic P. Only a very small fraction of soil P exists in the soil solution because of its extreme reactivity. The concentration of P in the soil solution is commonly approximately 0.05 mg/liter and seldom exceeds 0.3 mg/liter in unfertilized soil (Ozanne, 1980). Inorganic P in solution exists mainly as primary or secondary orthophosphate, depending on soil pH. Organic P may also make up a large fraction of soluble P, as much as 50% in soils with high organic matter content (Barber, 1984). Inorganic P associated with the solid phase can be sorbed to the surfaces of soil constituents (Sample et al., 1980) or can occur in calcium, iron,

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or aluminum minerals (Tiessen and Stewart, 1983; Barber, 1984). Organic P, occurring in humified organic matter, organic residues, and microbial biomass, can account for 2-80% of the P content of surface soils (Dalal, 1977). Most of the organic P occurs as ester P (Stewart and McKercher, 1982). Only the P dissolved in the soil solution is directly accessible to plants. Since the concentration of P in the soil solution is normally insufficient to support plant growth, crop growth depends on continual replenishment of soluble P from inorganic and organic sources (Chauhan et al., 1979; Anderson, 1980; Barrow, 1980; Ozanne, 1980; McGill and Cole, 1981; Tisdale et al., 1985; Stevenson, 1986). While most soils contain substantial reserves of total P, most of it remains relatively inert. According to Ozanne (1980), less than 10% of soil P enters the plant-animal cycle. Consequently, P deficiency is a widespread problem and P fertilizers are almost universally required to maintain crop production.

B. ADDEDPHOSPHATE FERTILIZERS In many agricultural systems, P fertilizers are routinely applied to promote crop yields. Most of these fertilizers contain P in water-soluble forms as salts of ammonium, calcium, and potassium (Tisdale et al., 1985). Although the P in these fertilizers is initially plant-available, it rapidly reacts with soil and becomes progressively less available for plant uptake. These forms become subject to the same forces as native soil P. As a result of the various retention mechanisms, most of the fertilizer P applied (often as much as 90%) is rendered unavailable for crop uptake but is retained in insoluble form (Stevenson, 1986). Although this P may have some residual benefit, further annual applications are often necessary to maintain adequate labile P. Because P applications usually substantially exceed crop uptake, the total P concentration of many soils has increased markedly over time (Barber, 1979). Thus, soils commonly have large reserves of “fixed” P that could support long-term crop requirements if it could be economically exploited.

c. ROCK PHOSPHATE AS A FERTILIZER SOURCE The direct application of rock phosphate as a fertilizer source has received renewed interest in recent years. Upon application to the soil, rock phosphate gradually releases soluble P by various solubilization reactions. As time progresses, the unsolubilized rock phosphate becomes progressively more recalcitrant. The rate of plant-available P release from

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rock phosphate has been shown to be quite variable owing to differences in rock phosphate source, particle size of the rock phosphate, soil pH, and other soil chemical properties (Barnes and Kamprath, 1975; Kucey and Bole, 1984; Hammond et al., 1986). In general, rock phosphate is not a reliable source of plant-available P in soils with pH greater than 5.56.0 (Engelstad and Terman, 1980) and even in acidic soils, the rock phosphate must be extremely finely divided to ensure an adequate rate of soluble P release (Tisdale et d., 1985). In practice, addition rates for rock phosphate are generally 10 times that recommended for manufactured P fertilizers (Kucey and Bole, 1984). A recent comprehensive review on the benefits of direct application of rock phosphates as fertilizers has been published by Hammond et al. (1986). The uptake of P from relatively insoluble sources can be affected by the type of plant growing in the soil. It has been determined that plants vary in the cation exchange capacity (CEC) of their root systems, and plants with high CEC levels, such as ragweed (Ambrosia artemisiifolia L.) or smartweed (Polygonum coccineum Muhl), are more effective at obtaining P from rock or soil P sources than those with low CEC, such as wheat (Triticum aestivum L.) and oats (Avena sativa L.) (Drake and Steckel, 1955). Furthermore, it was found that growing a plant with low CEC root systems beside a plant with a high root CEC resulted in increased P uptake by the former. It was hypothesized that the roots caused P release by binding calcium, iron, and aluminum with organic anions (Nye and Kirk, 1987). Buckwheat (Fayopyrom esculentum L.) was able to acidify its rhizosphere and cause dissolution of rock phosphates, but maize (Zea mays L.) was unable to do so (Bekele et al., 1983). Rapeseed (Brassica nupa oleiferu) plants in that study were able to utilize rock phosphates by absorbing high amounts of calcium, which shifted the mass equilibrium in favor of soluble ions. Van Ray and Van Diest (1979) also observed a relation between the rhizosphere pH of various plant species and their utilization of rock phosphate, superphosphate, or calcined aluminum phosphate. A variety of plants has been found to use one or more of these mechanisms in the solubilization of P in their rhizospheres (Johnston and Olsen, 1972).

Ill. MYCORRHIZAL EFFECTS ON PLANT PHOSPHATE AVAILABILITY The effect of mycorrhizal fungi on plant growth and nutrient uptake has been extensively studied and reviewed (Gianinazzi-Pearson and Gianinazzi, 1981, 1985; Barea and Azcon-Aguilar, 1983; Tinker, 1984). It is


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not the purpose of this chapter to review extensively the myriad effects of mycorrhizal fungi on plant growth. Rather, our discussion will concentrate on the effect of mycorrhizae on P uptake by plants. The mycorrhizal fungi are an important part of the soil microbial system because the prevalence of these associations on plants is so common under natural soil conditions that a nonmycorrhizal plant is the exception rather than the rule. Only a few groups of plants do not normally form mycorrhizal associations (Marx and Krupa, 1978). Endomycorrhizae, which include vesicular-arbuscular (VA) mycorrhizae, and ectomycorrhizae are noteworthy for their potential economic importance (Gerdemann, 1968). The effect of mycorrhizae on plant P uptake and the effect of soil P on mycorrhizae were among the first aspects of these symbioses studied. The influence of mycorrhizae on plant P uptake has become obvious and well known (Gianinazzi-Pearsonand Gianinazzi, 1985). The relative benefits of mycorrhizal infection decrease as P availability increases (Ross, 1971),partially due to the negative effect of P on the levels of mycorrhizal infection (Hayman et al., 1975; Kucey and Paul, 1983). Infected roots and high numbers of spores are found most commonly in soils of low to moderate P-availability status, whereas soils high in P have been found to contain few spores or infected roots (Azcon et al., 1978). Split-root techniques with sudangrass (Sorghum vulgare L.) colonized by GIomus fasciculatus have shown that the concentration of P within the plant, not P levels in the soil, reduces root infection and spore production levels (Menge et al., 1978b). Studies concerning fungi-assisted uptake of P should consider that plants growing in more fertile soil are associated with low fungal biomass and so derive less benefit than a plant associated with high levels of fungi. Because phosphate ions in the soil are relatively immobile, plant roots must expend considerable energy producing enough root material for adequate P absorption in soils with low P levels (Bieleski, 1973). Mycorrhizae appear to be able to assist the plant in absorbing the P it needs. There are several possible mechanisms by which the fungus could assist host uptake of P. Research has concentrated on three areas: (i) absorption of P from sources unavailable to the uninfected plant, (ii) alteration of plant growth such that the plant produces a larger root system or alters its enzymes for absorbing P, and (iii) extension of the P-depletion zone away from the root (Gerdemann, 1975; Gianinazzi- Pearson and Gianinazzi, 1981, 1985).

A. ABSORFWONOF UNAVAILABLE PHOSPHATE The possibility that mycorrhizal fungi may be able to use forms of P unavailable to plants is most interesting from an economic point of view.

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Many forms of P, although plentiful, are essentially unavailable for plant uptake. The possibility that mycorrhizal fungi may have a mechanism by which they can assimilate these forms and pass the P onto the host has been studied by adding various inorganic P compounds to P-deficient soils and measuring symbiotic and nonsymbiotic plant P uptake. For instance, mycorrhizal soybean (Glycine max L. Merr.) shoot weights were increased by up to 56% when monocalcium phosphate was added (Ross and Gilliam, 1973). Ali (1976) obtained positive plant responses from the addition of calcium or iron phosphates. Jackson et al. (1972) found no response of mycorrhizal and nonmycorrhizal corn roots to rock phosphate addition; however, increased P uptake from rock phosphates by mycorrhizal plants has been observed by others (Waidyanatha et al., 1979; Powell et al., 1980; Cabala-Rosand and Wild, 1982). Although initially it appears that plants colonized by mycorrhizal fungi can use these “unavailable” sources of P, it must be remembered that the precipitated forms of P are in chemical equilibrium with P in solution (Russell, 1973). If solution P is removed, precipitated P can replenish the solution P. In this way, unavailable forms of P may contribute to the solution P pool, albeit to a small extent, and become available for plant uptake (Gianinazzi-Pearson and Gianinazzi, 1985). Phosphorus uptake studies using ”P-labeled phosphate have shown that the specific activities of P in VA mycorrhizal and nonmycorrhizal plants are the same, indicating that both plants utilize the same sources of P (Mosse et ul., 1973; Gianinazzi-Pearson et al., 1981; Raj et al., 1981; Asea et al., 1988). Similar findings were observed for ectomycorrhizal and nonmycorrhizal pines (Thomas et al., 1982). The conclusion, therefore, is that the mycorrhizae do not utilize unavailable P sources, they utilize the available solution forms more efficiently (Gerdemann, 1975). Long-term experiments indicate that both mycorrhizal and nonmycorrhizal plants are less able to extract P from the soil as the levels of solution P become increasingly more depleted (Powell, 1977). Certain species of host benefit greatly from mycorrhizal infection because their uninfected roots are unable to take up P present in very low concentrations (Mosse, 1973; Plenchette et al., 1983). Hosts of this type depend heavily upon the mycorrhizal fungi to absorb the solution P and transport it to the host.

B. ALTERATION OF PLANT GROWTH OR ENZYME ACTIVITY Mycorrhizae may alter the physical or physiological activity of a host root system, which in turn may allow the host itself to take up more P. The possibility that mycorrhizae alter host shoot-root ratios (S:R) has


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been studied in some detail, although the results are conflicting. Smaller S:R of VAM-colonized plants have been reported in some cases (Ali, 1976), but larger S:R have also been obtained for infected plants (Crush, 1974; Daft and El Giahmi, 1976). The results of one experiment are generally inconsistent with those obtained in other experiments, so no general conclusion can be made on the effect of VA mycorrhizae on host shoot-root ratios. It appears that the results of a study involving mycorrhizal fungi may vary with the fungus and host used, as well as with the soil they are growing in. The fungi may affect plant roots by means other than increasing their physical size. Some groups have found that mycorrhizal onion roots (Allium cepa L.) removed a greater amount of 32Psolution from areas close to the root (Owusu-Bennoah and Wild, 1979). This has prompted several groups to explore the possibility of more efficient enzymes for hydrolyzing and/or absorbing P. A soluble alkaline phosphatase that is lacking in nonmycorrhizal roots has been found in onion roots infected with Glomus mosseae. The maximum levels of this enzyme’s activity appeared at the same time that the positive growth response to mycorrhizal infection appeared in the host, at which time the enzyme accounted for 32% of the total root phosphatase activity (Gianinazzi-Pearson and Gianinazzi, 1976, 1978). Allen et al. (1981) also observed increased alkaline phosphatase activity of mycorrhizal Bouteloua gracilis. The presence and activity of many enzymes in the fungi appear to change with the age of the fungus and its state of development (MacDonald and Lewis, 1978) and may increase the availability of certain organic P forms to host plants. Other theories assume that certain mycorrhiza-specificenzymes do not play roles in plant P uptake, but rather are involved in P assimilation by the fungus, either at the level of P absorption, in active transport by the fungus, or in active transport into the host plant (Gianinazzi-Pearson and Gianinazzi, 1978). The role of endomycorrhizae in organic P hydrolysis is still unclear; however, ectomycorrhizae have been shown to directly break down phytates in soils (Gianinazzi-Pearson and Gianinazzi, 1985).

c. EXTENSION OF PHOSPHATE DEPLETION ZONE Whether or not the host itself becomes more efficient at absorbing P, a major part of the increased uptake is directly due to fungal activity (Sanders and Tinker, 1971, 1973). Experimental evidence indicates that, as well as increasing P uptake from areas close to the root, mycorrhizal roots obtain P from areas far from the root (Gray and Gerdemann, 1969).

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It has been found that mycorrhizal onion roots removed 32Pfrom areas 8.0 cm from the root surface (Rhodes and Gerdemann, 1978), whereas nonmycorrhizal roots failed to absorb any label from sources placed 1.O cm from the root surface. Diffusion in this experiment accounted for movement of the isotope of less than 0.75 cm. Addition of a fungal toxicant to the soil restricted uptake of labeled P by mycorrhizal roots to that of control plants. Thus, it appears that the fungi extend the zone of P depletion from the root surface. The uptake of P by mycorrhizal fungi is not a passive action. The fungi appear to be specific in their uptake and translocation of nutrients. When equal amounts of 32P,35S,and 65Znwere injected into a soil, the molar ratios of the nutrients taken up and translocated to the host were 35:5:1 for P:S:Zn (Cooper and Tinker, 1978). Other workers found the hyphae to be more efficient at taking up and moving P than at moving calcium (Rhodes and Gerdemann, 1978). Thus it appears that the major action of mycorrhizal fungi in facilitating plant P uptake is to increase the absorptive surface area of the mycorrhizal root system and to extend the P depletion zone away from the root surface. To date, there is no evidence that mycorrhizal roots are able to absorb P from any sources of soil P not available to the nonmycorrhizal root systems. Nonetheless, mycorrhizal fungi play a very important role in aiding plant P uptake. Because mycorrhizal fungi increase plant uptake of P, it should be possible to substitute selected mycorrhizal fungi for part of the P fertilizer now added. Glomus fasciculatus was able to substitute for up to 56 ppm P in the greenhouse cultivation of Troyer citrange (Poncirus trifoliata L.) and 278 ppm in the cultivation of Brazilian sour orange (Citrus aurantium L.) (Menge et al., 1978a). These two cultivars depend heavily on fungi for adequate P uptake. Generally, the majority of work on the use of mycorrhiza has been conducted under controlled conditions. As most agricultural soils contain native VA mycorrhizae, research on the role of specific strains in increasing P uptake has, of necessity, been done in greenhouse or field soils treated to remove the indigenous VA mycorrhizae. Most of the conclusions about the ability of selected VA mycorrhizae to increase P uptake are therefore based on tests done under altered conditions. Vesicular-arbuscular mycorrhizal isolates vary in their ability to supply certain hosts with P (Abbott and Robson, 1978) and to compete with the indigenous VA mycorrhizae (Abbott and Robson, 1982). The use of these organisms to supplement commercial fertilizers will therefore depend on our ability to select strains that have the ability to increase P uptake and to compete with native microflora. The detailed experiments that have been conducted to deter-


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mine the mechanisms VA mycorrhizae use to increase P uptake should enable us to identify the characteristics we should look for to find effective strains. Experiments conducted in sterilized soils have relevance to practical situations in which the host species are routinely grown in soils sterilized to remove disease organisms, for example, in the case of containergrown seedlings. However, in order to select species or strains which are also highly competitive, experiments must be conducted under nonsterile field conditions.

IV. PHOSPHOBACTERINS AND ORGANIC PHOSPHATE MINERALIZATION During the 1950s, farmers in the USSR and several eastern European countries inoculated a large proportion of their agricultural soils with a fertilizer consisting of kaolin impregnated with spores of the bacterium Megatherium viphosphateum (USSR Min. Agric., 1953; Rubenchik, 1956). This bacterium was later renamed Bacillus megatherium var. phosphaticum and the fertilizer was termed phosphobacterin (Menkina, 1956; Cooper, 1979). The bacteria added were reputed to increase the rate of organic P mineralization in the soil, resulting in the release of plant-available P (Menkina, 1950, 1963; Yung, 1954; Kudzin and Yaroshevich, 1962; Kvaratskheliya, 1962). The mechanism of action was, however, not fully determined by these workers. Consequently, questions remained regarding the mode of action of B. megatherium in increasing plant growth and, indeed, whether the early experiments with phosphobacterin were properly analyzed (Mishustin and Naumova, 1962). Yield increases resulting from the addition of B. megatherium to Soviet soils were reported to range from 0-70%, with 10-20% yield increases being obtained from over half of the crops inoculated (Smith et a / . , 1961). Vegetable crops responded best to inoculation; however, grains and potatoes (Solanum tuberosum L.) were also found to respond (Smith et d., 1961). Soils that gave the best results with phosphobacterins were neutral to alkaline and high in organic matter (Yung, 1954). Lime and/or organic materials such as manure were supposed to be added to alleviate acid conditions or to enrich soils low in organic matter. Experiments on phosphobacterin effectiveness conducted in the United States did not show the positive results obtained in studies from the USSR. Wheat and tomatoes (Lycopersicon esculentum) grown in greenhouse tests using six Chernozemic or chernozem-like soils did not respond positively to inoculation with B. megatherium (Smith et al., 1961). Lack of plant

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response was also observed with wheat, oats, and sorghum (Sorghum vulgare) in field tests conducted in Alaska, Minnesota, Montana, North Dakota, and Texas (Smith and Allison, 1962). It was concluded that phosphobacterin was of no practical use for farming systems in the United States and, further, that B. megafherium inoculation should not be used on a practical scale because the degradation of soil organic matter would be detrimental to the soil system. Tests conducted in India, however, did show positive responses to phosphobacterin addition (Sundara Rao and Sinha, 1963; Sundara Rao at af., 1963; Kavimandan and Gaur, 1971). The positive effects of phosphobacterins, which contained Pseudomonas jlorescens strains as well as B. megatherium strains, increased when farmyard manure and mineral fertilizers were added in conjunction with bacterial inoculants (Sundara Rao et af., 1963). These inclusions, however, confound the experimental data since the amounts of mineral and organic fertilizers added were sufficient to give optimum yields in the absence of the bacteria (Brown, 1974). If mineralization did occur, the amounts of P released would have been hidden and insignificant in the presence of the manure and fertilizers added. Bacillus megatherium was shown to cause the mineralization of nucleic acid P (Menkina, 1963) and myoinositol phosphate (Greaves and Webley, 1969) in sand culture but was not shown to cause the release of myoinositol phosphate in soil (Greaves and Webley, 1969). Although phosphobacterin has been reported to release inorganic P from organic sources in soil (Molla et al., 1984), other studies have concluded that B. megatherium was unable to do so (Martin, 1973). Although other instances of plant growth increases resulting from the addition of B. megatherium have been reported, the mode of action was determined not to be due to organic P mineralization, but rather to the solubilization of inorganic P forms, and will be discussed in a later section. The literature on the use of bacteria to increase plant-available P through organic P mineralization is somewhat contradictory, partially owing to poorly designed and improperly analyzed experiments in some cases. Because of this, it is diffcult to determine the validity of the conclusions based on some trials. In many cases, insufficient attention was paid to designing experiments to determine the mode of action of the bacteria. Conclusions about the mode of action were based on empirical data, e.g., higher yields in treated versus untreated plots. As it is now known that bacteria can increase plant growth by other mechanisms, such as the production of plant growth regulators, these conclusions may not be valid. It thus appears as if the research into phosphobacterins may have been misdirected. Plant growth responses to B. megatherium addition undoubtedly have been obtained; however, the mechanism of action is un-


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likely to be via organic P mineralization. This does not reduce the potential use of phosphobacterins for agriculture, but it does shift the area of research into other directions.

V. INORGANIC PHOSPHATE-SOLUBILIZING MICROORGANISMS A. SOLUBILIZATION OF PHOSPHATE I N PURECULTURE Microbial solubilization of inorganic P under pure culture conditions has been shown many times. Indeed, solubilization of a precipitated calcium phosphate in agar medium has been used as the initial criterion for isolation and enumeration of P-solubilizing (PS) microorganisms (Sperber, 1958a; Katznelson and Bose, 1959). Organisms growing on such media and able to solubilize P produce a clear zone around themselves due to the solubilization of the fine particles of calcium phosphate. Rates of P solubilization vary with the source of inorganic P. Louw and Webley ( 1959) observed equal levels of solubilization between tricalcium phosphate and hydroxyapatite in liquid media by PS organisms isolated from oat plants. Solubilization of both compounds was less than from dicalcium phosphate and roughly equal to the solubilization of P from basic slag. Iron and aluminum phosphates also have been shown to be solubilized in liquid media (Banik and Dey, 1982),but levels of P released were less than that released from calcium phosphate. Solubilization of P from defined P sources is a convenient method for use in comparing isolates. For practical purposes, however, the organisms must be able to cause significant solubilization of rock phosphates or increased availability of phosphatic fertilizers. Louw and Webley (1959) found that most of the 26 PS isolates they tested solubilized Gafsa rock phosphate in liquid medium, but none of the isolates solubilized rock phosphate in agar medium or variscite, strengite, or taranikite in either medium. Duff et ul. (1963) also observed a lack of solubilizing effect of PS isolates on variscite and strengite. Khan and Bhatnagar (1977) observed that Aspergillus niger solubilized P from eight rock phosphate sources. They found that the presence of sodium fluoride in the medium or in the rock phosphate did not inhibit the solubilization of rock phosphates except at high levels. Agnihotri (1970) studied the ability of fungi occurring in nursery seedbeds to solubilize apatite and fluorapatite in solution culture and found the list of fungi able to do so to be quite limited in scope. Thomas ef al. (1985) found that the PS fungi isolated from coconut (Cocos nuciferu) plantation

210

R. M. N. KUCEY ETAL.

soils belonged to the Aspergillus or Penicillium genera. Kucey (1983) similarly found that most of the PS fungal isolates from prairie soils were either Penicillium or Aspergillus. This indicates that PS abilities are not common to soil fungi in general.

B. OCCURRENCE AND NUMBERS OF PS ORGANISMS IN SOIL Under soil conditions, potential benefits of adding P-solubilizing organisms would depend on several factors, one of the most important being the activity of the PS microbial population already in the soil. In almost all cases, the major sources of PS isolates have been soils. Another source of PS isolates has been the surfaces of seeds. Phosphate-solubilizing microorganisms have been found in almost all soils tested, although the numbers vary with the soil climate and history (Sperber, 1958a; Katznelson and Bose, 1959; Katznelson et a / . , 1962; Chhonkar and Subba-Rao, 1967; Agnihotri, 1970; Khan and Bhatnagar, 1977; Banik and Dey, 1982; Kucey, 1983; Kim et al., 1984; Thomas et al., 1985; U et al., 1985). Katznelson et ul. (1%2) found that 6 7 0 % of bacterial isolates obtained from seed coat surfaces of many plants showed the ability to solubilize P in agar media, but only 10% of the isolates from the rhizoplane and rhizosphere showed this ability. Further work, however, indicated the importance of the soil-borne PS organisms, since the colonization of plant roots was determined to be by soil-borne organisms rather than by seedborne organisms. Sperber (1958a) found that PS organisms in the rhizosphere of subterranean clover (Trfolium subterruneum L.), ryegrass (Lolium perenne), and wheat (Triticumaestivum L.) roots constituted 263% of the microbial population (Table I). Khan and Bhatnagar (1977) and Sperber (1958a) both concluded that there was no preferential effect of rhizospheric conditions on the incidence of PS organisms, since the general rhizospheric microbial population increased in proportion to the increases in the number of PS organisms. Katznelson and Bose (1959) also found approximately onethird of the bacteria from the rhizoplane of wheat to show P-solubilizing abilities, and although no preferential stimulating effect of the roots on PS bacteria was observed, these organisms were found to be more active metabolically than other bacteria isolated from the same soils. Baya et al. (1981) also observed the rhizospheric P-solubilizing bacteria to be more active than those isolated from nonrhizosphere soil. Although there appears to be no preferential effect of the rhizosphere on PS organisms, there is some evidence that legumes support greater numbers of PS organisms than do nonlegumes (Sobieszczanski, 1%1) and that some legumes support more PS organisms than others (Paul and Sundara Rao, 1971).


21 I

Table I Numbers of Phosphate-SolubilizingMicroorganisms and Total Microbial Populations in Soils Source Rhizosphere Nonrhizosphere Fallow soil Rhizosphere Root-free soil Cultivated soil Grassed soil Degraded rock Sandy soil Alluvial soil Clayey soil Lateritic soil Seed coat Rhizosphere soil Root-free soil Rhizoplane Alluvial soil Desert soil

Total microbial population" 3.1-51.2 x lo9 0.1-1.5 x

0.02-0.04 x ND ND 1.5-19.0 X 0.65-115 X 0.15-0.30 x ND ND ND ND 0.2-340 X ND ND ND 1.0 x 10' ND

lo9 lo9 lo7

lo7 10'

lo'

P-solubilizing population 26-39% 26-39% 10-17% 4.5-120.0 X lo6 0.6-2.0 x lo6 0.14-4.8 x 10' 0.23-11.5 x lo' 0.08-0.5 x 10' 1.1-3.7 x lo3 0.7-14.5 x lo3 1.1-11.6 X lo' 0.7-23.7 x 10' 0.1-202 x lo4 0.6-500 x lo6 0.3-1.7 x 10' 10-1 I%

1.6 x lo6 107-106

Reference Sperber (1958a) Sperber (1958a) Sperber (1958a) Khan and Bhatnagar (1977) Khan and Bhatnagar (1977) Kucey (1983) Kucey (1983) Kucey (1983) Thomas er a / . (1985) Thomas e r a / . (1985) Thomas er a / . (1985) Thomas er a / . (1985) Katznelson et a / . (1962) Katznelson er a / . (1962) Katznelson et a / . (1962) Katznelson et a / . (1962) Banik and Dey (1982) Saber er a / . (1977)

"ND. not determined.

Phosphate-solubilizing bacteria and fungi constituted 0.5 and 0.1%, respectively, of the general soil microbial population in prairie soils, with PS bacteria outnumbering PS fungi two-fold to 150-fold (Kucey, 1983). Banik and Dey (1982) observed that PS bacteria outnumbered PS actinomycetes and fungi in Indian soils three-fold and 50-fold, respectively. Thomas et al. (1985) were able to isolate more PS fungi from alluvial, lateritic, and clayey soils than from sandy soils. After subculturing of PS isolates, many of the bacterial isolates have been observed to lose their PS activity (Sperber, 1958a; Kucey, 1983). Once the P-solubilizing ability has been lost, it cannot be regained. Fungal isolates have not been observed to lose their P-solubilizing ability over many successive subculturings (Kucey, 1983). Fungal isolates in general showed greater PS activity in agar and liquid media than did bacterial isolates in the studies by Kucey (1983), Banik and Dey (1982), and Guar et al. (1973); however, Taha et al. (1969) reported that aerobic sporeforming bacteria were the predominant P solubilizers in the Egyptian soils tested. In spite of the high numbers of PS organisms in some soils (Table I),

212

R. M. N. KUCEY ET AL.

inoculation of some soils with PS organisms has been shown to result in increases in the rhizospheric population of P solubilizers. Khalafallah et al. (1982) observed an increase in the incidence of PS bacteria following inoculation of fava beans (Vicia faba) with a PS isolate of B. megatherium, and although the population increase slowly declined, the effect remained for the duration of their experiment ( 1 16 days). Saber et al. (1977) found similar results for B . megatherium inoculated onto pea plants (Pisum sativum L). Similar observations were made after addition of a PS isolate of a Pseudomonas spp. to the rhizosphere of red pine (Pinus resinosa) seedlings (Ralston and McBride, 1976), and for Bacillus polymyxa and Pseudomonas striata in the rhizosphere of inoculated wheat (Kundu and Gaur, 1980). Kucey (1988) found that inoculation of soil with Penicillium biluji resulted in a nearly four-fold increase in the number of PS Penicillium spp. in the rhizospheres of wheat plants under greenhouse conditions. Inoculation of partially sterilized soils has produced greater responses than inoculation of unsterilized soils, presumably because the native P-solubilizing organisms were greatly reduced by the sterilization treatment (Taha et al., 1969; Kundu and Gaur, 1980). Not all studies have shown increased numbers of PS organisms following inoculation. Ocampo et al. (1978) observed bacteriostatic action after 5 weeks following inoculation of lavender (Lavendulu spica var. Vera) with PS isolates of Pseudomonas and Agrobacterium. This resulted in cessation of increases in the numbers of rhizospheric PS bacteria. In contrast to the results of Saber et al. (1977), Badr El-Din et al. (1986) did not observe any increase in the incidence of P-solubilizing organisms in the rhizosphere of rice following inoculation of field soils with B. megatherium.

C. EFFECTOF INOCULATION IN SOILS

Because of the presence of PS organisms in the vicinity of plant roots, it could be argued that the addition of a few extra P solubilizers would not be of benefit to plant growth (Brown, 1974; Tinker, 1984). Solubilization of P in soil under greenhouse or field conditions is also much more difficult to prove than solubilization of P in solution culture. Nonetheless, several studies have shown plant growth responses to the addition of PS microorganisms to soils (Tables 11,111, IV). Several groups rely primarily on the use of P-solubilizing bacteria (Taha et al., 1969; Azcon et al., 1976; Ralston and McBride, 1976; Saber et al., 1977; Kundu and Gaur, 1980; Raj et a/., 1981; Khalafallaha et a/., 1982; Azcon-Aguilar et al., 1986; Badr El-Din et al., 1986; Piccini and Azcon, 1987) (Table 11). Other researchers (Gaur ef al., 1980; Banik and Dey, 1981a-c, 1982) used a combination of P-solubilizing bacteria and fungi (Table 111). Still other studies have relied primarily on the use of PS fungi (Kucey, 1987, 1988; Asea et

213


Table 11 Summary of Effects of Inoculation of Soils with Phosphate-SolubilizingBacteria _____

~~

Conditions

Crop

Greenhouse Greenhouse Greenhouse

Greenhouse


Field Field

Responses and observations

No effect on available P in soil Pseudomonas spp. increased plant weights and P uptake in soils with added Ca phospate B . circulans No effect Tomato Increased VAM colonization Unidentified Soybean Millet increased plant weight and P B . circulans uptake, increase in P available in soil Increased available P in soil, Brrcillrts spp. Rice increased plant P uptake Peas Increased plant weight and P B. inegatlierium uptake var. phosphuticrrm Fava bean B . inegatheriirm Increased plant weight and P uptake and concentration var. phospliuricctm from superphosphatefertilized soil Lavender Pseitdomonas spp. increased plant weight and P uptake and concentration Agrohacterium sp. from soils amended with rock phosphate Increased plant weight and P B. megatherium Barley uptake and concentration var. phosphaticirm Increased plant weights and P B . polymyxa Wheat uptake and concentration P. striuta Increased plant weight and P Unidentified Alfalfa uptake from rock phosphate amended with sandvermiculite No effect B. megatherium Soybean var. pliosphuticum Increased plant weights from B. jirniirs Rice soils with added rock phosphate

Laboratory None Greenhouse Red pine


Organism Bacillus spp.

Reference" I

2 3 4 5

6 7 8

9

10

II

12

13 14

"(I) Banik and Dey (1982); (2) Ralston and McBride (1976); (3) Lee and Bagyaraj (1986); (4) Azcon-Aguilar et al. (1986); ( 5 ) Raj et ul. (1981); (6) Banik and Dey (1982); (7) Saber et ul. (1977); (8) Khalafallah et a / . (1982); (9) A x o n et ul. (1976); (10) Taha et ul. (1969); ( I I ) Kundu and Gaur (1980); (12) Piccini and Azcon (1987); (13) Badr El-Din et 01. (1986); (14) Datta et a/. (1982).

al., 1988) (Table IV). Phosphate-solubilizing actinomycetes have been isolated (Rao et al., 1982), but inoculation studies with this group of organisms have not been reported. Comparison of results from different tests is difficult because of the variability in the experimental designs, soils, and PS organisms used.

214

R. M. N. KUCEY ET A L . Table 111 Summary of Plant Responses to Inoculation of Soils with Mixed Cultures of Phosphate-SolubilizingBacteria and Fungi

Conditions

Crop

Organisms


Flask

None

Greenhouse

Rice

Greenhouse

Rice

Field

Wheat

Aspergillus spp. Penicillium spp. Bacillus spp. Aspergillus spp. Penicillium spp. Bacillus spp. Pseudo. srriara Asp. awamori Pseudo. srriata Asp. awamori

Reference"

No effect on available P content of soil

I

Increased plant weight and P uptake and concentration increased available P in soil Increased plant weight and P uptake and concentration Increased plant weights in soils with added rock phosphate and N

2

3 4

"(I) Banik and Dey (1982); (2) Banik and Dey (1981~);(3) Kundu and Gaur (1984); (4) Gaur er al. (1980).

Generally, however, they can be grouped into those studies that measured the effects of inoculation on soil-available P levels, those that measured plant responses under greenhouse conditions, and those that studied plant responses under field conditions. The availability of P in soils inoculated with PS organisms has been Table IV Summary of Plant Responses to Inoculation of Soils with Cultures of Phosphate-SolubilizingFungi

Conditions

Crop

Organism

Flask

None

Greenhouse Greenhouse

Wheat + beans Wheat

Penicillium spp. Aspergillus spp. Penicillium bilaji P . bilaji

Greenhouse

Wheat

P . bilaji

Field

Wheat

P . bilaji

Field

Wheat

P. bilaji


Reference"

Increased availability of P in soil

1

Increased plant weight and P uptake

2

Increased plant weight and P uptake in soils with added rock phosphate Increased plant weights and P, Cu, and Zn uptake, increased P availability in soil with added rock phosphate Increased plant weights, yields, and P uptake in soils with added rock phosphate Increased plant weights, yields, and P uptake in soils with added rock phosphate

3

~

~~

"(1) Banik and Dey (1981b); (2) Kucey (1987); (3) Asea er al. (1988); (4) Kucey (1988).

4

2 4


215

shown to be increased in several cases. Banik and Dey (1981b,c) measured increased levels of available P in soils to which had been added farmyard manure, rock phosphate, and PS isolates of Bacillus, Streptomyces, Penicillium, and Aspergillus spp. The levels of NaHC0,-extractable P were determined to be increased after addition of P . bilaji in soils both with and without added rock phosphate (Kucey, 1988). Banik and Dey (1982), however, did not observe increases in P availability in inoculated soils in response to the addition of PS organisms in conjuction with manure and rock phosphate. In this case, the inoculum used was different from that used in the studies reported in 1981 (Banik and Dey, 1981b,c). The fixation of 32P-labeledsuperphosphate and tricalcium phosphate in soils inoculated with B. circulans has been shown to be decreased relative to that in uninoculated control soils (Raj et a / . , 1981). The majority of plant growth tests on P solubilization in soil have been conducted under greenhouse conditions. In the greenhouse rooting volumes are usually restricted, the contribution of soil-borne P to plant nutrition is reduced, and consequently, if P is solubilized by microorganisms the plant response will be greater. Under these conditions, Ralston and McBride (1976), Kundu and Gaur (1980, 1984),and Khalafallah et al. (1982) all reported increased P uptake and plant growth in various crops inoculated with PS organisms. In these cases, the P uptake from rock phosphate by inoculated plants was equal to or greater than that from superphosphate. Asea et a / . (1988),using a '*P isotope dilution method, found that greenhouse-grown wheat inoculated with P . bilaji was able to obtain 18% of its P from sources unavailable to uninoculated plants and was also able to solubilize added rock phosphate. Kucey (1988) observed that wheat dry matter production and P uptake increased under field and greenhouse conditions in response to P . bilaji inoculation in the absence of added rock phosphate and that addition of rock phosphate resulted in a further increase in dry matter production. Banik and Dey (19814 and Taha et al. (1969) both found increased plant growth and P uptake in response to the addition of PS bacteria. Several studies of PS microorganisms have included the effect of VA mycorrhizal fungi. Since the mycorrhizal fungi play an important role in the ability of the host plant to absorb P, this is a logical inclusion in experiments on P solubilization. The individual effects of mycorrhizal fungi and P-solubilizing Pseudomonas and Agrobacterium spp. have been found to be additive, such that lavender plants with both inocula received greater benefit from rock phosphate addition than plants receiving only one of the organisms (Azcon et al., 1976). Similar findings were reported for finger millet (Eleusine coracana) inoculated with rnycorrhizal fungi and PS Bacillus circulans (Raj et al., 1981). Kucey (1987) and Asea et al. (1988) also observed that maximum plant growth and P uptake in sterilized soils were obtained in treatments in which plants received both mycorrhizal

216

R. M.

N. KUCEY E T A L .

fungi and P. bifaji. Piccini and Azcon (1987) observed similar results for alfalfa grown in a sand-vermiculite mixture and inoculated with VA mycorrhizal fungi and a PS bacterium. Again, however, not all studies show positive responses to inoculation. Azcon-Aguilar et a f .(1986) and Lee and Bagyaraj (1986) found that although mycorrhizal fungi increased plant growth and P uptake, further addition of PS bacteria had no effect. Under field conditions, Kucey (1987, 1988) observed an increase in dry matter production and P uptake by wheat from 10 to 27% and 15 to 34%, respectively, as a result of the addition of P. bifaji to Chernozemic soils with low P availability and observed a further increase in growth (up to 47% greater than control) and P uptake (up to 55% greater than control) when rock phosphate plus fungus was added. Gaur et al. (1980) also observed increased wheat growth and P uptake in response to the addition of rock phosphate and a PS culture of Pseudomonas striata and Aspergiffus awamori. Badr El-Din et al. (1986), however, did not observe any positive responses of soybeans to the addition of B . megatherium to a field soil. D. MECHANISM OF ACTION OF P s MICROORGANISMS Phosphate-solubilizing organisms have been reported to solubilize inorganic forms of P by excreting organic acids that directly dissolve phosphatic materials and/or chelate cationic partners of the P ion (Sperber, 1958b; Katznelson and Bose, 1959). Analysis of culture filtrates of pure isolates of these microorganisms has revealed a number of organic acid products including lactic, glycolic, citric, 2-ketogluconic, malic, oxalic, malonic, tartaric, and succinic acids, all of which have chelating properties and could serve as active components of P solubilization (Sperber, 1958b; Louw and Webley, 1959; Duff et a f . , 1963; Taha et al., 1969; Banik and Dey, 1981a, 1982). Table V shows the predominant organic acids found in culture filtrates of various PS microorganisms tested. The quantities of acids produced by these organisms are in some cases equal to more than 5% of the carbohydrate consumed by the organism (Banik and Dey, 1982). It has been reported that conversion of root exudates into 2-ketogluconate by rhizosphere organisms may account for up to 20% of root exudates released into the rhizosphere (Moghimi et a f . , 1978b). Organic acids have several effects in the media. As acids, they have the effect of decreasing medium pH, e.g., to pH 3.8 for Aspergiffusawamori (Khan and Bhatnagar, 1977) and pH 2.7 for Aspergilfus carbonum (Gaur et a f . , 1973). Asea et al. (1988) found that P . bilaji was able to release more P from Idaho rock phosphate than that released by 0.1 N HCI added to achieve equivalent media pH levels. A lack of correlation between the ability to


217

Table V Principal Organic Acids Produced by Phosphate-Solubilizing Microorganisms

Organism Arthrobacter sp. Bacillus sp. Bacillus firmus B-7650 Bacillus firmus B-7651 Gram -short rod Pseudomonas sp. B. megatherium B. megatherium var. phosphaticum B. subtilis Bacterium S470 Escherichia freundii Micrococcus sp. Micrococcus sp. Streptomyces sp. Streptomyces sp. Nocardia sp. Aspergillus fumigatus Aspergillus candidus Aspergillus niger Penicillium sp. Soil yeasts

Predominant acids produced

Reference

Oxalic, malonic Oxalic, succinic 2-Ketogluconic, succinic Oxalic, succinic Lactic Citric, gluconic Lactic, malic

Banik and Dey (1982) Banik and Dey (1982) Banik and Dey (1982) Banik and Dey (1982) Taha et al. (1969) Taha et al. (1969) Taha er al. (1%9)

Lactic Lactic, citric Lactic, glycolic Lactic Oxalic Lactic, succinic Lactic 2-Ketogluconic Succinic, glycolic Oxalic, tartaric, citric Oxalic, tartaric Citric, glycolic, succinic, gluconic, oxalic Lactic, glycolic Lactic

Taha et a / . (1969) Taha et a / . (1969) Sperber (l958b) Sperber (1958b) Banik and Dey (1982) Taha et a / . (1969) Banik and Dey (1982) Banik and Dey (1982) Sperber (1958b) Banik and Dey (1982) Banik and Dey (1982) Sperber (l958b) Sperber (3958b) Taha et a / . (1969)

reduce media pH and the ability of the isolates to solubilize P has been observed by Chhonkar and Subba-Rao (1967),Gaur et a f .(1973),and Surange (1985). It thus appears that the PS ability of microorganisms is manifested via mechanisms other than strict acidification of the surrounding environment. Kucey (1988) showed that addition of 0.05 M EDTA to solutions containing insoluble copper and zinc compounds had the same solubilizing effect as inoculation with P. bifaji. Reduction of the solution pH to 4.0 by the addition of 0.1 N HCI did not result in metal ion solubilization. Sperber (1958b) was able to duplicate the effects of inoculation with PS microorganisms by the addition of lactic, glycolic, or citric acids to solutions containing apatite. Irrigation of greenhouse soils with 0.001 M EDTA has been shown to increase the uptake by Italian rye grass (Lolium muftiforum L.) of P, aluminum, and iron (Hartikainen, 1981). Duffer a f . (1963) observed that 2-ketogluconic acid produced by several PS bacteria and fungi effected the release in solution of numerous phosphate and silicate

218

R. M.N. KUCEY E T A L .

materials. Moghimi and Tate (1978) concluded that the main action of 2ketogluconic acid was to act as a source of hydrogen ions in the dissolution of calcium phosphates; however, Berrow et al. (1982) concluded that 2ketogluconate obtained from a batch culture of an Erwiniurn species was able to extract more cobalt, nickel, zinc, iron, titanium, and vanadium than ammonium acetate and was equal to EDTA and DTPA in extracting copper, manganese, molybdenum, nickel, and zinc. Certainly, although 2-ketogluconate may be a major component of rhizosphere products, other acids are produced as well. Thus it appears that a mechanism relying on the production and use of organic acids can be used as a PS system by microorganisms. Doubt regarding the role of PS microorganisms in liberating P under soil conditions has been expressed (Brown, 1974; Tinker, 1980, 1984). These doubts were founded upon theoretical limitations and lack of direct evidence of P solubilization. Indeed, many of the earlier experiments were conducted in nonbuffered conditions such as in solution culture or in sand. Many of these experiments included PS organisms as part of a combined treatment in which organic materials and/or phosphatic materials were added, so the effect of PS organisms, if present, could not be separated from the effects of the other amendments. Theoretical arguments have considered the processes necessary for P solubilization to result in increased plant P uptake (Rovira and Davey , 1974; Hayman, 1975; Tinker and Sanders, 1975). The organisms first would have to cause the release of P from the unavailable sources, then the P would have to be available for plant uptake. The first process has been shown to occur in unbuffered systems many times. Nye and Tinker (1977), however, have calculated the quantity of organic acid that would be necessary to solubilize inorganic P in bulk soil and concluded that microorganisms could not produce the quantities necessary and, further, that the plant could produce the necessary pH drop by releasing acids as exudates without the presence of the microorganisms (Hedley et al., 1982; Bekele et al., 1983). However, it is likely that the nature of the acids released is more important to the amount of P released than is the quantity of acid produced (Sperber, 1958a; Louw and Webley, 1959; Chhonkar and SubbaRao, 1967). In addition, although the plants may be able to release large quantities of exudate, it is unlikely that the exuded materials would remain in the rhizosphere untouched for long enough to affect P release (Hale et al., 1971; Moghimi et al., 1978a). It is possible that PS organisms can produce effective chelating materials in a microenvironment such as in the immediate vicinity of rock phosphate or phosphatic fertilizer materials, or in the rhizosphere (Moghimi et al., 1978a; Tinker, 1980). Under these conditions, P could be solubilized and be present in an available form in high enough concentrations to be avail-


219

able to plant roots. As for plant uptake of the material, if chelating materials are being released it is unlikely that all the P solubilized would be absorbed by the organisms, so most of it would remain in solution. It has already been shown that mycorrhizal fungi aid in plant uptake of P and that, in many cases, the addition of mycorrhizal fungi along with effective PS organisms increases the effect beyond that observed from the addition of either organism alone (Azcon et al., 1976; Kucey, 1987; Asea e f al., 1988). It thus seems plausible that mycorrhizal plant roots could efficiently absorb solubilized P over a period of time and that the increase could be equivalent to that observed from the addition of phosphatic fertilizers. A suggestion has been made that the action of PS organisms is not due to the release of plant-available P but rather to the production of plant growth substances (Brown, 1974; Tinker, 1980). These substances could cause plant growth stimulation, which would result in a larger plant that naturally would contain larger amounts of P and other materials than a smaller plant. Indeed, a number of Soviet scientists supported this idea with respect to the action of phosphobacterins (Dorosinski, 1%2; Mishustin and Naumova, 1%2; Samtsevich, 1%2; Mishustin, 1963; Voznyakovskaya, 1963), and other authors have stated that the responses they received to the addition of PS bacteria might be due to the production of biologically active substances (Barea e f al., 1975; Azcon e f al., 1976; Kucey, 1988). Barea et al. (1976) measured levels of indoleacetic acid, gibberellins, and cytokinins in culture filtrates of PS bacteria. Datta et al. (1982) were able to obtain positive growth responses of field-grown rice to the addition of a Bacillus firmus isolate that showed both P-solubilizing ability and indoleacetic acid-producing ability. In this case, the response was greater in the presence of Mussoorie rock phosphate, indicating that the PS activity shown in pure culture was also shown under field conditions. Jarrel and Beverly (1981) state that definitive evidence that a factor is increasing the availability of a nutrient for plant uptake is if the amount of nutrient in the plant increases and if the concentration of nutrient within the plant tissues increases as well. As previously shown in several studies, the concentration of P within plants inoculated with PS organisms has been found to be increased relative to uninoculated controls (Taha et al., 1969; Azcon et al., 1976; Kundu and Gaur, 1980, 1984; Banik and Dey, 1981~;Khalafallah et al., 1982). In other cases, in which increased plant growth and P uptake were observed but the concentration of P within plant tissues remained the same, the soils used were specifically chosen for their poor P availability indices, so that increases in plant-available P resulted in increases in plant dry matter production similar to increases observed in response to the addition of P fertilizers (Kucey, 1987, 1988; Asea et al., 1988). As further evidence that one of the mechanisms of PS organisms can

220


be the release of plant-available P, Asea et al. (1988) reported that addition of P . bilaji to 32P-labeledsoils resulted in dilution of 32P-labeledphosphate in the plant with unlabeled P, which could only have come from added rock phosphate or from unlabeled sources of soil P unavailabe to the uninoculated plants. These experiments were similar to those used to determine that VA mycorrhizal fungi did not solubilize unavailable forms of P (Mosse et af., 1973; Gianinazzi-Pearson et al., 1981). Asea et af. (1988) also observed that the VA mycorrhizae in their test did not cause 32 P isotope dilution. It is possible that PS microorganisms may produce biologically active substances and that these substances may play a role in the plant responses to the addition of these organisms. Barea et al. (1976) state that whereas plant growth stimulators may affect plant growth, P solubilizers would have at least a secondary role in making extra P available for the increased plant demands, and that inoculants that would produce both plant growth stimulants and be able to provide the extra P necessary would be most beneficial for crop production. Indeed, even mycorrhizal fungi, which have been proven to increase plant P uptake, have been shown to produce biologically active substances that alter plant growth (Barea, 1987). A plant growth-promoting isolate of Pseudomonas putida inoculated onto subterranean clover was shown to increase plant uptake of iron, copper, aluminum, zinc, cobalt, and nickel (Meyer and Linderman, 1986), and it was suggested that this organism was able to solubilize sufficient nutrient to meet the increased plant demand caused by the other growthpromoting factors produced by the bacterium. Kucey (1988) also suggested that P . bifaji may have other effects on plant growth and that the fungus was able to solubilize copper and zinc to meet the increased demands. The evidence points out that release of P in a plant-available form can also be one of the results from inoculation of soils with these organisms. Because of the diversity of organisms and soils used in the studies reported here, it is difficult to generalize about PS organisms. However, it is certain that some of the organisms that have been tested do show PS activity in soils as well as in unbuffered systems.

VI. SULFUR OXIDATION AND ROCK PHOSPHATE-SULFUR MIXTURES Lipman and his associates were among the first to mix ground rock phosphate with sulfur in an attempt to increase the availability of the P contained in the rock (Lipman et a f . , 1916a,b; Lipman and Mclean, 1918). The theory was proposed that the sulfur in the mixtures would be oxidized


22 1

to sulfuric acid by soil microorganisms (Wainright, 1984) and that the sulfuric acid would dissolve the rock phosphate in situ (Swaby, 1975). Wainright (1984) and others have published excellent reports on the many factors that affect the rates of S oxidation in soils. Addition of sulfur to rock phosphates has been tested under soil conditions. Kittams and Attoe (1965) found that application of phosphatesulfur mixtures increased the yields of ryegrass more than did the addition of superphosphate or rock phosphate alone in one of three soils tested but did not affect P uptake in the other two soils. Nimgade (1968) also observed greater P uptake from rock phosphate-sulfur mixtures than from rock phosphate alone, and Neller (1956) observed increased P availability of soils if sulfur was added along with rock phosphate. Bromfield (1975) found that a rock phosphate-sulfur mixture added to peanuts resulted in greater yields and P uptake than those obtained from ground rock phosphate alone and that the mixture was equal to superphosphate. The availability of P from rock phosphate-sulfur mixtures appears to be greatly affected by many factors, including soil temperature and moisture and the granule size of the final product (Terman et al., 1964, 1969; Kittams and Attoe, 1965; Attoe and Olson, 1966; Li and Caldwell, 1966; Nor and Tabatabai, 1977). Although several groups of soil bacteria are able to oxidize sulfur, the most important are the chemautotrophic bacteria Thiobacilfus thiooxidans and Thiobucillus thioparus (Starkey, 1966). For this reason, Swaby (1975) suggested that rock phosphate-sulfur mixtures could be inoculated with these bacteria to provide increased P availability of the mixtures. Rock phosphate availability was found to be highest at a soil pH of 6.0 or lower (Ellis er a f . , 1955), and the action of these bacteria is able to produce these acidic conditions in microsites. The inoculated mixtures (1S/5RP) used by Swaby (1975) were called biosupers and found to be superior to uninoculated mixtures for pasture production in tropical soils. Other studies in tropical conditions showed biosupers to be effective in tropical pasture conditions (Fisher and Norman, 1970; Gillman, 1973; Jones and Field, 1976; Partridge, 1980; Rajan and Edge, 1980; Rajan, 1981, 1982). Whitehouse and Strong (1977), however, found biosupers to be unsatisfactory as a wheat fertilizer in some Australian soils, and Lee and Bagyaraj (1986) found that the addition of thiobacilli to the soil in their greenhouse study resulted in a decrease in plant growth. They also suggested that the organisms naturally present in their soil were effective sulfur oxidizers and that inoculation with thiobacilli did not increase sulfur oxidation rates. Rajan (1982) also determined that inoculation with thiobacilli did not increase the effectiveness of Chatham Rise or Christmas Island biosupers. ,

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The data on the use of biosupers are somewhat contradictory. As with many systems requiring the use of living organisms, the success of the system probably depends on the specific environmental conditions. In the case of biosupers, the activity of both added thiobacilli and indigenous S-oxidizing organisms would be affected by environmental stresses. In addition, since these systems require two steps, S oxidation followed by P dissolution, the second chemical reaction step will also be affected by the soil environment. The greatest effects of biosupers were observed under tropical conditions, and it may be that the use of biosupers will be restricted to these areas. There is a need, however, to determine accurately the conditions that are essential for the rock phosphate-sulfur system to work.

VII. FUTURE OF TECHNOLOGIES The future of the biological technologies outlined in this review will depend greatly on the state of agricultural economies. Certainly, the development and use of phosphobacterins in the USSR was primarily initiated by the need of that country for increased production and the lack of sufficient phosphate fertilizer production plants. This situation is widespread in the so-called underdeveloped countries of the world today. Many of these countries have undeveloped reserves of rock phosphate that could be used if more economical means of exploiting them were available. At present, however, building phosphate fertilizer plants is too great an expense for the value of most of the deposits. North American agriculture could also benefit from some of these systems, since production costs of phosphate fertilizers would be reduced, which would, hopefully, also reduce the farmer’s costs for crop production. The use of VA mycorrhizal fungi is greatly limited by the inability of scientists to grow the fungi in pure culture to produce large amounts of inoculum (Gianinazzi and Gianinazzi-Pearson, 1986). This problem has been overcome in the use of ectomycorrhizal fungi, and at present, these fungi are commonly inoculated onto container-grown coniferous seedlings to increase growth and survival of the trees. Vesicular-arbuscular mycorrhizal fungi must be grown in a labor-intensive system of pot cultures in which the fungus is propagated in the presence of a host plant. Nonetheless, container-grown seedlings of crops such as oranges are inoculated because the mycorrhizal seedlings survive better than uninoculated ones. If a system of mass culture of VA mycorrhizal fungi were developed and specialized strains of VA mycorrhizal fungi were isolated, then the inoculation of certain soils could be of economic benefit. However, since


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the majority of soils contain native VA mycorrhizae, widespread inoculation of agricultural soils would most likely not be of economic benefit. If VA mycorrhizae could be cultured on synthetic media and strain selection could be performed, then introduction of specialized VA strains might prove useful for responsive crops (Gianinazzi and Gianinazzi-Pearson, 1986). The rock phosphate-sulfur mixtures, i.e., biosupers, appear to be useful in situations in which low amounts of P are necessary over a long period of time, such as in pastures. Their use in temperate areas would probably be limited by the S oxidation rate, which is much less than in tropical areas. Temperate annual crops require the release of P early in the growing season. Nonetheless, biosupers, if proved successful under field conditions, could provide a useful economical fertilizer for tropical pasture production. The use of phosphobacterins in North America was essentially terminated following the reports in the 1960s that tests conducted in the United States did not give positive results. If the bacteria were degrading soil organic matter to mineralize P, then it could be argued that the organic matter serves a much more valuable role as a soil structural component. If this is the case, then the use of these degradative bacteria should be discouraged. The study of B. megatherium var. phosphaticum in phosphobacterins led to their use in systems aimed at solubilization of inorganic P forms. The potential for using P-solubilizing bacteria and fungi for in situ processing of rock phosphates in soil has led to a resurgence of interest in biologically released P that has resulted in the submission of a number of patents dealing with these systems. It remains to be seen whether these biologically activated systems are successful under practical field situations. If so, and if the economics of agriculture in North America and elsewhere show that the production of inocula and addition of unprocessed rock phosphate with PS inocula is more economical than processing of rock phosphate into phosphate fertilizers, then we may see new fertilizer products on the market. In developing countries that have rock phosphate reserves, such fertilizers may provide a system by which these reserves can be developed without the expense of building phosphate fertilizer production plants.

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ENZYMOLOGY OF THE RECULTIVATION OF TECHNOGENIC SOILS S. Kiss, M. Dragan-Bularda, and D. Pasca Department of Plant Physiology Babes-Bolyai University 3400 Cluj-Napoca, Romania

I. 11.

Ill. IV. V. VI . VII.

VIII.

IX. X.

XI. XII.

XIII.

XIV.

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Introduction Technogenic Soils from Coal Mine Spoils A. Enzymological Research in the USSR B. Enzymological Research in the United States C. Enzymological Research in Poland D. Enzymological Research in Hungary E. Enzymological Research in the Federal Republic of Germany Technogenic Soils from Power Plant Wastes Enzymological Research in Poland Technogenic Soils from Retorted Oil Shale Enzymological Research in the United States Technogenic Soils from Iron Mine Spoils A. Enzymological Research in the USSR B. Enzymological Research in Romania Technogenic Soils from Manganese Mine Spoils Enzymological Research in the USSR Technogenic Soils from Lead and Zinc Mine Wastes A. Enzymological Research in the United Kingdom B. Enzymological Research in Romania Technogenic Soils from Sulfur Mine Spoils Enzymological Research in the USSR Technogenic Soils from Lime and Dolomite Mine Spoils Enzymological Research in the USSR Technogenic Soils from Refractory Clay Mine Spoils Enzymological Research in the USSR Technogenic Soils from Bentonitic Clay Mine Spoils Enzymological Research in the USSR Technogenic Soils on Sand Opencast Mine Floor Drift and Spoils A. Enzymological Research in Poland B. Enzymological Research in the USSR Technogenic Soils from Overburdens Remaining after Pipeline Construction A. Enzymological Research in the USSR B. Enzymological Research in the United States Recultivation of Soils Remaining after Topsoil "Mining" Enzymological Research in New Zealand Concluding Remarks References

229 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

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I. INTRODUCTION Technogenic soils are soils that form during the technical and biological recultivation of overburdens, tailings, and other spoils and wastes resulting from strip (opencast, surface) and shaft (underground, deep) mining and other industrial activities. The evolution of technogenic soils is, by definition, the process of transforming all these wastes into agricultural and forest soils or into soils used for other purposes (parks, sports fields, etc.). The practical importance of this process is growing because the development of mining and other industries leads to increasing amounts of wastes and, therefore, the recultivation of wastelands becomes more and more a major economic necessity. It is estimated that up to 1980 about 1600 x lo9 m3 of mine spoils accumulated on the earth’s surface and this amount has increased yearly by about 40 x lo9 m3. Water erosion affects a smaller amount of soil (about 13 x lo9 m3 per year). The evolution of technogenic soils presents questions of theoretical importance, too, which are related to a better understanding of the evolution of landscape as a whole (Greszta, 1973; Nastea et al., 1973; Khazanov, 1975; Szegi, 1983). The evolution of technogenic soils, which affects the efficiency of recultivation, is studied using many physical, chemical, and biological methods. Enzymological methods have also been applied, and it has been found that the level of enzymatic activity is a good indicator of the degree of evolution of technogenic soils. In the present review, data from the literature concerning the enzymology of technogenic soils will be grouped according to the nature of the raw material from the mining and processing of which the wastes resulted (coal; oil shale; iron, manganese, lead, and zinc ores; sulfur; lime and dolomite; refractory clay; bentonitic clay; and sand). Finally, the enzymology of the recultivation of overburdens remaining after pipeline construction and that of the recultivation of soils remaining after “mining” (removal) of their topsoil for use in landscape improvement will be dealt with.

II. TECHNOGENIC SOILS FROM COAL MINE SPOILS A. ENZYMOLOGICAL RESEARCH IN

THE

USSR

Keleberda (1973) determined the H,O,-splitting capacity in the Aleksandrii spoil heap at the Baidakov brown coal strip mine (located in the northern steppe zone of the Ukraine, Kirovograd region) and in the spoil

THE RECULTIVATION OF TECHNOGENIC SOILS

23 1

heaps of the abandoned Yurkov brown coal strip mine (located in the forest-steppe zone of the Ukraine, Cherkassy region). The spoils consist predominantly of loess. At both mines, a part of the spoils were recultivated with Scotch pine (Pinus sylvestris) for 8 and 3 years, respectively. In some spoil plots the plants grew well, whereas in others their growth was inhibited. The H,O,-splitting capacity, and especially its heat-labile component (catalase activity), was always higher in the plots with welldeveloped plants than in those on which the growth of plants was inhibited. Keleberda has drawn the conclusion that the forest-growing properties of recultivated spoils can be evaluated by catalase activity measurements. Verbin and Keleberda (1974) compared urease and catalase activities in the Y urkov spoil plots recultivated only with black alder (Alnus glutinosa), with black alder plus Scotch pine, or only with Scotch pine. In pure stands, the growth of black alder was good and that of Scotch pine bad. In mixed stands, the black alder stimulated the growth of Scotch pine. After 3 years of recultivation, enzyme activities, and nitrogen (N) content (total, NH4+.NO,-) in the 0-20- and 2040-cm layers of the spoil plots showed the following order: black alder plots > black alder plus Scotch pine plots > Scotch pine plots. The values were highest in the rhizosphere and especially in the zone of root nodules of black alder. Keleberda et al. (1974) and Keleberda and Dan’ko (1975) reported on a new enzymological study of the recultivated spoils at the Yurkov mine. Spoils (sandy loams or loamy sands) of the Strizhev coal strip mine (located in the central forest zone of the Ukraine, Zhitomir region) were also studied. Invertase, urease, and catalase activities were determined in soils of spoil plots cultivated with a green manure crop, perennial lupine (at both mines), or with black alder (at Yurkov). Uncultivated spoil plots served as controls. It was found that the enzyme activities decreased with the sampling depth in each plot. Recultivation led to evident, sometimes manifold, increases of enzyme activities as compared to the uncultivated controls. The effect of perennial lupine was stronger than that of the black alder. Owing to the lupine, the humus and total N contents also increased, 2.54-2.87 and 2.94-3.30 times, respectively. In the alder plots, the increase of humus and N contents of soils was 1.95- and 1.81-fold, respectively. The results indicate that recultivation with black alder or, even better, with the green manure plant perennial lupine makes it possible to increase, in a relatively short period of time, the fertility level of technogenic soils. In another study, Keleberda (1976) found that the soils of the Yurkov spoil plots cultivated with Scotch pine for 9 years or with black locust (Robiniu pseudoacacia) for 4 years also manifested higher invertase activity than did the uncultivated spoil. The activity increase in the Scotch pine soil was close to that in the lupine soil, but in the locust soil the activity increase was lower as compared with that in the lupine soil. The

232

S. KISS ET AL.

humus and total N contents increased in the Scotch pine and locust soils, also. It was also found (Keleberda and Dan’ko, 1975; Keleberda, 1976) that jack pine (Pinus banksiana), Scotch pine, weeping birch (Betula verrucosa), and pedunculated oak (Quercus pedunculata) grew much better on the green (perennial lupine)-manured than on the nonmanured Strizhev spoil plots. Keleberda (1977) has pointed out that in addition to invertase, urease, and catalase activities proteinase and amylase activities also reflect the evolution of spoil heaps toward technogenic soils. The Yurkov spoil heaps recultivated with different tree and shrub species became primitive soils in a relatively short time (9-14 years), acquiring a humus layer and an increased N content and enzymatic potential (Keleberda, 1978) (Table I). The spoils, consisting predominantly of loess, at the Baidakov brown coal strip mine located in the northern steppe zone of the Ukraine, were also studied multilaterally by Mikhnovskaya (1981) and Eterevskaya et al. (1985). They found that invertase, urease, and proteinase activities and respiration (CO, evolution) were very low in the loess forming the walls of the quarry and in the loess heap not covered by vegetation and increased very much with the age of the indigenous vegetation covering the loess heaps, but even 23 years after the growth of indigenous vegetation the activity values were lower than those measured in the undisturbed zonal soil (Table 11). The spoil heaps (medium loams; pH -7) at the brown coal strip mines located in the forest-steppe zone in the Nazarovo Basin (which belongs to the Kansk-Achinsk Fuel-Energetic Complex, Siberia) were studied enzymologically by two research groups. Naprasnikova et al. (1982) and Naprasnikova (1983, 1985a,b, 1987) determined enzyme activities in spoil heaps covered with 3-, 5-, and 15-yearold indigenous plant communities, in spoil heaps recultivated with pines, larches, willows, or sweetclover for a maximum of 15 years, and in zonal (gray forest) soils that had not been affected by the strip mining. Proteinase activity was always lower in the recultivated spoils and much lower in the spontaneously revegetated spoils than in the zonal soils. Peroxidase activity remained low in both recultivated and spontaneously revegetated spoils as compared with that in the zonal soils. Invertase and acid, neutral, and alkaline phosphatase activities in the recultivated spoils and in spoils under 15-year-old indigenous vegetation, however, approached or even exceeded those found in the zonal soils. Polyphenol oxidase and the other enzymes were most active in the rhizosphere of the dominant plant species. The results obtained underline the advantages of recultivation over uncontrolled revegetation of the coal strip mine spoils. Based on the finding that the enzyme activities increased twice in non-

233

T H E RECULTIVATION O F TECHNOGENIC SOILS Table I

Humus and N Contents and Enzyme Activities in Primitive Soils under Forest Vegetation Developed on Spoil Plots at the Yurkov Coal Strip Mine"

Plant stand and its age Scotch pine (14 years)

Black locust (9 years)

Sea buckthorn (9 years)

Black alder (9 years)

Control (uncultivated)

Depth (cm)

Humus

N

(%)

(76)

0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30

4.74 2.33 1.20 0.41 0.36 2.08 1.02 0.73 0.16 0.11 1.65 0.75 0.43 0.26 0.18 1.30 1.03 0.84 0.17 0.19 0.72 0.43 0.24 0.23 0.21

0.24 0.12 0.07 0.02 0.02 0.18 0.09 0.07 0.01 0.01 0.19 0.06 0.04 0.02 0.02 0.13 0.12 0.08 0.03 0.03 0.05 0.02 0.01 0.01 0.01

N in humus (96) 6.0 5.1 5.8

5.0 5.5

8.6 9.0 9.6 6.2 9.9 11.5 8.0 9.5 8.0 11.1

10.0 11.6 9.5 17.6 15.8 6.9 4.6 4.1 4.3 5.0

lnvertaseh

Ureaseh

Proteinaseh

29.70 10.00 6.62 4.42 0.60 22.30 8.02 4.90 1.12 0.60 44.50 11.20 2.25 I .71 I .02 28.50 13.50 7.50 4.70 0.80 10.80 5.20 2.35 0.10 0

1.96 0.56 0.45 0.34 0.26

0.56 0.30 0.20 0.16 0 0.48 0.28 0.28 0.16 0 0.96 0.34 0.26 0. I5 0 0.76 0.36 0.26 0.12 0 0.34 0. I5 0.08 0 0

I.64

0.62 0.33 0.28 0.15 3.68 0.72 0.37 0.16 0.11 2.86 0.84 0.42 0.32 0.25 1.08 0.71 0.32 0.20 0

"From Keleberda (1978). "lnvertase activity is expressed as milligrams of inverted sugar, urease activity as milligrams of NH,' N, and proteinase activity as milligrams of NHZ N produced by I g of soil in 40, 40, and 72 hr, respectively.

topsoiled spoils during their agricultural recultivation for 3.5 years, Naprasnikova and Makarova ( 1986) recommend this recultivation method without covering the spoils with a fertile soil layer. Shugalei et ul. (1984, 1983, and Korsunova and Shugalei (1986) studied the enzyme activity and chemical composition of the topsoil (50-cm layer) stockpiled before surface mining of coal. The stockpiles are 1.5-4 m high and, after 3-5 years of storage, they are used to cover the leveled spoils for their recultivation. Catalase activity in the stored topsoil remained nearly at the same level as in the 0-50-cm layer of the adjacent undisturbed

234

S. KISS ET AL. Table I1

Enzyme Activities and Respiration in Spoils (Loess)at the Baidakov Coal Strip Mine”

Analyzed material

lnvertase”

Urease”

Proteinase”

Respiration”

Loess from the walls of quarry Loess (spoil) heap without vegetation Loess (spoil) heaps spontaneously vegetated. age of vegetation: I year 2 years 4 years 23 years Zonal soil (common chernozem)

0.3 0.3

1.1

I .4

2.3 2.4

4.4 4.6

I .2 2.2 4. I 7.0 20.5

I .5 2.0 2.3 4.5 15.6

2.7 3.8 5.1 9.1 28.7

5. I N.D.‘ 28.7 35.3 217.0

.

~~

“From Eterevskaya et ul. (1985). ”Invertase activity is expressed as milligrams of inverted sugars produced by I g of material, urease activity as milligrams of NH3 produced by 100 g of material, proteinase activity as milligrams of tyrosine produced by 100 g of material in 24 hr, and respiration as milligrams of CO, evolved from I kg of material/hr. ‘Not determined.

leached chernozem, but urease activity decreased and proteinase activity increased to some extent in the stored topsoil. The quantity and quality of humic substances in the stored topsoil and in the undisturbed soil were nearly the same. The topsoil, after its replacement on the leveled spoils and recultivation with perennial grasses, showed catalase and urease activities approaching the seasonal mean values recorded in the 0-50-cm layer of the undisturbed soil, whereas proteinase activity was three times higher in the replaced topsoil than in the undisturbed soil. In different raw spoil samples, Shugalei and Yashikhin (1985) registered high variation coefficients of catalase, urease, and proteinase activities, respiration (CO, evolution) rate, organic matter, total N , NH,’ N , and NO,- N contents (77, 90,66, 58, 69, 72,82, and 151%, respectively) which was attributed to nonhomogeneous mixing of the different overburden layers. Leveling of raw spoils led to diminution of the variation coefficients, except that of the urease activity which increased. Shugalei et al. (1985) determined enzyme activities also in the 1-1.5cm humus layer that had formed in 10 years on leveled spoils not covered with stored topsoil but cultivated with Scotch pine or Siberian larch. It was found that catalase, urease, and proteinase activities in the newly formed humus layer exceeded by 5-20 times the activities measured in the 0-20-cm layer of the undisturbed soil.


235

According to Naplekova et al. (1983) and Trofimov et af. (1986), the soil-forming processes in the technogenic landscapes created by the coal strip mining in the Kuznetsk Basin (Kuzbass, Siberia) take place very vigorously, owing to the spontaneous revegetation and forest recultivation of spoil heaps (slightly alkaline, calcareous sandy loams and clay shales). In 18-year-old spoil heaps in the mountain taiga zone and in 8-12-yearold spoil heaps in the steppe zone the number and composition of microflora approach those in the zonal soils (pseudopodzolic soil and leached chernozem, respectively). This causes the intensification of respiration (CO, evolution); catalase, dehydrogenase, amylase, and phosphatase activities; gelatin and cellulose decomposition; and amino acid accumulation in the technogenic soils and leads, finally, to an increase in their fertility. In the forest-steppe zone, Naplekova ef al. (1985) determined the polyphenol oxidase and peroxidase activities of the technogenic soils formed during the development of spontaneous indigenous plant communities on the spoil heaps. They established that the nature of the dominant plant had a decisive influence on these activities and on their ratio. The polyphenol oxidase-peroxidase ratio, expressed as a percentage, is considered an indicator of the intensity of humification processes (humification coefficient). High values of the humification coefficient were registered in technogenic soils under perennial legumes and grasses. Klevenskaya et al. (1986) emphasize that the associations between microorganisms and plants, owing to accumulation of enzymes, especially of polyphenol oxidase and peroxidase in the rhizosphere, speed up the elementary pedogenetic processes in the technogenic soils. Some enzymological aspects of the recultivation of coal shaft mine spoils situated in the valley of the Samara River (western Donets Basin, Donbass) have been dealt with by Gel’tser and co-workers (Gel’tser and Tsvetkova, 1982; Tsvetkova et af., 1982; Ras’kova et al., 1984; Gel’tser et al., 1986). The four experimental plots had the following structure (from top to bottom): I , mine spoils (silts and clay shales with sands and coal inclusions); 11, 0.5 m of loess plus 0.5 m of sand plus mine spoils; 111, 0.5 m of sand plus 1 m of loess plus mine spoils; IV, 0.5 m of chernozem plus 0.5 m of sand plus 0.5 m of loess plus mine spoils. The thickness of the mine spoil layer was 7-10 m. The plots had all been planted to black locust 5 years before. In plot I the growth of locust was unsatisfactory. In the other plots the locust grew well. For comparison, a zonal soil (common chernozem) under natural vegetation or a 15-year-old black locust plantation was used. Five enzyme activities were measured in the 0.5-m layer of the plots. In plot I the activities were present only in traces. The highest activities were found in the litter of plots I1 and IV. Proteinase, urease, and dehydrogenase activities were no lower in the 0-2 and 2-10-cm layers of these plots than in the humus-enriched horizon of the zonal soil. The opposite was true for the invertase and catalase activities. In the 10-50-cm

236

S. KISS ETAL.

layer of plot I1 (loess) the activities decreased sharply, whereas in the same layer of plot IV (chernozem) they remained, after a slight initial decrease, practically constant. The enzymological measurements made possible a more accurate differentiation of the layers and places of soilforming processes within the soil profile than did its morphological description. In another study, Gel’tser et al. (1985) have found that proteinase and invertase activities were nearly twice as high in the litter of the black locust plots on loess and chernozem overlying mine spoils as in the litter of the black locust plantation on zonal soil. B. ENZYMOLOGICAL RESEARCH IN

THE

UNITED STATES

Reviewing the role of microorganisms in the revegetation of strip-mined land in the western United States, Cundell (1977) emphasized the observations of Pancholy et al. (1979, according to which the low urease and dehydrogenase activities in a completely denuded area near an abandoned zinc smelter in Oklahoma were an excellent indicator of the inability of this soil to revegetate. Therefore, Cundell advocated enzyme activity determinations for studies of mine soil recovery. Miller ( 1978) described complex recultivation investigations in which enzymological methods were also applied. Four sites were studied. The first three were strip coal mine sites and the fourth, a coal refuse site, resulted from deep coal mining. The sites are located in different parts of the United States. At the Indian Head mine (Zap, North Dakota), proteolytic and dehydrogenase activities were determined in topsoil stockpiled prior to replacement on the spoil. In the 0-2.5-cm layer of a single topsoil storage pile at 10 and 22 months of age, the proteolytic activity was only -30 and 25%, respectively, in comparison with that measured in the same surface layer of the adjacent undisturbed soil. At 30-300-cm depths, enzymatic activity was also lower in the topsoil pile at 22 months of age than at 10 months of age. Dehydrogenase activity in the 0-2.5-cm layer was 20% greater in the 29-month-old topsoil pile examined than in the undisturbed soil, but in the 30-400-cm depths of the pile, dehydrogenase activity represented only -1-15% of that found in the 0-2.5-cm layer of the same pile. In other words, during storage the enzymatic potential diminishes to a large extent in most parts of the topsoil pile. At the Jim Bridger mine near Rock Springs, Wyoming, 3 plots were studies enzymologically. In the first plot the spoil was covered with stored topsoil. In the second plot a native soil was immediately reapplied to the spoil without prior storage. Adjacent to these plots was the third (control) plot, an undisturbed native area composed of an Atriplex community and


237

an Artemisia community. All plots had a west-facing aspect and a 30% slope. Plots 1 and 2 were established during the spring and fall of 1976, with the reapplied stored topsoil plot having extra growing season. They were seeded with wheatgrasses (Agropyron spp.) and fourwing saltbush (Atriplex canescens). No fertilizers were applied. On both plots, the dominant vegetation was Halogeton glomeratus followed by Salsola kali. Almost no wheatgrass survived, and no fourwing saltbush was evident. However, volunteer Atriplex gardneri and Atriplex confertifolia were encountered, with A . gardneri predominating. In July 1977, soils were sampled to determine their cellulase, urease, and dehydrogenase activities, which were found to decrease in the following order: undisturbed soil > stored topsoil applied on spoil > native topsoil applied on spoil. It should be mentioned that in a sample from the native topsoil plot, cellulase activity was not detectable at all. Proteolytic and dehydrogenase activities were assayed in the surface layer of orphaned (nonrecultivated) spoil heaps at the Big Horn mine and at the abandoned Hidden Water Creek mine near Sheridan, Wyoming. The spoils were composed of semiconsolidated shale and sandstone. Surface soil samples collected from undisturbed grazed rangeland sites near the Big Horn mine served for comparison. The mean value of the proteolytic activity was -7.5 times lower in the spoil than in the soil samples and dehydrogenase activity was lacking in the spoil samples. A coal refuse pile generated in the coal-cleaning process at a deep coal mine near the city of Staunton in Macoupin County, Illinois was submitted, many decades after the abandonment of the mine, to technical and biological recultivation. The enzymological analyses performed at the end of the growing season in the first year of recultivation (1977) indicated the presence of two activities (urease and dehydrogenase) and the absence of proteolytic and cellulase activities in the coal refuse. These four activities gave high values in the adjacent field soil. During the summers of 1975 and 1977, Hersman and Temple (1979) collected samples from six reclamation plots of the coal strip mine spoils in the Colstrip area (eastern Montana) and determined their ATP content, phosphatase and pectinolyase activities, and rate of respiration (0,uptake). ATP content correlated significantly with respiration rate and pectinolyase activity for the 1975 samples, and with respiration rate and phosphatase activity for the 1977 samples. Of the two enzymes tested, phosphatase gave more positive values for correlation. Nevertheless, its positive correlation with both ATP content and respiration rate in one set of samples, but not in the other, suggests that phosphatase activity is more variable than either respiration rate or ATP content. Pectinolyase is more specifically related to plant decomposition and may have some implications for spoils that are in the early stage of pedogenesis. Pectinolyase activity showed only one signiticant correlation, suggesting that it may be primarily

238

S. KISS ET AL.

useful as an adjunct measurement, but not as a general indicator of microbial activity. We think that this conclusion of Hersman and Temple’s would not be valid if the evolution of a given reclamation spoil plot were considered, because the six plots studied by these investigators, as they emphasize, were distributed over an area of many square miles and had been submitted to a variety of treatments, but the differences between plots (and treatments) were not examined. At the San Juan strip coal mine near Farmington, New Mexico, Fresquez and Lindemann (1982) determined dehydrogenase activity and several microbiological parameters (numbers of aerobic heterotrophic bacteria, streptomycetes, ammonium oxidizers, azotobacter, and fungi, and distribution of fungal genera) in representative samples from four sites: (1) a spoil bank (-1 year in age; nonvegetated); (2) a topsoil stockpile (at least I year in age); (3) a reclaimed area (revegetated 3 years earlier by grading and leveling the spoil, spreading stockpile topsoil 18 cm deep, mulching with native hay, fertilizing with 67 kg of N and 100 kg of P,O,/ ha, seeding with native grasses and shrubs, and irrigating for 2 years); (4) an undisturbed surface soil (near the reclaimed area). At each site the samples were collected to a depth of 18 cm. The results of determinations showed that dehydrogenase activity decreased in the following order: undisturbed soil >> reclaimed area > stockpiled topsoil > nonvegetated spoil. Microbial numbers and distribution of fungal genera were greater in the undisturbed soil and reclaimed area than in the stockpiled topsoil or nonvegetated spoil, the lowest values being registered in the nonvegetated spoil. Azotobacter was not found at any of the sites. One can draw the conclusion that stockpiling of topsoil leads to the diminution of enzymactic activity, whereas reclamation of spoils leads to an increase of their enzymatic and microbial potential. In agreement with this conclusion, Fresquez et al. (1985) found that, except for arylsulfatase, the other enzymes analyzed (dehydrogenase, nitrogenase, urease, phosphatase, amylase, cellulase, invertase, and protease) were less active in an older, 3- to 4-year topsoil stockpile than in the undisturbed soil. In February 1979, Fresquez and Lindemann (1982) initiated a greenhouse experiment to study the influence of amendments on the enzymatic and microbial parameters of the nonvegetated spoil. The experimental variants, each in four repetitions, were as follows:

1 . spoil; 2. spoil plus topsoil inoculant (224 t/ha); 3. spoil plus alfalfa hay (22.4 t/ha) plus fertilizers (336 kg of urea and 336 kg of P,O,/ha);


239

4. spoil plus alfalfa hay plus fertilizers plus topsoil inoculant; 5. spoil plus sewage sludge (y-irradiated sludge at a rate of 89.6 t/ha); and 6. spoil plus sewage sludge plus topsoil inoculant. The spoil and the spoil-amendment mixtures placed in pots were seeded to blue grama grass (Boutelom grucifis) and kept under favorable humidity and temperature conditions. In June 1979, nonrhizosphere spoil samples were taken for enzymatic and microbial analyses. In September 1979, the same pots were replanted to fourwing saltbush. In April 1980, rhizosphere samples were collected for microbial analyses. The analytical data indicated that dehydrogenase activity remained at the same low level in pots with spoil and in those with topsoil-inoculated spoil but increased in the other pots, the highest increase being found in pots containing spoil plus sludge plus topsoil inoculant. In this experiment, azotobacter could be detected in each pot, both in nonrhizosphere and rhizosphere samples. Comparison of potmixtures 1 and 2 showed that inoculation of spoil with stockpiled soil did not lead to greater microbial numbers in nonrhizosphere and rhizosphere samples, except in the case of fungal number, which increased significantly in the nonrhizosphere sample. Inoculation slightly increased the distribution of fungal genera, also, in the nonrhizosphere sample. In the other mixtures, as compared to those in potmixtures 1 and 2, the microbiological parameters gave higher values (except for azotobacter number in nonrhizosphere samples from mixtures 5 and 6). Under the influence of the treatment with alfalfa hay plus fertilizers, the highest increase occurred in the numbers of ammonium oxidizers and azotobacter in both nonrhizosphere and rhizosphere samples, whereas sewage sludge had the strongest effect on the increase in numbers of aerobic heterotrophic bacteria and streptomycetes in rhizosphere samples. Thus, it is evident that amendment with organic matter (alfalfa hay or sewage sludge) was more effective for increasing the enzymatic and microbial potential of spoil than was topsoil inoculation alone. At the San Juan mine, a reclamation field experiment was also carried out (Lindemann et al., 1984). In May 1979, plots were established on graded and leveled spoil. To increase water movement into the spoil, sterile bottom ash from an electrical generating plant was spread to a depth of 10 cm over the entire area and incorporated to a depth of 20 cm. Nine treatments, each in four repetitions, were applied to the spoil and bottom ash mixtures: 1 . control: unamended spoil; 2. topsoil: stockpiled topsoil at least 1 year old was applied to a depth of 30 cm;

240

S. KISS E T A L .

3. topsoil inoculum: topsoil collected from around plants in an undisturbed area and applied as a source of microorganisms, including spores of vesicular-arbuscular (VA) mycorrhizal fungi, primarily Glomus fasciculatum, at a rate of 14.5 t of topsoiVha; 4. hay: native hay (mostly grass) at a rate of 2.2 t/ha; 5. sludge: dried and y-irradiated sewage sludge at a rate of 2.2 t/ha; 6. Glomus mosseae root inoculum: sorghum roots containing G. mosseae mycelium, vesicles, and spores at a rate of 1.2 t/ha; 7. Glomus mosseae soil inoculum: soil from sorghum pots containing G. mosseae spores at a rate of 9.1 t/ha; 8. Glomus fasciculatum root inoculum: sorghum roots containing G. fasciculatum mycelium, vesicles, and spores a t a density of 0.7 t/ha; and 9. Glomus fasciculatum soil inoculum: soil from sorghum pots containing G. fasciculatum spores at a density of 14.5 t/ha.

In addition to these amendments, 34 kg/ha each of N (NH,NO,) and P,O, (triple superphosphate) was applied to each plot. The plots were planted with a mixture of native grasses and shrubs in May 1979 and replanted in July 1979. In August 1979 and May 1980, nonrhizosphere spoil (or soil) samples were taken from the 0-20-cm depth of plots 1-5 for enzymatic and microbial analyses. In March and September 1980, samples were collected from roots of fourwing saltbush (Atriplex canescens) and alkali sacaton (Sporobofus aeroides) growing on plots 1-5 for enumeration of rhizosphere dcroorganisms. In March and September 1980, the mycorrhizal formation was also evaluated in all plots. The analyses of nonrhizosphere samples indicated that dehydrogenase activity, numbers of streptomycetes and fungi, and distribution of fungal genera were higher in the hay- and sludge-amended and topsoiled plots than in the unamended or topsoil-inoculated plots. The number of ammonium oxidizers increased significantly only in the hay- and sludgeamended plots (August 1979) or in the sludge-amended plots (May 1980). The aerobic heterotrophic bacteria were least affected by treatments. It is clear from the results of this field experiment that hay or sludge amendments were more effective in increasing dehydrogenase activity and microbial parameters than was topsoil inoculation, which is in good agreement with results of the greenhouse experiment of Fresquez and Lindemann ( 1982). The analyses of rhizosphere samples showed that microbial numbers and distribution of fungal genera in the rhizosphere of A. canescens and S . aeroides were not significantly affected by any of the treatments. Azotobacter was absent in the nonrhizosphere samples and in those from the


24 1

A . c a n e x e n s rhizosphere, but was present in great number i n the S. aeroides rhizosphere; its growth was strongly stimulated in the organically amended and topsoiled plots. Formation of VA mycorrhizae on the planted grass species was, in general, much weaker in plots 3 and 6-9 (spoil inoculated with topsoil or with sorghum soil and roots containing mycorrhizal fungi) than in plot 2 (spoil covered with stockpiled topsoil). Practically no mycorrhizal infection occurred on plants from plots 1, 4, and 5 . Stroo and Jencks (1982) have studied 11 coal strip mine soils within 3 km of each other in Preston County, West Virginia. These mine soils were or were not excessively acid depending upon how the pyritic and nonpyritic materials were mixed when replaced. They tended to be high in sandstone fragments. The eleven mine soil sites studied were varied in age, type, and degree of plant cover and in the type of postmining treatments. Two adjacent native soils were included for comparison. Amylase, phosphatase, and urease activities and respiration (0, uptake) rate were measured in samples taken in early April 1980. The top layer (0-10 cm) was collected after removing the loose unincorporated litter at the surface. Selected physical and chemical soil properties (pH, clay, oxidizable C, total N, C:N, mineralizable N , acid extractable P and K) were also analyzed. Descriptions of the study sites and the results of the enzyme activity and respiration measurements are presented in Tables I11 and IV. One can deduce from these tables that the enzyme activities and respiration rate were generally lower in the mine soils than in the adjacent native soils. The activities and respiration recovered with time, which was attributed to organic matter and N accumulation, but these indices in a 20-year-old mine soil were still lower than in native soils. Vegetation was critical to the recovery of activities and respiration. As long as legumes were present and actively fixing N2, there seemed to be little difference between grassland and locust vegetation. The only vegetated site with activity levels as low as the barren sites was a mine soil (L-2) with a high clay content that was heavily compacted, resulting in slow organic matter and N accumulation. On the unamended locust sites, amylase and phosphatase activities and respiration rate were all significantly correlated with each other. These three indices were dependent on the levels of oxidizable C and total and mineralizable N. Significant correlations were found between amylase and phosphatase activities and mine soil age. Amylase activity was also correlated with clay content, whereas urease activity correlated only with respiration rate. There were no significant correlations between activities or respiration rate and pH or acid-extractable P or K. When all sites were considered together, amylase activity correlated

Table Ill Description of Mine Soil Sites in Preston County, West Virginiia"

Treatments

Age (years)

B- 1 B-2

17 18

Barren Barren

L- 1 L-2 L-3 L-4 L-5 L-6

9 11

Black locust, goldenrod Black locust, blackberry Black locust, tall fescue Black locust, tire cherry Black locust, blackberry Black locust, joe-pye weed

Locust mine soils Sandy loam Unamended, Clay loam Unamended, Silty clay Unamended, Clay loam Unamended, Clay loam Unamended, Clay Unamended,

Tall fescue, birdsfoot trefoil Tall fescue, orchard grass, mixed clovers Same as above

Grass-legume mine soils Sandy clay loam Unamended, planted to grass-legume mixture Limed, fertilized, topsoil replaced, planted to grass-clover mixture; Clay cut for mulch Clay Same as above

Red oak, black cherry Maize

Silt loam Silt loam

G- 1

11 17

18 20

G-2

17 3

G-3

5

N-l N-2

-

Dominant vegetation

Surface texture

Site

"From Stroo and Jencks (1982).

Barren mine soils Sandy clay loam Unamended, unplanted Limed and fertilized after mining, planted to tall fescue and birdsfoot Sandy loam trefoil; the latter died out within the first 5 years after seeding planted planted planted planted planted planted

to locust to locust; heavily compacted to locust to locust to locust to locust

Native soils Native soil, undisturbed, forest Limed, fertilized, manure added annually; continuous corn production with annual ploughing

E v1 v1


243

Table IV Average Enzyme Activities and Respiration of Mine Soils in Preston County, West Virginia”

Site”

Amylase‘

Urease‘

Phosphatase‘

Respiration‘

0.6 a” 1.0 b 1.8 c 1.1 b 2.3 d 3.4 e 2.4 d 3.5 e 3.6 e 6.5 h 4.4 f 4.6 f

13.6 bc 13.6 bc 20.4 de 6.8 a 23.6 fg 19.3 def 17.0 cd 11.2 b 18.6 de 17.2 cd 19.1 def 17.0 cd 25.1 g

1.2 b 0.8 a 2.3 d 0.6 a 1.9 c 3.6 f 2.6 de 4.8 g 2.8 e 2.9 e 2.8 e 8.6 h

0.58 a 1.65 b 1.88 b 0.84 a 1.80 b 2.83 e 1.59 b 1.96 b 2.07 bc 5.17 g 3.53 f 2.77 cd 2.74 cd

~

B- 1 8-2 L- I L-2 L-3 L-4 L-5 L-6 G- I G-2 G-3 N- 1 N-2

5.5 g

5.1 g

“From Stroo and Jencks (1982). “See Table 111. ‘Expression of enzyme activities: amylase as pmoles of reducing sugars produced by 100 g of soil/hr; urease as pgrams of urea hydrolyzed by I g of soil/ hr; phosphatase as pmoles of p-nitrophenol produced by I g of soillhr. Respiration is given as pliters of O2 consumed by I g of soillhr. “Numbers followed by the same letter in a column are not significantly different at p = 0.05.

with urease activity, pH, and acid-extractable P. Correlations also appeared between urease activity and oxidizable C and total N , whereas respiration rate correlated only with amylase activity, pH, and P. The results obtained suggest several potential problems in reclamation. Compaction, particularly of fine-textured overburdens, should be avoided because it slows recovery. Phosphatase activity was too low in the studied mine soils, which indicates possible difficulties in phosphorus mineralization. The case of the B-2 barren mine soil suggests that the goal of recreating productive, self-sustaining ecosystems is not being achieved on such sites. For a further study, Stroo and Jencks (1985) selected the B-2 barren minesoil (see Tables 111 and IV). This acidic, infertile soil was sparsely vegetated with tall fescue (Festuca arundinacea). Different plots of this mine soil were treated in four replications with lime, fertilizers, lime plus fertilizers, and lime plus fertilizers plus sewage sludge. Limestone sufficient to raise the pH from 4.6 to 6.2 was broadcast in late February. Ammonium nitrate, triple superphosphate, and KCI sufficient to raise N , P, and K to

244

S. KISS E T A L .

average levels of 40, 34, and 66 kg per ha, respectively, were broadcast in mid-March. Fifty or 100 kg of air-dried, aerobically digested sewage sludge were added per hectare, 14 days after fertilization, as a slurry. In August, soil was sampled from top 10 cm of each plot for determining enzyme activities and respiration rate. The yield of tall fescue was also estimated. The data of Table V show that the treatments, except for liming, led to significantly increased amylase activity as compared to that in the untreated control. Urease and phosphates activities increased significantly only in the plots treated with lime plus fertilizers plus sludge applied at the higher rate. All treatments produced signif cant increases in respiration rate and tall fescue yield, this effect being strongest in the plots treated with lime, fertilizers, and sludge at the higher rate. It is evident from this study that lime, mineral fertilizers, and sewage sludge might be used profitably to restore seeded vegetation diminished over time because of acidity and low fertility in mine soil. As amylase activity was in general strongly, whereas urease and phosphatase activities were poorly, related to respiration rate, it can be concluded that respiration rate andor amylase activity might serve as a reliable indicator of microbial activity in amended mine soils, although urease and phosphatase activities appear to be of little value in evaluating mine soil microbial activity. This Table V Average Enzyme Activities, Respiration, and Tall Fescue Yield in an Amended Mine Soil in Preston County, West Virginia" Treatment

Amylaseh

Ureaseh

Phosphatase'

Respirationh

Yieldh

Control Lime Fertilizers Lime fertilizers Lime fertilizers 50 kg of sludge per ha Lime + fertilizers 100 kg of sludge per ha

26 a' 24 a 43 bc 40 b 48 cd

35.5 cd 35.5 cd 20.9 b 9.0 a 45.0 d

2.2 bc 0.6 a 1.5 b 1.9 bc 2.5 c

0.89 a 1.31 b 2.17 c 2.71 c 4.42 d

217 a 337 b 669 c 703 c 838 d

55 d

66.6 e

5.3 d

6.38 e

1043 e

+ +

+ +

"From Stroo and Jencks (1985). "Expression of enzyme activities: amylase as pmoles of reducing sugars produced by 1 kg of soil/hr; urease as milligrams of urea hydrolyzed by 1 kg of soil/hr; phosphatase as pmoles of p-nitrophenol produced by I g of soil/hr. Respiration is given as milliliters of O2 consumed by 1 kg of soiVhr. Yield is recorded as kg/ha. 'Numbers followed by the same letter in a column are not significantly different at p = 0.05.


245

conclusion is not surprising, because only a part of the activity of these enzymes is due to the proliferating microorganisms that determine the respiration of soil: the other part is the result of the accumulated enzymes, which are independent of the momentary proliferation of soil microorganisms (Kiss et al., 1975). Persson and Funke (1986) studied, enzymologically and microbiologically, the topsoil pile R26T03, which is located on the Baukol-Noonan lignite strip mine near Center, North Dakota. The pile was formed in July 1983 and vegetative cover was established on its surface. Samples were taken from depths of 0-7.6 and 114-122 cm in May 1984 and July 1985. Alkaline phosphatase and dehydrogenase activities were found to decrease at increasing depths and to show further declines during the storage of the topsoil pile. Counts of bacteria and actinomycetes were similar in all samples, whereas the number of fungi decreased with depth but increased with storage time.

c. ENZYMOLOGICAL RESEARCH IN POLAND Gilewska and Bender (1978, 1979) carried out detailed studies of the Patnow strip mine spoil heap located in the Konin Brown Coal Basin. They have determined the cellulolytic, invertase, urease, and catalase activities in the ploughed layer (0-25 cm) of spoil plots (medium-heavy loam; pH in H,O 7.6) cultivated with spring barley (in the third year of their recultivation) and not fertilized or fertilized with NH,NO,, triple superphosphate, and potash salt in single ( 1 NPK: N, 130; P205,240; and K 2 0 , 140 kg/ha) or double (2 NPK) dosage. An adjacent arable soil that had not been affected by the strip mining was used for a control. Each activity was demonstrable, although at a low level, in the unfertilized plots and each gave higher values in the fertilized plots, particularly in those treated with the double NPK dosage. Cellulolytic activity in these plots exceeded, whereas invertase and urease activities approached, that found in the arable soil. Catalase activity in fertilized plots increased to a lesser extent, reaching only 50% of that measured in the arable soil. The barley grain yields were also higher in the fertilized spoil plots than in the unfertilized ones. The increased enzyme activities and barley yields were accompanied by the accumulation of humus and the beginning of cloddy structure formation in the ploughed layer of the spoil plots (Bender and Strzyszcz, 1978). Mineral fertilization, especially with the double NPK dosage, increased cellulolytic, invertase, urease, and catalase activities and led to humus accumulation also in those spoil plots that were cultivated with other plants (winter rape, winter wheat, alfalfa) (Bender, 1980).

246

S. KISS E T A L .

Other plots of the Patndw spoil heap as well as plots of other spoil heaps in the Konin Brown Coal Basin (Wschodnie and Goslawice) were also studied by Bender and Gilewska (1980, 1984a,b,d) and Gilewska and Bender (1983, 1984). The newly studied Patnow plots were brought into agricultural recultivation 3 or 4 years before the studies began. (The last two crops were winter wheat in some plots and alfalfa with orchard grass, Dactylis glomerata, in some others.) Both the wheat and the alfalfa-orchard grass plots included unfertilized, 1-NPK-and 2-NPK-fertilized variants. The Wschodnie plots had been recultivated for 10 years (last crop: winter wheat) and all were 1-NPK-fertilized.The Goslawice plots were 6-year-old mine spoils without any plant cover, fertilized with the smaller NPK dosage. “Raw and fresh” (unplanted and unfertilized) Patnbw spoils and an adjacent arable soil fertilized with 1 NPK and cultivated with wheat served as controls. Four activities (cellulolytic, invertase, urease, and catalase) were measured in the ploughed layer (0-25 cm) of the spoil plots and arable soil. The lowest activities were registered in the raw spoils. The main factor determining the increase of enzymatic activity in coal mine spoils proved to be mineral fertilization with a high NPK dosage. The presence of plants strongly enhanced the effect of fertilizers, which increased with recultivation time. Thus, after 10 years of recultivation the enzymatic potential of the mine spoils reached that of the arable soil. The nature of the crop plant used for recultivation had also an evident influence: e.g., invertase activity of the recultivated spoils was highest under the alfalfa-orchard grass mixture; the highest urease activity was recorded in the wheat plots. The optimum moisture content for the enzyme activity was in the 4040% range of the maximum water-holding capacity of the spoils. The recultivated spoils containing higher amounts of clay minerals showed higher catalase activity. In parallel with increasing enzyme activity, the production of plant biomass, humification of plant residues, accumulation of microbial biomass, soil respiration (CO, evolution), N, fixation, and formation of a humus layer have intensified. The C:N ratio in spoil decreased from 22-26 (raw spoil) to 15 (spoil recultivated for 10 years). Owing to the agricultural recultivation techniques applied, it became possible to transform the spoils into fertile soils in a relatively short period of time (10-15 years) (Bender and Gilewska, 1980, 1984b,d,e; Golebiowska and Bender, 1980, 1983). Bender and Gilewska (1983) have demonstrated that invertase, urease, and catalase activities are good indicators of the soil-forming processes occumng even in those Patndw spoils that had been submitted to technical recultivation only 1 year before. Samples were taken from the 0-25, 2550, 50-75, and 75-100-cm depths of 15 representative profiles. Invertase and urease activities were always highest in the 0-25-cm layer and de-


247

creased with depth. Catalase activity of the 0-25-cm layer did not give the highest value in each profile and showed only a trend to decrease with depth. This differentiation of the enzyme activity is a noticeable symptom of the soil profile variation and indicates that the soil-forming processes are most intense in the top layer, which is due to the microorganisms penetrating the spoils from the surrounding agroecosystem. These soilforming processes are, however, limited because of the shortage of biogenic elements, especially N and P, in the spoils. Therefore, the initiated soil-forming processes would develop very slowly; the transformation of spoils into soils would take many decades. This is why the NPK fertilization and biological recultivation of spoils are required. Some enzymological aspects of the forest recultivation of strip mine spoils in the Konin Brown Coal Basin have also been studied (Bender and Gilewska, 1984c; Gilewska and Wojcik, 1984). Black locust was used as a pioneer forest plant on the slopes of the Goslawice and Nieslusz spoil heaps (medium-heavy loam and sandy loam). At the time of study, the locust plantations were 15 years old. The average tree height was 10.8 m at Goslawice and 11.6 m at Nieslusz. The stand density was medium on both spoil heaps. Black cherry (Prunus serotina), purging buckthorn (Rhamnus cathartica), golden elder (Sambucus nigra), and aspen (Populus tremula) occur sporadically in the plantations. Twenty profiles were examined; Nos. 1-10 are located on Goslawice and Nos. 11-20 on Nieslusz. The results showed that invertase activity in all profiles and urease and catalase activities in most profiles were highest in the 0-25-cm layer (see Table VI, containing the analytical data obtained in profiles 1, 8, 10, 15, and 20). In general, organic matter, total N , P,05, and K,O content were also highest in this layer, but in most cases, there was no distinct outline of the humus horizon in contrast to the agriculturally recultivated coal strip mine spoils, in which the 0-25-cm humus layer, after 10 years of recultivation, was clearly outlined. Six enzyme activities (dehydrogenase, catalase, invertase, p-glucosidase, urease, and asparaginase) were determined in the Smolnica black coal mine spoil heap (Silesia), which had been submitted to three recultivation experiments (Osmariczyk, 1980; Osmanczyk-Krasa, 1984a,b). The spoils consisted of sandstones and, mainly, of shales; pH in H,O was 4-5. In the first experiment, which began in 1973, 10 mixtures of grasses and legumes (Arrhenatherum elatius, Bromus inermis, Ductylis glomerata, Festuca capillata, F. heterophylla, F. pratensis, F. rubra, Lolium multijlorum, L. perenne, L . westerwolicum, Lotus corniculatus, and Trifolium repens) were used for revegetating spoil plots. The second and third experiments started in 1975, and the spoil plots were recultivated with four tree species: European ash (Fraxinus excelsior), red oak (Quercus rubra),

248

S. KISS ET AL. Table VI Enzyme Activities in Technogenic Soils Formed in Some Plots Planted to Black Locust on the Goslawice and Nieslusz Coal Strip Mine Spoil Heaps‘

Profile No.

Depth (cm)

Invertase”

Ureaseh

CataIaseh

I

0-5 (A,) 5-25 25-40 40-70 70-100 0-5 (A,) 5-25 25-40 40-70 70-100 0-25 25-40 40-70 70- I00 0-25 25-40 40-70 70- I00 0-5 (A,) 5-25 25-40 40-70 70-100

322 156 35 45 47 565 240 80 57 65 I I5 22 65 75 175 60 55 47 520 233 83 70 87

38.0 18.2 10.0 9.5 8.7 90.0 49.4 19.0 9.5 7.7 15.5 9.0 13.2 18.2 20.5 11.8 8.7 10.0 80.0 41.8 7.4 10.0 8.2

132 22 26 20 20 340 108 10 2 4 14 14 10 26 40 16 6 6 312 68 12 2 4

8

10

15

20

“From Gilewska and W6jcik (1984). ”Expression of enzyme activities: invertase as milligrams of “glucose” produced by 100 g of soil in 24 hr; urease as milligrams of NH,’ N produced by 100 g of soil in 3 hr; catalase as milliliters of 0, consumed by 100 g of soil in 15 min.

sycamore (Acer pseudoplatanus), and black alder (Alnus glutinosa). The enzymological analyses were carried out in 1977 and 1978 (every 6 weeks during the vegetation period). The plots submitted to recultivation with grass-legume mixtures were fertilized with 120 kg of N, 75 kg of P205,and 25 kg of K,O/ha in the first year (1973), only with N and P in the next years, and with the N dosage being reduced to 60 kgha in the third year; later no fertilizers were applied. For enzymological analyses samples were taken from the 0-10-cm layer of each spoil plot. The results show that each of the six enzymatic activities determined increased in the recultivated plots as compared to the control (unreclaimed, stored raw spoil). The increase was highest in the case of


249

invertase and P-glucosidase activities. Of the 10 plant mixtures tested, that consisting of Bromus inermis, Festuca pratensis, F. rubra, Lolium perenne, and Lotus corniculatus manifested the strongest positive effect on the enzymatic potential in recultivated spoil (Osmanczyk, 1980). The plots submitted to recultivation with tree species were fertilized with 100 kg of N , 50 kg of P205,and 25 kg of K20/ha in the first year (1975), with the same N and P dosages but without K in the second and third years, and with doubled N and P dosages in the fourth year. The samples for enzyme analyses were collected from below the root system (at 30-cm depth). In the case of the control plot (not recultivated, not revegetated), the samples were taken from the 0-10-cm layer. It appears from the analytical data that the enzyme activities, except for catalase activity, increased in the recultivated plots in comparison with those measured in the control. The highest increase occurred in dehydrogenase activity. The increase was remarkable in the invertase and P-glucosidase activities also. The trees enhanced the enzymatic potential of the recultivated spoil plots in the following order: alder > sycamore = ash > oak (OsmaAczyk-Krasa, I984a). Tree seedlings were planted on spoil plots in three different ways. The first tree seedlings were planted directly into the spoil. The second ones were planted in holes made on the spoil surface and filled with about 6 liters of podzolic soil taken from pine forests. The third ones were planted in holes filled with about 6 liters of fly ash derived from a slag dump. The fertilizers applied were 100 kg of N and 50 kg of P,O,/ha in 1975 and 1976, 200 kg of N and 100 kg of P,O,/ha in 1977, and 100 kg of N and 40 kg of P,O,/ha in 1978. As in the second experiment, the samples were taken from below the root system (at 30 cm depth). Comparison of enzyme activities in the untreated, soil-treated, and flyash-treated spoils has shown that treatment of spoils with podzolic soil or fly ash led to an increased dehydrogenase activity. The effect of soil, as compared with that of the fly ash, was stronger in the plots with ash tree and oak, weaker in the plot with sycamore, and identical in the alder plot. Catalase activity also increased under the influence of soil and fly ash treatments. The increase was more pronounced in the fly-ash-treated plots than in those treated with soil, except for the alder plot. Invertase and P-glucosidase activities behaved like catalase activity. Urease activity was not influenced by treating the spoils with soil or fly ash; the only exception was the soil-treated ash tree plot, in which urease activity increased. Asparaginase activity was favorably influenced by both soil and fly ash treatments in each plot, but the effect of fly ash was stronger than that of the soil. Of the four species planted, alder caused the greatest increase in the activity of the majority of enzymes in both untreated plots and those treated with soil or fly ash. The effect of alder should be

250

S . KISS ET AL.

attributed to the N,-fixing capacity of its root nodules, which finally leads to improved nutritional conditions for microorganisms and, implicitly, to increased enzyme production. The enzyme activities were high also under ash trees planted in fly ash (Osmanczyk-Krasa, 198413). Reviewing the results of these experiments, Osmanczyk-Krasa ( 1987) emphasizes that the enzyme activities appearing in unreclaimed spoil after its 3- to 5-year storage indicate the initiation of the soil-forming process in it, while the increase in enzyme activities of the revegetated spoil proves further development of pedogenesis. D. ENZYMOLOGICAL RESEARCH IN HUNGARY In a pot experiment carried out by Sulyok and Voros (1983), refuse soils (yellow sand, yellow clay, gray clay, and andesitic tuff), resulted from the strip mining of lignite in Gyongyos-Visonta and the original topsoil (brown chernozem) were fertilized with NPK (at a rate equivalent to 309 kg of N, 189 kg of P,05, and 180 kg of K,O/ha) or with NPK plus 10 t of wheat straw per ha and sown with alfalfa. The controls were: (1) unfertilized and unsown and (2) unfertilized but sown refuse soils and topsoil. After 2 years of recultivation, invertase activity of both controls and fertilized variants was highest in the topsoil and lowest in the yellow sand. Rate of respiration (CO, evolution) and intensity of cellulose decomposition were also higher in the unfertilized and unsown topsoil than in the unfertilized and unsown refuse soils. In the case of the sown controls and fertilized variants, however, the largest amount of CO, was produced by the yellow clay. Cellulose decomposition was most intense in the yellow clay fertilized with NPK and straw, whereas the highest alfalfa yields were obtained on the yellow clay fertilized with NPK and on the gray clay fertilized with NPK and straw. The results suggest that enzyme (invertase) accumulation in the recultivated sandy refuse soil is a very slow process. In the recultivated clayey refuse soils this process is somewhat faster but not so fast as the microbial cellulose decomposition or CO, production. Therefore, enzyme accumulation should be considered a more reliable indicator of the transformation of mine spoils into soils than cellulose decomposition or CO, evolution.

E. ENZYMOLOGICAL RESEARCH IN THE FEDERAL REPUBLIC OF GERMANY Schroder et al. (1985) studied, from physical, chemical, microbiological, enzymological, and micromorphological viewpoints, seven profiles of


25 I

technogenic soils formed during the agricultural recultivation of spoils (loess; pH in 0.01 M CaCI, solution was 7.1-7.9) that resulted from the strip mining of brown coal in the Rhine region. The soils studied were located near Grefrath and Berrenrath. The recultivation started about 20 years ago through redeposition of stored loess either as dry material, followed by leveling (five soils), or as wet material (pumping of water-loess mixture into depressions, i.e., slurry poldering), followed by partial evaporation of water (two soils). Recultivation led to humus and K accumulation and to Na and Mg loss but due to tillage, the 30-50-cm layer of soils became compacted and impermeable. In the soils formed through redeposition of dry loess, the 50-120-cm layer also frequently became compacted. Dehydrogenase activity was detectable, in general, only from the 0-30cm layer but even in this layer, the activity reached only about 10% of that of the native soils. Respiration (CO, evolution) was more intense in the upper layers than in the deeper ones, the lowest values being recorded in the compacted layers. In each layer of the native soils, respiration rate was greater as compared with the corresponding layers of the seven technogenic soils. Similar results were obtained also in regard to cellulose decomposition. For improving the compacted soils deep loosening and drainage were recommended. In the same coal mine region, but in another locality (Gustorf near Grevenbroich), Lessmann and Kramer (1985) determined, in 1983, some enzymological and microbiological parameters in a leveled spoil (loess) recultivated with alfalfa for 2 years. For comparison they used a native, vegetated loamy riverside soil. This study site was located at Kirchhoven (at approximately 33 km from Gustorf). The Gustorf plot was never fertilized and the Kirchhoven plot was not treated with mineral fertilizers in the last 2 years. Both plots are serving as controls for an experiment on organic fertilization. The studies will continue for many years, and the organically fertilized plots will also be analyzed. The unfertilized plots differed from each other in the enzymological and microbiological parameters of their 10-20-cm layer. In this layer, the loess plot had lower dehydrogenase and urease activities, contained less bacteria, respired less strongly (produced less CO,), and degraded the cellulose more slowly than the plot of native soil. In the 3545-cm layer, there were no significant differences between the two plots. The profile differentiation as indicated by enzymological and microbiological data was reduced in the loess plot. Dealing with the problems of soil assessment in the recultivated brown coal area located in the “Niederrheinische Bucht,” Schroder (1986) compared a native soil, characteristic for this area, with two representative technogenic soils on loess, the agricultural recultivation of which began in 1970. Recultivation of one of these two soils was considered good and

252

S. KISS ET AL.

that of the other bad. Thus, in each horizon, the bulk density was lower than 1.65 g/cm3in the good recultivated soil and had a higher value in the bad recultivated soil. In the 0-40-cm layer, dehydrogenase activity, like CO, production and cellulose decomposition, showed the following order: native soil > good recultivated soil > bad recultivated soil. Haubold et al. (1987) performed a similar comparison using 15 good and 15 bad recultivated soils on loess, also from the brown coal area in the Rhine region. They found that dehydrogenase activity, microbial biomass, and cellulose decomposition in the 0-40-cm layer were, in general, about 50-100% lower in the recultivated soils than in the native soils. At the same time, the mean values of these microbial parameters and those of the chemical parameters analyzed (C, N, Na, K, Mg, and Ca contents, and cation-exchange capacity) indicated no remarkable differences between the good and bad recultivated soils. We think that this finding, although valid for dehydrogenase activity (an activity depending on the momentary proliferation of microorganisms), cannot be applied to those enzymes that are able to accumulate in soil and to be, in the accumulated state, independent of the momentary microbial proliferation. The activity of such enzymes was not assayed by these investigators. In other studies carried out in the same brown coal strip mining area, Schroder et af. (1987) and Schroder (1988) have determined, among other things, the dehydrogenase activity in the 0-40 cm layer of 13 technogenic loamy-silty loess soils formed after slurry poldering of spoils followed by their agricultural recultivation for 6-25 years, and have found that the activity was significantly higher in the older soils than in the younger ones. The C, N, K, and Ca contents, cation-exchange capacity, and microbial biomass also increased significantly over time, while the time-dependent decrease of carbonate, Na, and Mg contents and increase of cellulose decomposition were insignificant.

Ill. TECHNOGENIC SOILS FROM POWER PLANT WASTES ENZYMOLOGICAL RESEARCH IN POLAND Field experiments were carried out over 10 years to plant the ash-containing wastes of the Halemba power plant with different species. Plots fertilized and cultivated with potato, barley, field bean, and flax and an unfertilized and uncultivated plot were selected by Balicka and Wegrzyn ( 1984) for microbiological and enzymological analyses. For this purpose, the 0-10- and 10-20-cm layers were sampled two or three times yearly. Owing to cultivation, the bacterial counts increased in each plot; the


253

highest increase occurred in the potato plot. The increase of dehydrogenase and catalase activities and of nitrification potential was similar in plots with different crop plants. It has also been found that the low biological activity in the unfertilized and uncultivated plot is the result of the lack of N ; these wastes are not toxic for soil microorganisms.

IV. TECHNOGENIC SOILS FROM RETORTED OIL SHALE

ENZYMOLOGICAL RESEARCH IN

THE UNITED STATES

Hersman and Klein (1979) studied, under laboratory conditions, the effects of retorted oil shale additions on some enzymological and microbiological characteristics of surface soils. The retorted oil shale used was produced by the Paraho process. Samples of this retorted oil shale transported to the study site (the Piceance Basin, northwestern Colorado) and samples of surface soil collected from the study site were sieved and mixed in different proportions by weight, obtaining five variants: 100% soil (control); 95% soil plus 5% shale; 90% soil plus 10% shale; 75% soil plus 25% shale; 100% shale (control). The mixtures were stored at 23-25°C with the soil-water content maintained at 10%weight and sampled and analyzed at 2-week intervals over a 10-week period. The results showed that retorted shale additions caused, for all sampling times, significant decreases in acetylene reduction (N, fixation) rate, dehydrogenase activity, ATP content, mineralization rate of uniformly I4Clabeled glucose, respiration (0,uptake) rate, and fungal counts, although counts of bacteria and actinomycetes were not significantly affected by the presence of up to 25% retorted shale. In another experiment (Klein et al., 1982) retorted oil shale produced by the Lurgi process was used. Its addition to soil samples in 5, 10, and 25% proportions also brought about significant diminutions in dehydrogenase and phosphatase activities as compared to those in the control soil. These results have a great importance for the revegetation programs in areas disturbed by oil shale processing. If retorted oil shales are covered with stored surface soils, it may be necessary to insure that little physical mixing of the surface soil and retorted oil shale occurs. It may even be necessary to consider the construction of capillary barriers below the surface soil column to minimize upward movement of soluble fractions from the retorted oil shale. These suggestions were verified by Sorensen et al. (1981) and Klein et af.(1982, 1985), under field conditions using experimental plots installed

254

S. KISS ETAL.

in the Piceance Basin in the fall of 1977 and designed to allow the study of vegetation succession on surface soil (a fine loam) of various depths (thicknesses) overlying Paraho retorted oil shale and on surface soil separated from retorted shale by a gravel barrier. The profile configuration for five panels included panels with soil depths of 30,61, and 91 cm over compacted shale, a capillary barrier panel (61 cm of soil over 30 cm of fine and coarse gravel), and a control panel (soil). The surface of each panel was divided into three replicate plots. Within each of these plots nine possible combinations of three seed mixtures (native species, introduced species, and native plus introduced species) and three fertilization rates (1 12 kg of N plus 56 kg of P per ha, 56 kg of N plus 28 kg of P per ha, and no N and P) were applied randomly to subplots. In the summers of 1979, 1980, and 1981, samples were taken from the 5-10-cm depth after removal of the 0-5-cm layer. The samples were analyzed to determine their acetylene reduction, dehydrogenase, and phosphatase activities. The average yearly activities across the panels from 1979 to 1981 are shown in Fig. 1, from which one can see that over the 3 years acetylene reduction decreased to a large extent in all experimental variants. In each year, this activity was most intense in the control soil and weakest in the 30- or 61-cm deep soil over retorted shale. In 1979, this activity in soil overlying the capillary barrier was not significantly lower than that in the control soil.

h

-

C D E 1981

1979 1980 FIG.1. (a) Acetylene reduction, (b) potential dehydrogenase, and (c) phosphatase activities of soil of various depths over retorted oil shale and capillary banier. The error bars indicate least significant differences between means. A, control; B, capillary barrier; C, 91 cm; D, 61 cm; E, 30 cm. (Redrawn from Klein et al., 1985.)


255

Potential (zymogenous) dehydrogenase activity and phosphatase activity behaved contrarily over the 3-year period: potential dehydrogenase activity increased, whereas phosphatase activity showed a consistent downward trend since 1979 in all variants. In each year, both activities had the highest values in the control soil and in the soil overlying the capillary barrier, whereas the soil 30 cm deep over shale was, in general, the least active. The acetylene reduction data manifested a significant interaction

A B C D E

t B C D E

1979

1980

A B C D E

1979

A B C D E

I C D E

1981

A B C D E

1980 1981 Panel treatment and year FIG. 1. (conrinued)

256

S. KISS E T A L .

between seed mixture (or the ensuing plant community) and fertilization in 1979, although in 1980 and 1981 only a simple effect of fertilizer on acetylene reduction occurred. In 198 1, potential dehydrogenase activity showed a significant three-way interaction between seed mixture, fertilization, and soil-shale arrangements. In the case of phosphatase activity, a significant interaction between seed mixture, fertilization, and soil depth was observed when the activity values from 1979, 1980, and 1981 were analyzed together. Measurements of percentage mean plant cover over the 3-year period in the control plot, corresponding capillary barrier plot with 61 cm of soil, and plots with 91, 61, and 30 cm of soil directly over shale indicated no significant decrease in percentage mean plant cover when comparing the control plot with the plot with only 30 cm of soil. These measurements also suggested an improved plant development in the plot with the capillary barrier. In comparison, acetylene reduction, potential dehydrogenase, and phosphatase activities decreased to a greater extent than the corresponding values for percentage mean plant cover on the plots with less surface soil overlying the retorted shale. Thus, this field study confirms the laboratory results of Hersman and Klein ( 1979). Appreciable diminution of microbial and enzymatic activities occurred in soil up to 91 cm in depth when it was placed directly over retorted oil shale. Suppression of these activities in such soils might lead to a long-term reduction in productivity. A capillary barrier composed of fine and coarse gravel helped maintain enzyme activities in soils placed over retorted oil shale during revegetation. In 1981, the retorted shale over which soil had been placed was also analyzed enzymologically . Potential dehydrogenase and phosphatase activities were essentially absent in the shale. In contrast, actual dehydrogenase activity in the shale showed values equivalent to those found in the control soil (Klein et al., 1982), but the possibility that this activity was due to nonenzymatic factors is not excluded. Also in 1981, Klein ef al. (1982) analyzed enzymologically a stored topsoil pile. The planted north end of the soil pile in comparison with its unplanted south end manifested higher dehydrogenase and phosphatase activities. Contrarily, numbers of actinomycetes, fungi, and bacteria and the soil water content were higher in the unplanted south end of the pile. In several industry-constructed reclamation plots in which the surface soil was placed over shale materials retorted by different processes (TOSCO 11, USBM, Union B, Union Decarbonized), dehydrogenase activity decreased with soil depth and the shale-to-surface had the lowest activity. The TOSCO I1 shale-to-surface was less dehydrogenase-active than the USBM shale-to-surface. The plots were constructed in western Colorado during 1970-1975 and analyzed enzymologically in 1981 (Klein et al., 1982, 1985).


257

V. TECHNOGENIC SOILS FROM IRON MINE SPOILS A. ENZYMOLOGICAL RESEARCHIN THE USSR Catalase and invertase activities in the revegetated spoil heaps around the Lebedin strip mine (located in the Starooskol iron ore zone within the Kursk Magnetic Anomaly region) were determined by Sviridova and Panozishvili (1979). The spoil heaps here are of three types: sandy, loamy, and cretaceous-marly. They were revegetated with spontaneous and introduced grasses or with forest plants (sea buckthorn, black locust, and oleaster). After 8-10 years the heaps revegetated with grasses were covered with a 1.5-2-cm sod, protecting them against erosion. In the 8-10year-old forest stands the mass of litter reached densities of 0.5-0.9 t/ha. Catalase activity was measurable in the soil of all revegetated heaps. This activity, like respiration (CO, evolution) and humus accumulation, was more intense under herbaceous vegetation than under forest. Invertase activity was influenced not only by the nature of vegetation but also by the nature of spoil heaps. This activity was highest in the rhizosphere of sea buckthorn growing on the cretaceous-marly heap. Zasorina (1985a,b) has studied, enzymologically, the spoil heaps near the Stoilensk iron strip mine (located within the Kursk Magnetic Anomaly region). These spoil heaps are of two types: sandy and cretaceous. The age of natural vegetation growing on heaps varied between 3 and 20 years. Invertase, urease, and catalase activities in the 0-5-cm layer of spoil heaps increased in parallel with the age of vegetation. The increase was more pronounced in the sandy spoils than in the cretaceous one. During the growing season, the maximum activity values were registered in midsummer. In both young and old spoil heaps, after their revegetation with a mixture of six grasses and legumes (bromegrass, fescue, wheatgrass, alfalfa, clover, and sainfoin), the enzymatic activities increased l .5-3 times.

B. ENZYMOLOGICAL RESEARCHIN ROMANIA Blaga et al. (1981) compared dehydrogenase, catalase, and invertase activities in spoils (calcareous sandy loams or clays; pH in H,O was 7.78.3) leveled with the aim of their agricultural recultivation in the northern zone of the iron strip mine in C%pus(Cluj district) and in adjacent soils. In the soils the activities decreased with depth (0-70 or 0-80 cm), whereas in the spoils the activities were approximately the same in the 0-20- and 50-80-cm layers. In the 0-20-cm layer each activity was many times lower in the spoils than in the soils. In the 50-80-cm layer the differences between spoils and soils in dehydrogenaseactivity were great, but differences

258

S. KISS ET A L .

were not so pronounced in the case of their catalase and invertase activities. In another study carried out in the same zone by Bunescu and Blaga (1980), similar results (i.e., very low and higher activities, respectively) were registered in different spoil and soil profiles, except for a spoil profile that showed relatively high dehydrogenase and catalase activities. The activities correlated with the total N content of spoils and soils. On the leveled spoils in the southern zone of the CBpus iron strip mine, recultivation plots were installed. Some plots were recultivated with sainfoin (Onobrychis viciaefolia) and others with orchard grass (Dactylis glomerata). After 3 years of recultivation the spoils of these plots were analyzed enzymologically by Drggan-Bularda et al. (1983). For comparison, the 0-15-cm layer of an adjacent native soil (rendzina) and the same layer of a spoil plot not submitted to recultivation were also analyzed. Some of the results are presented in Fig. 2. They show that recultivation led to increased enzyme activities in iron mine spoils during their transformation into technogenic soils. Potential dehydrogenase activity increased to a lesser extent than phosphatase activity, which reached values similar to that of the native soil. Both activities were higher in the sainfoin plots than in those recultivated with orchard grass. In each case, the 020-cm layer was more active than the 2040-cm one. In the Satra zone of the Cilpus iron strip mine, Blaga et al. (1984) recorded very low values of dehydrogenase, catalase, and invertase activities in the 0-20- and 50-80-cm layers of three profiles of spoils leveled for their recultivation. In the fall of 1985, the ninth year of a fertilization and crop rotation experiment on the southern zone of the CBpus iron strip mine, samples were taken for enzymological analyses from the 5-20-cm depth of unfertilized, farmyard-manured (40 t/ha), lightly or heavily NPK-fertilized ( 100 kg of N as NH,NO, plus 60 kg of P as simple superphosphate plus 40 kg of K as potash salt or 300 kg of N plus 180 kg of P plus 120 kg of K/ha, respectively), and complexly fertilized (40 t of farmyard manure plus 100 kg of N plus 60 kg of P plus 40 kg of K/ha) spoil plots planted in maize, oats, or sainfoin. The enzymatic and nonenzymatic catalytic potential (actual and potential dehydrogenase, invertase, phosphatase, urease, and nonenzymatic H,O,-splitting capacity), like the yield of crops, was highest in the complexly fertilized spoil plots and lowest in the unfertilized ones. Significant correlations were found between invertase activity and corn yield and between phosphatase activity and sainfoin yield. But under the influence of long-term fertilization, the crop production capacity of the studied technogenic soil increased to a larger extent than its biological potential reflected by its enzymatic activities. This means that long-term fertilization is able to greatly enhance the crop production capacity of the

T H E RECULTIVATION OF TECHNOGENIC SOILS

259

I 2 RG.2. Potential dehydrogenase and phosphatase activities in recultivated iron strip mine spoils. (I)Adjacent native soil; (2) spoil not submitted to recultivation; (3) spoil recultivated with sainfoin (depth: 0-20 cm); (4) spoil recultivated with sainfoin (depth: 2 0 4 0 cm); ( 5 ) spoil recultivated with orchard grass (depth: 0-20 cm); (6) spoil recultivated with orchard grass (depth: 20-40 cm). (Redrawn from Dragan-Bularda et a/.,1983.) 1

2

3

4

5

6

0

technogenic soil, but the increasing effect of fertilization on the biological potential of the technogenic soil, as reflected by its enzymatic activities, is the result of much slower processes (Dragan-Bularda et al., 1987).

VI. TECHNOGENIC SOILS FROM MANGANESE MINE SPOILS ENZYMOLOGICAL RESEARCHI N

THE

USSR

According to Keleberda (1973), the 0-20-cm layer of the spoil heap (consisting of medium loams) at the Aleksandrov manganese quarry (Dnepropetrovsk region, Ukraine) contains more humus and shows higher H,O,-splitting (enzymatic plus nonenzymatic) capacity than the 20-40-cm

260

S . KISS E T A L .

layer. In another study, carried out in the same area, Keleberda (1978) has found that recultivation of spoil plots with oleaster for 11 years resulted in the formation of a technogenic soil with increased humus and N contents and invertase, urease, and proteinase activities in the 0-5-, 5-lo-, and 1020-cm layers as compared with the uncultivated control plot. At the same manganese quarry, Uzbek (1986) determined several enzyme activities in different layers of a 20-year-old spontaneously revegetated spoil plot, now covered with a stable phytocoenosis made up of meadowgrass (Poa angustifolia) and sagebrush (Artemisia austriaca) and found that the activities were much higher (catalase 1.5, phosphatase 13, urease 36, invertase 46, and dehydrogenase 72 times) in the 1-cm-deep surface layer rich in roots than at a 6-cm depth and in deeper layers of the spoil plot. Microbial counts as well as humus, total N, mobile P, and exchangeable K contents were also highest in the surface layer. The enzymological properties of the recultivated strip mine spoils in the Chiatura manganese ore zone (situated in the Kvirila Basin, western Georgia) were described by Daraseliya and Kalatozova (1973, 1976) and Daraseliya (1979). The experimental variants were the following: (1) adjacent native (brown forest) soil; (2) mine spoils without plants; (3) perennial grass with legume (ryegrass-alfalfa) on mine spoils; (4) grapevines on mine spoils; and (5) grapevines on stored topsoil (40-45-cm layer) reapplied on mine spoils. The mean values of the analytical data over 3 years (1969-1971) indicated that the native soil had much higher enzyme activities than the uncultivated spoils (calcareous sands and clays; pH in H,O was 8.4). The activities increased in the recultivated spoils but did not reach the values registered in the native soil. The perennial grasslegume mixture was more efficient than grapevines. Reapplying the stored topsoil on the surface of spoils had beneficial effects on the accumulation of enzymes (invertase, phosphatase) in the grapevine plots. The 0-20-cm layer was more active and richer in humus and microorganisms than the 2040-cm one. No relationship was found between catalase activity in spoils and the recultivation measures applied.

VII. TECHNOGENIC SOILS FROM LEAD AND ZINC MINE WASTES A. ENZYMOLOGICAL RESEARCH IN

THE

UNITED KINGDOM

Studying the decomposition of vegetation growing on metal mine waste, Williams et al. (1977) also carried out enzymological analyses. The waste studied was located around the disused mine at Y Fan (Powys, Wales)


26 1

and contained high concentrations of lead and zinc. After the abandonment of the mine (1928), the waste was partially colonized naturally by metaltolerant Agrostis tenuis. An evenly colonized area was selected for study. A similar but uncontaminated area was also selected on a pasture situated about 500 m from the mine. The vegetation on this site consisted primarily of A . tenuis and Festuca ovina. Urease activity in soil, microbial populations in litter, and soil and microfauna in litter from both sites were compared. Accumulation of litter was greater on the waste, which also contained significantly less humic and fulvic acids in the soil immediately beneath the litter layer. Urease activity was also significantly lower in the mine soil than in the nearby pasture soil (Table VII). Microbial counts from litter at the two sites were not markedly different, although numbers of fungi were lower on litter from the mine waste, while those of bacteria and actinomycetes were higher. In contrast, counts of all groups in the mine soil were considerably lower than those in the pasture soil. Similarly, there were fewer animals in the litter on the waste. The low biological activity in the litter and soil of the studied mine waste, caused by the high Pb and Zn concentrations, explains the retarded decomposition of vegetation growing on this site. Clark and Clark (1981) have applied soil-enzymological methods, in order to determine the reasons for the differences in the floras of adjacent species-poor and species-rich areas of a limestone terrace in the lead-mining complex on Grassington Moor, in the Yorkshire Pennines (England). The northern half of the terrace received drainage water and fine-textured, Pb- and Zn-containing mine waste from abandoned mine workings up slope, and the vegetation there was sparse, floristically impoverished, and composed of species typical of heavy-metal mine areas in the British

Table VIl Urease Activity in a Mine Soil and a Pasture Soil at Y Fan, Powys, Wales"

mg of NH,' N released from 100 g of soil (on air-dry basis) at 37" C in 3 hr

Reaction mixture Soil Soil

+ urea solution + water

Urease activity ~~

Mine soil'

Pasture soil'

2.55 2 0.14 1.97 t 0.25

42.62 t 2.38 3.32 2 0.58

0.58

39.31

2

0.19

?

2.17

~

"From Williams p t ul. (1977). 'Means of two soils significantly different at p

=

0.05.

262

S. KISS ETAL.

Isles, i.e., Minuartia verna, Agrostis tenuis, and Festuca ovina. There was no direct input of mine waste on the southern half of the terrace, and there the vegetation was floristically rich and continuous, except where the limestone cropped out. The mean number of species per 0.25 m2 on the species-poor area was 2.4, in contrast to the species-rich area, where it was 10.1. The soils of the species-poor area had lower pH values and contained less humus, N03--N, NH,+-N, available P, and exchangeable K, compared with those of the species-rich area. The total lead content averaged in the soils of the species-poor and species-rich areas 78,000 and 8,000 pg/g of soil, respectively, far above the 350 pg/g threshold value above which lead levels are anomalously high. The soils of both these areas would therefore normally be expected to be toxic to all but tolerant races. The average level of ammonium acetate-extractable Pb was 21,800 pg/g of soil on the species-poor area and only 311 pg/g of soil on the other area. Zinc levels were mainly lower than those of lead on both areas and the difference in the levels of total and ammonium acetate-extractable Zn between the areas was less marked than for Pb. Acid phosphatase, dehydrogenase, and urease activities and respiration (CO, evolution) were measured in soil samples taken in the root zone, 29 cm below the surface. When expressed on an air-dry soil basis, they were higher in the species-rich soil. However, when expressed on an airdry organic matter basis, the differences were reduced or eliminated (Table VIII). In each half of the terrace there were significant correlations between density of species, amounts of plant nutrients, and enzyme activities, and all were related inversely to the levels of extractable lead. The conclusion has been drawn that nutrient enrichment is involved in the formation of the species-rich area on Grassington Moor; the higher enzyme activities in the species-rich area indicated that metal detoxification was taking place there, and the higher organic matter content of this area is related to the enzyme activities. B. ENZYMOLOGICAL RESEARCHIN ROMANIA The raw and the revegetated wastes at the Sgsar mine (Baia Mare, Maramures district), the ores of which contain Pb and Zn as well as Cu, Cd, and some other heavy metals, were studied enzymologically by Soreanu (1983). An adjacent native meadow soil served for comparison. The revegetation experiment started in 1975 and comprised unfertilized and NPKfertilized mine waste plots seeded with a mixture of perennial grasses and legumes or with individual grass and legume species or sunflower. In 1980, samples were taken from the 0-20-cm depth of each plot and native soil

263

THE RECULTIVATION OF TECHNOGENIC SOILS Table VIII

Enzyme Activities and Respiration in Soils from a Limestone Terrace Contaminated by Pb- and ZnContaining Mine Waste on Grassington Maor, Yorkshire" Area Species-poor (SP) Activity or respiration

Acid phosphatase (pg of p-nitrophenol) Dehydrogenase (pg of triphenylformazan) Urease (mg of urea) Respiration at current moisture content' (mg of C) Respiration at field capacity (mg of C)

Sb 9.9 (2.7)" 38.3 (2 I .5) I .5 (0.4) 0.085'

0.067

OM' 67.4 260.4

-

Species-rich (SR) Sh

18.1 (2.2) 2503.O

10.0 0.578

6.9 (2.7) 0.093

0.456

0.090

OM'

Ratio (SWSP) S'

OM'

1.8

1.2

65.4

42.3

30.4 0.409

4.6

3.0

1.1

0.7

0.396

1.3

Ok9

79.5 11026.4

"From Clark and Clark (1981). 'Activity or respiration registered in 1 g of air-dry soil in 24 hr. 'Activity or respiration reported for I g of air-dry organic matter in 24 hr. dFor activity values, standard deviations are given in parentheses. 'For respiration values, the least significant difference is 0.016 at p < 0.05. 'Considerably less than field capacity.

for determining invertase, dehydrogenase, phosphatase, and urease activities in wastes and soil, respectively. It was found that revegetation caused an increase in each activity as compared with those measured in the raw waste, but except for urease activity, the other activities did not reach the values recorded in the native soil. The fertilized plots revegetated with the grass-legume mixture gave the best results in respect of plant cover percentage, herbage yield, and enzyme activities of wastes.

VIII. TECHNOGENIC SOILS FROM SULFUR MINE SPOILS ENZYMOLOGICAL RESEARCHIN THE USSR Peterson et al. (1976, 1979) determined the actual dehydrogenase and catalase activities, counts of microorganisms, and respiration (COz evolution) in sulfur strip mine spoils (the Podorozhnen mine, Rozdol, Ukraine), submitted to agricultural recultivation. Spring wheat, pea, a vetch-oats

264

S. KISS E T A L .

mixture, spring barley, sweetclover, and trefoil were used for recultivation in unfertilized and NPK-fertilized plots. Unfertilized spoil heaps (clays and sandy loams; pH in KCI solution was 4.6-5.6) under ruderal vegetation served as controls. The published analytical data were obtained with spoil samples collected in spring, summer, and fall during the first 3 years of recultivation (19761976). In the first year, dehydrogenase activity was lacking in the control heaps. In the recultivated plots, the activity was measurable in samples collected in summer. In the second and third years, the activity in control heaps appeared in summer and fall, whereas it was present in the recultivated plots in spring also, the highest values being found in summer. In general, the fertilizers applied in spring caused a decrease of activity in spring and an increase in summer and fall. Of the crop plants tested, sweetclover gave the best results in increasing the dehydrogenase activity of spoils. This activity was strongly related to the number of heterotrophic microorganisms growing on starch-ammonium-agar and of the oligotrophic ones growing on soil extract-agar. There was no significant correlation between dehydrogenase activity and respiration rate of the spoils and between their catalase activity and plant species or fertilization rate.

IX. TECHNOGENIC SOILS FROM LIME AND DOLOMITE MINE SPOILS ENZYMOLOGICAL

RESEARCHIN

THE

USSR

Keleberda (1973) collected samples from the G20- and 2 W - c m depths of the spoil heaps at the Ol’gin lime and dolomite quarry (Novotroitsk, Donetsk region, Ukraine). The spoils are sandy loams mixed with lime and dolomite in form of rubbles. Both humus content and H,O,-splitting (enzymatic plus nonenzymatic) capacity were higher in the upper than in the deeper layer.

X. TECHNOGENIC SOILS FROM REFRACTORY CLAY MINE SPOILS

ENZYMOLOGICAL RESEARCHIN

THE

USSR

Keleberda and Dan’ko (1975) studied the Dneprov spoil heap that resulted from the strip mining of the Chasov-Yar refractory clays (Donetsk region). The spoils are loamy sands. This spoil heap was recultivated with sweetclover (Melilotus volgicus) as a green manure plant. Uncultivated


265

plots served as controls. It was found that invertase, urease, and catalase activities and respiration (CO, evolution), like humus and total N contents, increased significantly in the 0-5- and 10-20-cm layers of the recultivated spoil heap as compared with the control plots. Invertase activity of the 20-30-cm layer was also higher in the sweetclover plots than in the controls (Keleberda, 1976), and proteinase activity also increased in the top layer of the sweetclover plots (Keleberda, 1977). Afforestation of some spoil plots in this area was performed with black locust and oleaster. It has been established (Keleberda, 1978) that after 11 years the spoil was transformed into a primitive soil, characterized by increased humus and N contents and invertase, urease, and catalase activities in its 0-20-cm layer as compared with the uncultivated control plot (Table IX). In the same area, Keleberda (1979) has also studied the influence of black locust on the development of other tree species: green ash (Fraxinus viridis), small-leaf linden (Tifiu corduta), and elm (Ufmuspinnato-rumosa). When these species were planted in rows having contact with locust, they developed better, even in the first years of their plantation, than the plants having no contact with locust. Their better development was accompanied by increased invertase, urease, and proteinase activities (Table X); humus

Table IX Humus and N Contents and Enzyme Activities in Primitive Soils under Forest Vegetation Developed on a Spoil Heap Resulting from the Strip Mining of the Chasov Yar Refractory Clays"

Plant stand and its age Black locust ( I I years)

Depth (cm)

Humus

N

N in humus

(%)

(%)

(%)

Invertase'

Ureaseh

Proteinaseh

0-5 5-10

2.23 0.87 0.60 0.54 2.61 1.03 0.46 0.33 0.98 0.67 0.57 0.49

0.23 0.11 0.04 0.03 0.41 0.17 0.05 0.03 0.08 0.06 0.02 0.02

10.3 12.6 6.6 5.4 15.7 17.0 10.8 9.0 8.1 8.9 3.5 4.0

26.40 10.00 7.15 4.20 39.30 16.70 8.31 2.32 10.80 5.30 3.25 0.20

2.02 0.82 0.49 0.42 3.92

0.76 0.35 0.26 0.20 I .09 0.81 0.22 0. I4 0.36 0.15 0.04 0

10-20 20-30

Oleaster ( I I years)

0-5 5-10 10-20 20-30

Control (uncultivated)

0-5 5-10 10-20 20-30

1.60

0.77 0.73 I .06 0.57 0.34 0.16

"From Keleberda (1978). "Invertase activity is expressed as milligrams of inverted sugar, urease activity as milligrams of NH,' N, and proteinase activity as milligrams of NH, N produced by I g of soil in 40, 40, and 72 hr, respectively.

266

S. KISS E T A L . Table X Enzyme Activities in Technogenic Soils Resulting from the Recultivation of the Chasov Yar Refractory Clay Strip Mine Spoils“

Contact with black locust

Depth (cm)

Green ash

t

Small-leaf linden

+

0-5 5-10 10-20 0-5 5-10 10-20 0-5

Tree species

5-10 -

+

Elm

10-20 0-5 5-10 10-20 0-5 5-10 10-20 0-5 5-10

Black locust

Pure stand

10-20 0-5 5-10

10-20

Invertaseh

Urease”

Proteinase”

24.8 5.1 1.3 2.9 2.3 2.1 28.9 5.5 5.4 17.9 7.2 6.3 21.9 14.7 11.7 19.8 10.0 9.6 26.4 10.0 7.2

I .97 0.78 0.52 0.72 0.56 0.47 1.79 0.66 0.63 1.10 0.64 0.64 I .79 0.71 0.65 1.14 0.67 0.63 I .89 0.74 0.50

0.69 0.32 0.09 0.02 0.07 0.02 0.68 0.32 0.24 0.51 0.10 0.03 0.68 0.3 I 0.09 0.45 0.33 0.09 0.76 0.26 0.35

“From Keleberda (1979). ”Invertase activity is expressed as milligrams of inverted sugar, urease activity as milligrams of NH,’ N, and proteinase activity as milligrams of NH2 N produced by 1 g of soil in 40, 42, and 72 hr, respectively.

levels; amounts of total and hydrolyzable N ; and mobile P and K contents of their soils (especially in the 0-5-cm layer).

XI. TECHNOGENIC SOILS FROM BENTONITIC CLAY MINE SPOILS

ENZYMOLOGICAL RESEARCHIN

THE

USSR

Daraseliya et al. (1978) applied enzymological and microbiological methods to evaluate the efficiency of the recultivation of spoils that resulted from the strip mining of bentonitic clays (gumbrine, askanite, etc.) at Gumbra (Tskhaltubo district, Georgia). The spoil heaps were reculti-


267

vated with maize (unfertilized or fertilized with 300 kg of N, 240 kg of P, and 90 kg of m a ) or with a ryegrass-alfalfa mixture. Naturally revegetated spoil heaps and an adjacent native yellow soil under forest cover were used for comparison. The results have shown that the spoils, in comparison with the original soil, are characterized by lower invertase and phosphatase activities, which are related to the reduced humus content of spoils. Owing to recultivation, the activities increased. Fertilization of the spoil heaps recultivated with maize had a beneficial effect on the enzyme activities. The highest activities were found in the spoil heaps recultivated with the ryegrass-alfalfa mixture. The activities in the 0-20-cm layer of these spoil heaps approached those measured in the native soil. In each variant, the 0-20-cm layer was more active than the 20-40-cm one. Recultivation also led to substantial increases in the number of the main groups of microorganisms in the spoil heaps. Studying the same spoil heaps, Rtskhiladze et al. (1981) found that dehydrogenase activity behaved like invertase activity.

XII. TECHNOGENIC SOILS ON SAND OPENCAST MINE FLOOR DRIFT AND SPOILS A. ENZYMOLOGICAL RESEARCHIN POLAND Hazuk (1%7) was the first to utilize enzymological methods for studying the recultivation of the floor drift of sand opencasts formerly used for the mining of filling sand (necessary for the hydraulic filling of the workings in coal mines). The study area, the Szczakova filling sand quarry, is situated in the western part of the Little Bledowska Desert (Cracow region). The alluvial “soil” of this former sand opencast consists of deep (thick), medium-grain loose sands (pH in H,O was 8.3). Only the top layer (1-10 cm) of these sands contains nutrients ( P , K), but in very small amounts. For recultivation, a fertilization experiment was carried out, in five variants. It has been established that fertilization with a sorbent based on bentonite and NPK without or with the addition of peat led to increased enzyme activities in the “soil”: invertase, urease, and asparaginase activities approximately doubled, whereas catalase activity increased twofour times as compared with the unfertilized control. These investigations were continued, developed, and described in detail by Greszta and Olszowski (1974). They studied seven experimental variants (each in four plots): 1. controls (no fertilization); 2. mineral fertilizers;

268

S. KISS E T A L .

3. humus (60 t/ha) plus mineral fertilizers; 4. sorbent fertilizer (30 t/ha); 5 . sorbent fertilizer (60 t/ha); 6. sorbent fertilizer (60 t/ha) plus peat dust (12 t/ha); and 7. cinders (60 t/ha plus mineral fertilizers). The dosage of mineral fertilizers in variants 2, 3, and 7 was: N 20, P,O, 65, and K,O 60 kg/ha. The humus used came from the caprock of the sand pit hole and consisted of the material from the humus-mineral horizon mixed with forest leafmold. The sorbent was produced from bentonite containing 73-74% clayey components (montmorillonite, kaolin, illite). The sorbent fertilizer consisted of bentonite to which lime was added in the proportion 1 part of lime per each 20 parts of clay. It was subsequently mixed with mineral fertilizers (N 4, P,O, 16.2, and K,O 24 kg/ha). The cinders came from a power plant. Following these treatments, a seed mixture of perennial plants, predominantly legumes, were sown in all the plots. Prior to sowing, the seeds of leguminous plants were treated with “Nitragina” (specific Rhizobium culture). The mixture consisted of: Lupinus luteus 100, Lupinus polyphyllus 10, Melilotus albus 10, Lotus corniculatus 5 , Trifolium repens 5, Anthyllis vulneraria 3, Festuca ovina 1, and Bromus inermis 1 kg/ha. For 2 years (1966-1967), soil samples were collected throughout the vegetative season at 3-week intervals for the determination of enzyme activities (invertase, P-glucosidase, urease, asparaginase, and catalase) and respiration (CO, evolution) rate. Counts of bacteria, actinomycetes, and fungi as well as several physical and chemical properties were also determined periodically. The results have shown that the activity of enzymes was highest in the soil of the plots with a sorbent fertilizer, especially when this was applied with added peat. In plots fertilized with cinders the activity recorded during the first year did not differ from the activity values determined from plots treated with the sorbent fertilizer, but it appeared to be lower in the second year, showing a tendency to decrease steadily. Treatment with humus containing an addition of mineral fertilizers and with mineral fertilizers alone did not cause any significant change in the activity of enzymes. The highest enzyme activities were found in the 1-5-cm layer. At the 6-10-cm depth a marked decrease in activities was observed. The results obtained for the plots with the sorbent fertilizer containing added peat indicate that it is advisable to combine mineral and organic fertilization. The results also indicate that there exists a causative relationship between the activity of soil enzymes and the herbage yield of the recultivated


269

plots. Thus, the highest enzyme activities and the best yields were recorded for plots treated with sorbent fertilizer containing added peat. This correlation seems to be the result of a stimulating effect of the applied fertilizer treatment on the metabolism of soil microorganisms expressed by the activity of the enzymes studied, due to which the growth of the plants was better. Another possible explanation is that the sorbent fertilizer retained more available nutrients, so the plants grew better and their residues stimulated the microorganisms. The highest rate of CO, evohtion and the largest number of microorganisms, especially bacteria and fungi, were recorded also in the soil of plots treated with the sorbent fertilizer containing an addition of peat. In a recultivation experiment started in 1977 on the area of an exhausted sand mine in Cheszczdwka, dehydrogenase and urease activities in the technogenic soil formed were analyzed in 1984 and it was found (Zukowska-Wieszczek et al., 1985) that the enzyme activities like the biomass of grasses were highest in the experimental variants treated with clay plus sewage sludge, fly ash plus sewage sludge, and waste fungal mycelia from pharmaceutical plants. The variants treated with sewage sludge alone, municipal refuse, and compost were less efficient.

B. ENZYMOLOGICAL RESEARCHIN THE USSR Enzymological and microbiological study of the spoils resulted from the opencast mining of quartz sand in the Chiatura district (western Georgia) and recultivated with grapevine, fruit trees, forest trees, and perennial grasses has proved (Rtskhiladze et af., 1981) that dehydrogenase and invertase activities and counts of microorganisms from different physiological groups can be used as indicators of the efficiency of the recultivation measures applied.

XIII. TECHNOGENIC SOILS FROM OVERBURDENS REMAINING AFTER PIPELINE CONSTRUCTION A. ENZYMOLOGICAL RESEARCHIN THE USSR The enzymological study of the recultivation of overburdens remaining after pipeline construction was initiated by ldrisova (1984). In 1981, she modeled, in laboratory and field, a recultivation technology based on the

270

S. KISS E T A L .

spreading of overburdens on the surface of adjacent agricultural fields, in different proportions: overburdens (calcareous loam) remaining after the construction of 100 m of pipeline (diameter 1420 mm) were spread on 0.5, 1, 1.5, and 2 ha of agricultural field (leached chernozem on the foreststeppe of the Bashkir Pre-Urals). The control field was not treated with overburdens. Analysis of the ploughed (0-30-cm) layer of soil showed that catalase activity was practically unaffected by overburdens. In contrast, invertase and phosphatase activities and respiration (CO, evolution) rate of soil decreased in parallel with the diminution of the field surface on which the overburdens were spread. Thus, the maximum values were found in the control soil and the minimum ones in the field with the smallest surface (0.5 ha) on which the overburdens were broadcast. The contents of humus, mineral N, mobile P, and exchangeable K in soil as well as crop yields (in 1981: vetch-oats mixture; in 1982: winter rye) decreased in a similar manner. The decrease in crop yields was lower in the plots fertilized with 40 t of farmyard manure plus 90 kg of N as NH,NO, plus 60 kg of P as double superphosphate plus 45 kg of K as KCl/ha than in the unfertilized plots. In addition to the technology of spreading overburdens on soil surface, in 1981 Ishem’yarov ef al. (1984) and Idrisova et al. (1986a,b, 1987) applied, under both laboratory and field conditions, another technology: mixing of the ploughed soil layer with overburdens in the following proportions: 87.5, 75.0, 62.5, 50.0, 37.5, 25.0, and 12.5% soil plus overburdens up to 100%.

As expected, in the soil-overburden mixtures, in comparison with the control soil, catalase, invertase, phosphatase, and urease activities and the contents of humus, mineral N, mobile P, and exchangeable K decreased with increasing proportions of overburdens. In this case, too, some plots were fertilized with the same amounts of farmyard manure plus NPK as specified above. Other unfertilized plots served for comparison. In the 1981-1984 period, the crop yields were estimated in a rotation composed of vetch-oats mixture, winter rye, spring wheat, and maize. In the soiloverburden mixtures, the crop yields decreased, mainly in the unfertilized plots. Comparison of crop yields after application of the two recultivation technologies (spreading or mixing of overburdens on or with soil, respectively) indicated that diminution of crop yields was lowest with the spreading technology applied in association with organic and mineral fertilization. In addition, the cost of recultivation was also lower when this technology was used. At the same time, it has been emphasized that the ratio between the amounts of overburden and soil (ploughed layer) should never exceed 1:1.


27 1

B. ENZYMOLOGICAL RESEARCHIN THE UNITED STATES The work edited by Redente and Cook (1986) contained information concerning revegetation of soils disturbed by pipeline construction in the Piceance Basin, northwestern Colorado. In 1985, three high-elevation sites (2250 m) and a low-elevation site (2040 m) disturbed by pipeline construction 2-27 years ago were compared with undisturbed native controls. The soils were sampled from the 5-10-cm depth. The data included in Table XI indicate distinct direct relationships between i.icreases in soil organic matter, mineralizable N, and phosphatase activity that occurred in relation to plant community age. In contrast, dehydrogenase activities showed a tendency to decrease in the older sites, which may be due to the fact that some of the areas that were sampled have been heavily grazed.

Table XI Influence of Revegetation on Some Parameters of Sites Disturbed by Pipeline Construction as Compared to Native Sites in the Piceance Basin, Colorado’

High-elevation sites Disturbed (years ago)

Low-elevation sites Native

Disturbed

Native

(23 years ago)

Parameters

2

4

27

PH Organic matter (%) Phosphatase activity (pg of p-nitrophenoll g of soil/hr) Mineralizable N (wg of NH,’ N/g of soil) Deh ydrogenase activity (pg of triphenylformazan/g of soil in 24 hr) Actual (autochthonous) Potential (zymogenous) Litter cover (%) Total plant cover (%)

8.36 0.95 193

7.72 1.49 467

7.35 1.04 690

6.70 2.66 781

8.54 0.53 70

7.13 0.93 377

47.4

63.7

62.7

91.9

29.2

46.1

37.1

17.7

11.5

13.6

13.7

3.8

25.3

25.9

9.8

15.4

13.9

8.0

2 19

0 20

63 67

6 38

“From Redente and Cook (1986).

47 60

4 20

272

S. KISS E T A L .

XIV. RECULTIVATION OF SOILS REMAINING AFTER TOPSOIL “MINING” ENZYMOLOGICAL RESEARCH IN NEWZEALAND Topsoil is “mined”, i.e., removed, around many urban areas for use in landscape improvement. Recultivation of the remaining soil for restoring its fertility is required with the same emphasis as the recultivation of wastelands resulted from mining or other industrial activities. This problem was enzymologically studied by Ross et al. (1982). The soil studied, the Judgeford silt loam (Wellington area), was originally under grazed grassclover pasture, then used for topsoil “mining”, removal to depths of 10 cm (S,, plots) and 20 cm (S,, plots) in March 1978. The remaining soil was treated with lime and urea and with high amounts of P and K fertilizers, and resown in pasture species. Plots without topsoil removal (So) served for comparison. Between March 1978 and March 1981, seven enzymatic activities and several other biochemical as well as chemical and physical properties were determined periodically in the 0-10-cm layer of all the plots. Herbage yield was also recorded and taken as the criterion for soil fertility. The herbage yield and enzyme activity values obtained are presented in Fig. 3. They show that these values increased rapidly in the S,, plots but had not reached So levels after 3 years. Similar results were obtained in the measurements of CO, and mineral N production, biomass C, mineral N flush, and ATP. Organic C and total N contents increased only slowly. Enzymatic and other biochemical activities were, in general, significantly correlated with herbage yields in the S,, or S,, plots, but not in the So plots; organic C and total N contents were not generally correlated significantly with yields over the first 2 years. Overall, invertase, and then sulfatase, activity appeared to be the best indicators of soil fertility status in the stripped (“mined”) soil studied.

XV. CONCLUDING REMARKS The literature reviewed shows that application of enzymological methods makes it possible to indicate the degree of evolution of technogenic soils, the transformation of overburdens and other spoils and wastes into agricultural or forest soils, the efficiency of the recultivation measures applied. In comparison with microbiological parameters, the enzymes are


30

273

Urease

P

'2.

0 -0

> .-c > .-c

2

10

y" 2

0

0 lnvertase 0 1 Y

I

;250k

Amylase

.

^^^

loool /--

50

H

I

2000

I

I

0

.750

Phosphatase

v)

P 0

-k

I

I

I

1

A-1

I

I

1978 I1979 I198011981

1978 11979 I1980 11981

.-2

-3

FIG.3. Herbage yield and enzyme activities from plots of Judgeford soil that had been stripped of 0, 10, and 20 cm of the original topsoil in March 1978, reestablished in a grassclover pasture, and sampled periodically over the following 3 years. (1) Soplots (no topsoil removed); (2) S,, plots (10 cm of topsoil removed); (3) S,, plots (20 cm of topsoil removed). Herbage yield is expressed on a dry matter basis. Enzyme activities are given as pmoles of product released by 1 g of soil per second (products-urease: NH,' N; invertase, amylase, and cellulase: "glucose"; xylanase: "xylose"; phosphatase and sulfatase: p-nitrophenol). (Redrawn from Ross et al., 1982.)

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S. KISS ET AL.

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Persson, T., and Funke, B. 1986. Proc. North Dakota Acad. Sci. 40, 122. Peterson, N. V., Kurylyak, E. K., and Panas, R. N. 1976. Tez. Dokl. Vses. Soveshch. po Fizio1.-Biokhim. Osnovam Vzaimodeistviya Rastenii v Fitotsenozakh, 4th (Kiev) pp. 117-1 18. Peterson, N. V., Kurylyak, E . K., and Dubkovetskii, S. V. 1979. Mikrobiol. Zh. (Kiev)41, 129- 134. Ras’kova, N. V., Gel’tser, Yu. G., Tsvetkova, L. A., and Trofimov, S. Ya. 1984. Tez. Dokl. Vses. Shk. Vliyanie Promyshlennykh Predpriyatii na Okruzhayushchuyu Sredu (Zvenigorod) pp. 157-158. Redente, E. F., and Cook, C. W. (eds). 1986. “Structural and Functional Changes in Early Successional Stages of a Semiarid Ecosystem.” Dept. Range Sci., Colorado State Univ., Fort Collins. Ross, D. J., Speir, T . W., Tate, K. R., Cairns, A., Meyrick, K. F., and Pansier, E. A. 1982. Soil Biol. Biochem. 14, 575-581. Rtskhiladze, T. G., Rostiashvili, K. A., and Lapanashvili, E. F. 1981. Tez. Dokl. Deleg. S’ezda Vses. Obshch. Pochvov., 6th (Tiblis) 2, 189. Schroder, D. 1986. Z. Kulturtech. Flurberein. 21, 318-325. Schroder, D. 1988. Z. Pjlanzenernaehr. Bodenkd. 151, 3-8. Schroder, D., Stephan, S., and Schulte-Kamng, H. 1985. Z. Pjlanzenernaehr. Bodenkd. 148, 131-146. Schroder, D., Haubold, M., and Henkes, L. 1987. Landw. Z. (Bonn), 154, 1466-1469. Shugalei, L. S., and Yashikhin, G. I. 1985. In “Biologicheskaya Aktivnost’ Lesnykh Pochv” (V. M. Korsunov, ed.), pp. 80-88. Inst. Lesa i Drevesiny Sib. Otd. Akad. Nauk SSSR, Krasnoyarsk. Shugalei, L. S., Yashikhin, G. I., and Korsunova, T. M. 1984. Tez. Dokl. Vses. Shk. Vliyanie Promyshlennykh Predpriyatii nu Okruzhayushchuyu Sredu (Zvenigorod) pp. 224226. Shugalei, L. S., Korsunova, T. M., and Yashikhin, G. 1. 1985. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Biol. Nauki (13/2), 7 6 7 6 . Soreanu, 1. 1983. Bul. Stiiny. Inst. Invdtrimint Sup. Baia Mare, Ser. B 6, 93-98. Sorensen, D. L., Klein, D. A., Ruzzo, W. J., and Hersman, L. E . 1981. J. Environ. Qual. 10, 369-37 I. Stroo, H. F., and Jencks, E. M. 1982. Soil Sci. Soc. Am. J . 46,548-553. Stroo, H. F., and Jencks, E. M. 1985. J. Environ. Qual. 14, 301-304. Sulyok, L., and Voros, 1. 1983. In “Recultivation of Technogenous Areas” (J. Szegi, ed.), pp. 207-212. Mhtraalja Coal Mining, Gyongyos. Sviridova, 1. K., and Panozishvili, K. P. 1979. Tez. Dokl. Vses. Soveshch. Biol. Produktivnost’ Pochv i Ee Uvelichenie v Interesakh Narodnogo Khozyaistva (Moscow) pp. 141-142. Szegi, J. 1983. In “Recultivation of Technogenous Areas” (J. Szegi, ed.), pp. 105-1 11. Matraalja Coal Mining, Gyongyos. Trofimov, S. S., Naplekova, N. N., Kandrashin, E. R., Fatkulin, F. A., and Stebaeva, S. K. 1986. “Gumusoobrazovanie v Tekhnogennykh Ekosistemakh.” Nauka, Sib. Otd., Novosibirsk. Tsvetkova, L. A., Ras’kova, N. V., and Gel’tser, Yu. G. 1982. Mater. Vses. Simp. Mikroorganizmy kak Komponent Biogeotsenoza (Alma-Ata) pp. 109-1 10. Uzbek, I. Kh. 1986. Tez. Dokl. S’ezdu Pochvov. Agrokhim. Ukr.SSR, 2nd (Kharkov) p. 112.

Verbin, A. E., and Keleberda, T. N. 1974. Pochvovedenie (2), 116-120. Williams, S. T., McNeilly, T., and Wellington, E. M. H. 1977. SoilBiol. Biochem. 9, 271275.

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EFFECTS OF NITRIFICATION INHIBITORS ON NITROGEN TRANSFORMATIONS, OTHER THAN NITRIFICATION, IN SOILS' K. L. Sahrawat International Crops Research Institute for the Semi-Arid Tropics ICRISAT Patancheru P.O. Andhra Pradesh 502 324, India

I. Introduction 11. Effects of Nitrifcation Inhibitors on Physical and Chemical Processes Relevant to Nitrogen Transformations A. Transport and Movement of Nitrogen B. Ammonium Fixation and Release C. Ammonia Volatilization 111. Effects of Nitrification Inhibitors on Biological Nitrogen Transformations A. Mineralization and Immobilization B. Denitrification C. Nitrous Oxide Emission via Nitrification and Denitrification D. Urea Hydrolysis IV. Other Effects V. Perspectives References

1. INTRODUCTION Interest in nitrification inhibitors stems from the fact that retardation of nitrification reduces loss of nitrogen by leaching and denitrification following nitrification. This helps in some situations to achieve more efficient use of nitrogen for crop production and may also help in minimizing fertilizer nitrogen-related environmental stresses, especially accumulation of nitrate in surface and ground waters. Nitrification is generally used to mean biological oxidation of ammonium to nitrate via nitrite effected, respectively, by Nitrosomonas and Nitrobacter species of nitrifying bacteria, although nitrification inhibitors are defined as compounds or materials that specifically retard the oxidation of ammonium to nitrite without 'Approved for publication as ICRISAT Journal Article No: 705. 279 Copyright 0 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.

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affecting the subsequent oxidation of nitrite to nitrate. Inhibition of nitrification is referred to as retardation of nitrification because complete inhibition is seldom achieved with the use of nitrification inhibitors. The literature on nitrification inhibitors is very extensive (e.g., see Gasser, 1970; Prasad et al., 1971; Hauck, 1972, 1983,1984; Huber et al., 1977; Meisinger et al., 1980; Sahrawat, 1980, 1986; Hauck and Behnke, 1981; Mulvaney and Bremner, 1981; Slangen and Kerkhoff, 1984; Sahrawat and Keeney, 1984, 1985; Amberger, 1986). These reviews cover various aspects of the effects of nitrification inhibitors on (i) retardation of nitrification in soil, and (ii) crop production and some aspects of crop quality (e.g., see Sahrawat and Keeney, 1984). The interest in nitrification inhibitors followed the development of nitrapyrin [2-chloro-6-(trichloromethyl) pyridine] by the Dow Chemical Company of the United States as an effective inhibitor of nitrification (Goring, 1962a,b). Research has suggested that in addition to retarding nitrification, nitrification inhibitors may affect certain other processes of the nitrogen cycle in soils such as mineralization-immobilization, nitrous oxide production, ammonia volatilization, and denitrification (e.g., see Table I). The capacity of nitrification inhibitors to affect these processes depends on their bioactivity in soil, which is affected by soil texture, temperature, and the amount of inhibitor added. The half-lives of nitrification inhibitors such as nitrapyrin may vary from a few days to several weeks depending on the rate of application, soil type, and season (temperature) (e.g., see Meisinger et al., 1980; Sahrawat, 1980). This review summarizes the literature on the effects of nitrification inhibitors on nitrogen transformations other than nitrification in soil and identifies future directions for research. This field of research is developing in importance because of increasing interest in the use of these chemicals.

II. EFFECTS OF NITRIFICATION INHIBITORS ON PHYSICAL AND CHEMICAL PROCESSES RELEVANT TO NITROGEN TRANSFORMATIONS Since retardation of nitrification increases the persistence of ammonium in soils, it must be expected that retardation of nitrification affects ammonium nitrogen transformation processes such as fixation or adsorption and volatilization in some situations. Also, retardation of nitrification may result overall in less movement and transport of mineral nitrogen because of higher NHJNO, ratios in soils caused by retardation of nitrification (e.g., see Sahrawat and Keeney, 1984).

EFFECTS O F NITRIFICATION INHIBITORS

28 1

Table I Recent References on the Effects of Nitrification Inhibitors on Nitrogen Transformations, Other Than Nitrification, in Soils

Aspect of N transformation processes

References

Physical and chemical processes Huber et a/. (1969); Keeney et a/. (1979); Owens (1981); Papendick and Engibous (1980); Hergert and Wiese (1980); Onken (1980); Timmons (1984) Gin et a/. (1982); Juma and Paul (1983); Aulakh and Ammonium fixation and release Rennie (1984) Cornforth and Chasney (1971); Bundy and Bremner Ammonia volatilization (1974); Smith and Chalk (1978, 1980); Jain e r a / . (1981); Rodgers (1983); Simpson et a/. (1985); Magalhaes and Chalk (1987); Prakasa Rao and Puttanna (1987)

N transport and movement

Mineralization and immobilization Denitrification

Nitrous oxide production

Urea hydrolysis

Biological processes Dubey and Rodriguez (1970); Laskowksi et a / . (1975); Malhi and Nyborg (1979a, 1983); Juma and Paul (1983, 1984); Aulakh and Rennie (1984) Mitsui et a / . (1964); Sandhu and Moraghan (1972); Henninger and Bollag (1976); McElhannon and Mills (1981); Notton er a/. (1979); Yeomans and Bremner (1985a,b); Bremner and Yeomans (1986); Mills (1984); Mills and McElhannon (1984) Bremner and Blackmer (1978, 1979); Freney el a/. (1979); Smith and Chalk (1978, 1980); Bremner er a/. (1981); Aulakh ef a / . (1984); Magalhaes er a / . (1984); Yeomans and Bremner (1985a,b); Casella et a / . (1986); Bremner and Yeomans (1986); Davidson et a/. (1986); Magalhaes and Chalk (1987) Goring (1962b); Bremner and Douglas (1971); Bundy and Bremner (1974); Bremner and Bundy (1976); Reddy and Prasad (1975); Ashworth et a/. (1977, 1979, 1980); Amberger and Vilsmeier (1979); Guthrie and Bomke (1981); Rodgers (1983); Sahrawat (1979a,b); Mishra and Flaig (1979); Mishra et a/. (1980); Lethbridge and Burns (1976); Malhi and Nyborg (1979b); Coos (1985)

A. TRANSPORT AND MOVEMENTOF NITROGEN Keeping nitrogen in the ammonium form by retarding nitrification reduces movement of mineral nitrogen because (i) ammonium is retained by soil particles and thus is less mobile and (ii) if less nitrate is formed, this results in reduced amounts of nitrate N leached. For example, in a

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K. L. SAHRAWAT

3-year study Huber et al. (1969) showed that in the field inhibition of nitrification of fall-applied ammonium sulfate with nitrapyrin (0.56 kg/ha) prevented the movement of applied nitrogen below a depth of 30.5 cm (Table 11). These results show that the inhibition of nitrification reduced the amounts of nitrate formed and its subsequent leaching in the soil profile. A greater proportion of mineral N was present in the ammonium form in the inhibitor-treated plots. Keeney et al. (1979) found that nitrapyrin inhibited nitrification of ammonium in a loamy sand soil and also reduced the amounts of nitrate leached in soil columns over 2-5 weeks. However, by 20 weeks, the period for which the experiment was run, the amounts of nitrate leached were similar in soil columns with and without nitrification inhibitor treatment. This might have been due to degradation of nitrapyrin. Similarly, Owens (1981) showed that nitrapyrin reduced the amounts of mineral N (mostly nitrate) leached in 1-m-long soil columns. After 91 days, 1.0 and 9.7% of the applied urea nitrogen had leached from nitrapyrin-treated and untreated soil cores; however, after 144 days, 41.9 and 53.0%, respectively, of applied N had leached. Studies in the United States on the effect of nitrapyrin on transformations and movement of fertilizer nitrogen in soils indicated that in some situations it reduced the movement of fertilizer nitrogen over winter o r during irrigation (Hergert and Wiese, 1980; Onken, 1980; Papendick and Engibous, 1980; Timmons, 1984). In a 3-year field lysimeter study, Timmons, (1984)

Table I1 Extractable Ammonium and Nitrate N Content (kg/ba) in Southwick Silt Loam Soil in the Spring Following Fall N Fertilization with and without Nitrapyrin"'*

NO, N at depth (cm) Treatment

NH4 N at depth (cm)

0.0-30.5

30.5-61.0

Total

30.5-61.0

Total

17.3 21.2

18.9 18.9

36.2 40. I

2.9 8.5

8.3 5.4

11.2 13.9

19.3 35.3

22.5 20.7

41.8 56.0

11.3 8.8

7.6 8.1

18.9 16.9

28.7

22.2

50.9

27.6

9.2

36.8

0.0-30.5

~~

Control Calcium nitrate Calcium nitrate plus nitrapyrin Ammonium sulfate Ammonium sulfate plus nitrapyrin

"From Huber et a / . (1969). The study was conducted in the fall of 1965, 1966, and 1967. Data are average of 3 years. bNitrogen was applied at the rate of 67.5 kg N/ha in the fall of 1965 and 1966, and 84 kg N/ha in the fall of 1967. Nitrapyrin was added annually at a rate of 0.56 kg/ha.


283

found that nitrapyrin application with urea reduced the 10s of NO, N leached at the 1.2-m depth in soil planted to corn (Zea mays L.) (Table 111). In a 6-year lysimeter study, Owens (1987) found that nitrapyrin (1.12 kg/ha) application with urea (336 kg N/ha) reduced the loss of inorganic N in percolation water from Rayne silt loam (fine loamy, mixed, mesic; Typic Hapludult) planted to no-tillage corn. The average annual N loss by leaching in the untreated lysimeter was 160 kg N h a , which was reduced to 117 kg N/ha by nitrapyrin application. Nitrapyrin was found to be effective in reducing the leaching loss of inorganic N in spring, summer, autumn, and winter (Table IV).

Table 111 Leaching Loss of Nitrate N in a Field Lysimeter Sandy Loam Soil (Typic Hapludoll) Fertilized with Urea, with and without Nitrapyrin" _

_

~

Treatmentb Year 1977 1978 1979 Average I977 1978 1979 Average

Urea

Urea plus Nitrapyrin

Percolation (mm) 337 233 218 263

313 236 234 26 1

NO, N leached (kg/ha) I94 I48 161 157 127 142 161 I49

Flow-weighted NO, N Conc' (mmoVliter) 1977 4. I 3.4 1978 4.9 4.7 1979 4.2 4.3 Average 4.4 4.1 "From Timmons (1984). Each value is an average of three replications measured at 1.2-m depth. "Lysimeters were fertilized with 224 kg urea N/ha before planting to corn and nitrapyrin was added at the rate of 0.56 kg/ha. 'Flow-weighted concentration is total NO, N leached divided by total water percolated and converted to mmoVliter.

284

K. L. SAHRAWAT Table IV

Effect of Nitrapyrin on Nitrogen Loss in Percolation Water from Rayne Silt Loam (Typic Hapludult) in Lysimeters Fertilized with Urea and Planted to No-Tillage Corn"

Inorganic N lossb (kg Nlha) ~

Lysimeter

Treatment

Spring

Summer

Autumn

Winter

Annual

A

Urea Urea plus nitrapyrin Urea plus nitrapyrin

66.6 5 1.7 49.3

12.0 10.2 9.8

18.8 12.6

62.6 45.2 40.9

160.0 119.7 114.0

B C

14.0

"From Owens (1987); results presented are averages of 6 years' data, 1978-1984. Urea was applied at a rate of 336 kg N/ha and nitrapyrin at the rate of 1.12 kg/ha annually. 'Spring, April-June; Summer, July-September; Autumn, October-December; Winter, January-March.

B. AMMONIUM FIXATION AND RELEASE The retardation of nitrification can enhance immobilization of fertilizer nitrogen because the persistence of ammonium increases (i) its incorporation in the organic nitrogen fraction or (ii) its migration to fixed or nonexchangeable sites on clay minerals. For example, Juma and Paul (1983) found that under field conditions treatment of "N-aqueous NH, and "Nurea with a nitrification inhibitor, ATC (4-amino-l,2,4-triazole), caused enhanced recovery of fertilizer N in the soil surface layer (52-55% versus 28-30%). Between 5 and 8% of the fertilizer N was recovered in the nonexchangeable ammonium form in the A horizon of the soil treated with ATC as opposed to about 1% in the non-ATC treatments (Table V). Laboratory study these soil samples further revealed that the nonexchangeable "NH, was released at rates equivalent to a half-life of 38 weeks and the rate constant was 0.0Wweek at 28 ? 1°C at a soil water potential of - 34 kPa (kPascal). The clay fraction of the soil, consisting of mica, vermiculite, and smectites, contained 49% of the labeled nonexchangeable NH, whereas the coarse silt fraction accounted for 26% of the labeled nonexchangeable NH,. Aulakh and Rennie (1984) showed that nitrapyrin did not increase the fixation of NH, initially, but the release of recently fixed NH, was decreased and delayed by nitrapyrin application in a 2-year study of fallapplied "N-labeled urea in Canadian chernozemic soils (Typic Udic Haploborrolls) (Table VI). In another experiment, nitrapyrin application significantly increased the amount of fertilizer urea recovered as fixed NH, after 8 months of application to a clay loam soil (see Table XV). In some situations the changes in the amounts of fixed NH, could influence N loss and availability to plants.

285

EFFECTS O F NITRIFICATION INHIBITORS Table V

Percentage Recovery of "N-Labeled Aqueous Ammonia and Urea in a Loam Soil (0.43%total N, pH 7.4) with and without ATC Nitrification Inhibitor after Harvest of the Wheat Crop''b

"N recovered in soil depth

Treatments NH, OH

0-15 cm 15-30 cm 30-60 cm Total Nonexchangeable NH,'

+ ATC

NH, OH

28 10 2

56 3

40 1

60 5

Urea 31 10 2 43 I

1

Urea

+ ATC 52 2 I 55 8

"From Juma and Paul (1983). bThe fertilizers were added at a rate of 56 kg N/ha and the inhibitor at a rate of 4% of fertilizer N. The "N excess of each fertilizer was 5.6% 'Nonexchangeable NH, expressed as percentage of the remaining ''N.

C. AMMONIAVOLATILIZATION Retardation of nitrification in soil results in accumulation of ammonium and higher soil pH (Cornforth and Chasney, 1971; Hauck and Bremner, 1969; Bundy and Bremner, 1974; Smith and Chalk, 1978; Magalhaes and Chalk, 1987), which are conducive t o ammonia volatilization. In fact, Cornforth and Chasney (1971) showed in the field that application of AM (2-amino-4-chloro-6-methyl pyrimidine) nitrification inhibitor with ammonium sulfate (168 kg N h a ) increased the ammonia loss by volatilization Table VI Changes in Ammonium Fixation (kg N/ha) of Fall-Applied Urea in the Soil Profile to 30-cm Depth without and with Nitrapyrin Applied to Baline Lake Clay Loam Soil"

Treatmentb Sampling date

Urea

30 September 1981 20 October 1981 I December 1981 23 March 1982 27 April 1982 27 May 1982

10.6 8.6 9.6 6.9 6.2 I .o

Urea

+ Nitrapyrin 10.8 9.7 9. I 7.4 8.1 4.6

"From Aulakh and Rennie (1984). "Urea was applied at a rate of 100 kg N/ha and nitrapyrin at a rate of 1% of fertilizer N on 30 September 1981.

286

K. L. SAHRAWAT

from bare soil. The inhibitor increased by nearly eightfold the amount of ammonia volatilized from grass-covered soils in comparison with the control during 28 days of study, and nearly 22 kg N/ha was lost as ammonia. Less ammonia was lost when unamended ammonium sulfate or urea was applied to grass rather than to bare plots. Bundy and Bremner (1974) showed that nitrapyrin [2-chloro-6-(trichloromethyl) pyridine], ATC (4-amino-l,2,4-triazole) and CL-1580 (2,4-diamino-6-trichloromethyl-s-triazine) nitrification inhibitors retarded nitrification of urea in soil but increased the volatile loss of ammonia from soils in a laboratory study (Table VII). However, it should be mentioned that these losses were experienced when a sandy clay loam soil was treated with a relatively high rate of urea (400 pg N/g soil). This study, nevertheless, indicates the potential of high loss due to ammonia volatilization when nitrification inhibitors in conjunction with urea are surface-applied to coarse-textured calcareous soils. The increased ammonia volatilization from soils treated with nitrification inhibitors was due to the persistence of ammonium and higher soil pH (Table VIII), which created a soil environment conducive to ammonia volatilization. In another laboratory study, Rodgers (1983) determined the loss by ammonia volatilization from three soils fertilized with urea prills or urea prills containing 7% by weight of DCD (dicyandiamide), a nitrification inhibitor. It was found that the volatile loss of ammonia was less when urea or urea and DCD was incorporated than when it was applied to the surface. Soil type influenced the volatile loss of ammonia during 4 weeks of testing. The volatile loss of ammonia from a soil that did not nitrify was not affected by DCD application but volatilization was increased in the two other soils (Table IX). In general, the soils were quite slow in nitrification, and by Table VII

Effects of Three Nitrification Inhibitors on Nitfieation and Volatile Loss of Ammonia from a Sandy Clay Loam Soil (pH 7.2; organic C 1.65%)at 14 Days of Incubation‘** Inhibitor

Inhibition of nitrification (%)

Volatile loss as ammonia (% of urea N added)

None Nitrapyrin ATC CL-1580

94 92 88

9 34 30 28

“From Bundy and Bremner (1974). bSoil samples (10 g) were treated with 4 mg of urea N and with 0 or 100 pg of nitrification inhibitor and incubated at 30°C and 60% WHC (water holding capacity) moisture.

EFFECTS OF NITRIFICATION INHIBITORS

287

Table VIIl Effect of Nitrapyrin on Soil pH in a Sandy Clay Loam Soil Treated with UreaEvb

Soil pH (1:2.5 H,O)

Time (days)

-

With nitrapyrin

Without nitrapyrin

0 2 4 6 8 10 12 14 21

7.2 8.2 8.2 8.2 8.1 8.0 8.0 8.0 7.5

7.2 8.0 7.3 6.3 6.2 6.1 6.2 6.2 6.2

“From Bundy and Bremner (1974). ’Soil samples (I0 g) were treated with 4 mg of urea N and with 0 or 100 pg of nitrapyrin and incubated at 30°C and 60% WHC moisture.

4 weeks only 1-21% of the urea N added was recovered as nitrate N in soil samples not treated with DCD. The effects on ammonia volatilization due to retardation of nitrification in this study are not as dramatic as those obtained by Cornforth and Chasney (1971) in the field and Bundy and Bremner (1974) in the laboratory. These differences are probably due to the difference in nitrifying capacity of soils and persistence of ammonium in soil samples with and without the nitrification inhibitor treatment. Smith and Chalk (1978) found that in a calcareous soil treated with ammonia, nitrapyrin application only slightly increased the volatile loss of ammonia in 28 days. The volatile loss of ammonia amounted to 86 and 92 pg/g soil in treatments without and with nitrapyrin when the soil was fertilized with 1127 kg/g ammonia N. The pH of the nitrapyrin-treated soil was higher, as was the extractable NH, N, and nitrification was at a low ebb (Table X). The losses due to ammonia volatilization by retardation of nitrification were similar and small in the studies reported by Rodgers (1983) and Smith and Chalk (1978) although they used high rates of urea application (Tables IX and X). This could additionally be due to the different method of urea application used by these researchers (soil incorporation) as opposed to Bundy and Bremner (1974). Also, Rodgers (1983) used urea prills and Bundy and Bremner (1974) applied urea solution to the soil surface, and this might have affected urea hydrolysis and subsequent nitrification. As

K . L. SAHRAWAT

288

Table IX Effect of Dicyandiamide (DCD) on Urea Transformations in Three Soils"*b

Treatment Form of urea N recovered

Soil ~

~~

Urea

Urea

+ DCD

~

Rothamsted (PH 5.2)

Saxmundham (PH 7.7)

Woburn (PH 5.4)

Urea N

0.0

0.0

NH4 N NO2 N NO3 N NH3 N Urea N

17.8 0.0 I .o 15.4 0.0

78.2 0.0

NH, N NO, N NO, N NH, N Urea N

72.4 2.6 20.9 9.2 0.0

74.9 0. I 2.1 11.8 0.0

NH, N NO, N NO3 N NH, N

56.2 0.8 16.0 31.2

58.9 0.0 2.0 31.3

0.5.

14.6 0.0

"From Rodgers (1983). %oil samples (50 g) were treated with 50 mg urea N or urea containing 7.2% by weight DCD and incubated at 30°C under aerobic conditions for 4 weeks.

Table X Effects of Nitrapyrin on Inorganic N and Gaseous N Evolution ( p g N/g soil) from a Calcareous Soil (pH 8.5, organic C 1.3%)Treated with Ammonia"'*

Inorganic N (28 days)

Gaseous N evolved (28 days)

Treatment

Soil pH

NH4+

NO,-

NO,-

N2

N20

No nitrapyrin Nitrapyrin

7.8 8.2

792 1012

70 44

154 0

76 13

57 0

NO

+ NOz 9 1

NH3 86 92

"From Smith and Chalk (1978). "Soil samples were incubated at 30°C and 0.33 bar soil water potential after treatment with I127 pg ammonia N/g soil, and 0 t o 10 pg nitrapyridg soil.

289


mentioned earlier, the soils in these studies differed greatly in their capacity to produce nitrate from hydrolyzed urea. Simpson er al. (1985) studied the effects of phenylphosphorodiamidate (PPD), a urease inhibitor, and dicyandiamide, a nitrification inhibitor, on nitrogen losses, transformations, and recovery of nitrogen, when urea was applied to a flooded rice field. It was found that although PPD delayed urea hydrolysis and decreased loss via ammonia volatilization, DCD, the nitrification inhibitor, had no significant effect on nitrate concentrations in the flood water and ammonia loss. Of the 80 kg of urea N added, 20.6% was lost through ammonia volatilization from the control, followed by 18.8%from the urea plus DCD treatment, and 12.5% from the urea plus PPD treatment during the I I days after application of the fertilizer (Table XI). These results show that DCD was not effective in inhibiting nitrification in the flooded soil, in contrast to its effectiveness as a nitrification inhibitor in aerobic soils (Amberger, 1986). The pattern of ammonia loss from the urea plus PPD treatment was very different from that of the Table XI Effects of DCD and Phenylphosphoradiamidate (PPD) on Ammonia Volatilization Losses (kg N/ha/day) from Flooded Clay Soil (Pelloxerert, pH 8.2Fb

Treatment Days after urea application

Urea

0 I 2 3 4 5 6 7 8 9 10 I1 Total loss Loss as % of

0.11 2.70 3.00 3.67 1 .OO 1.08 1.30 1.12 0.49 0.77 0.86 0.39 16.49 20.6

Urea

+ DCD

0.28 2.24 1.32 2.38 I .06 0.96 1.12 1.47 1.11 1.14 1.43 0.56 15.07 18.8

Urea

+ PPD

0.00 0.07 0.05 0.22 0.20 0.50 0.94 1.88 I .74 I .35 1.81 1.21 9.97 12.5

applied N "From Simpson er a / . (1985). "Prilledurea applied at the rate of 80 kg N h a by uniformly broadcasting into the flood water. DCD was added at the rate of 10% urea N and PPD at the rate of 1% of urea N (w/w).

290

K. L. SAHRAWAT

control in that the losses were small in the beginning during the first 6 days but increased during the last 5 days. Prakasa Rao and Puttanna (1987) conducted laboratory and field experiments to study the effects of DCD on nitrification and ammonia volatilization from a sandy loam soil (pH, 7.3; organic C, 0.5%) fertilized with urea. It was found that in laboratory experiments 15 or 20 mg/kg of DCD effectively retarded the nitrification of urea but increased the volatile loss of ammonia and also extended the period of ammonia emission. In the field experiment, DCD (15 or 20% of urea N) application greatly increased the loss via ammonia volatilization when DCD-treated urea was surface-applied; however, the loss was minimized when the inhibitortreated urea was applied at a 5-cm depth. More than 30% of the applied urea (187 kg N/ha) was lost as ammonia when DCD-amended urea was applied to the soil surface, but this was decreased to less than 5% when DCD-amended urea was placed at a 5-cm depth in the soil. Placement of unamended urea also greatly reduced the volatile loss of ammonia as compared to its surface application. It would appear that if the benefit of retardation of nitrification is not to be offset by enhanced ammonia volatilization loss in light-textured calcareous soils, placement of inhibitor-amended urea or ammonium fertilizers at least at a 5-cm depth in soil would be a better strategy for efficient nitrogen management.

Ill. EFFECTS OF NITRIFICATION INHIBITORS ON BIOLOGICAL NITROGEN TRANSFORMATIONS A. MINERALIZATION AND IMMOBILIZATION Nitrification inhibitors may effect mineralization of soil nitrogen in some situations. They can also influence immobilization of nitrogen due to persistence of ammonium, which is preferentially immobilized over nitrate by soil microorganisms (Alexandra, 1977). Not only is ammonium preferentially immobilized over nitrate, but also remineralization of immobilized ammonium is relatively slower than that of immobilized nitrate (Bjarnason, 1987). Dubey and Rodriquez (1970) found that the fungicides dyrene [2,4-dichloro-6-(O-chloroanilino)-s-triazine] and maneb (manganese ethylene bisdithiocarbamate) did not affect ammonification of soil nitrogen at 60 pg/ g soil concentrations although nitrification of ammonium was greatly retarded. Only at a high rate of application (960 Kg/g soil) did dyrene and maneb retard ammonification. Laskowski et al. (1975) showed that 6-

29 1


chloropicolinic acid (GCPA), a hydrolysis product of nitrapyrin, had no effect on net mineralization of soil organic nitrogen at up to 1000 pg/g soil concentrations. Some data on the effects of nitrapyrin and 6-CPA on general microbiological activity of soils, as indicated by CO, production, are shown in Table XII. These results clearly establish that nitrapyrin has no effect on the general microbial activity of soils. Although 6-CPA did not affect general microbial activity in two soils, it significantly reduced the production of CO, in the loam soil that was low in organic carbon, even at the lowest concentration. The compound, however, did not affect the production of CO, at a loo0 pg/g soil concentration, although it significantly reduced CO, production at lower concentrations in the low-organic matter loam soil. Such an effect of 6-CPA is important to note because it would usually be attributed to nitrapyrin. Nitrification inhibitors can also influence mineralization of soil nitrogen. For example, Malhi and Nyborg (1979a,b, 1983) found that nitrapyrin, ATC (4-amino-I ,2,4-triazole), and CS, (carbon disulfide) reduced the amounts of nitrate formed but also reduced the amounts of ammonium released, i.e., ammonification in soils. The effect of ammonification inhibition was greater with ATC and CS, than with nitrapyrin. ATC and CS, at concentrations of 22 kglha suppressed both ammonification and nitrification of soil in the field and thus reduced nitrate formation during a wet spring (Table XIII). Juma and Paul (1984) used the soil samples from plots previously fertilized with "N-labeled urea or aqueous ammonia (NH,OH) with and

Table XI1 Effect of Nitrapyrin and 6-Chloropicolinic Acid (6-CPA) on General Microbiological Activity as Indicated by CO, Production in Soils in 291 Days of Incubation"'b Amounts of C 0 2 (mg) evolved 6-CPA (mg/kg soil)

Nitrapyrin (mg/kg soil) Organic C I

Texture

pH

(%)

0

1

10

Loam 7.6 Clay 7.7 Loam 7.2 Average all soils

0.5 0.8 2.4

221 320 422 321

242 274 403 306

236 188 335 253

100 lo00

241 298 384 310

204 209 270 228

0 384 469 421 425

I

10

100

lo00

243' 262' 406 441 524 446 391 383

257' 301 356 305

366 312 419 366

"From Laskowski e t a / . (1975). bSoil samples (50 g) were treated with dust formulation of nitrapyrin or solutions of K salt of 6-chloropicolinic acid to achieve the specified concentrations and incubated (optimum water content) at 20°C. 'Significantly different from control at p = 0.05.

292

K. L. SAHRAWAT

Table XIII The Effects of Nitrification Inhibitors on the Release of Mineral N over the Winter in Mdmo Silty Clay Loam Soil (pH 6.0; O.M. 9.7%) in 1978-1979" NH, N and NOp N in the 0-30-cm layer' NO, N (kg/ha)

NH4 N (kg/ha) Treatmentb 27 Oct

16Mar 10 May

Control ATC Nitrapyrin CS,

22 c 30 b 25 bc 38 a

14

-

-

22 b 36a 24 b 40a

27 Oct

16Mar

14

60a 33 b 56a 21c

-

(NH,

10 May 27 Oct 18a 19a 16a 19a

28

-

+ NO,) N (kg/ha) 16 Mar

10 May

82 a 63 b 81 a 59 b

40 b 55 a 40 b 59 a

"From Malhi and Nyborg (1983). bThe nitrification inhibitors were added a t a rate of 22 kg/ha. ATC and nitrapyrin were mixed into the soil to a depth of 10 cm, and carbon disulfide (CS,) was injected 10 cm deep in bands 23 cm apart. 'In each column, the values not followed by the same letter are significantly different ( p = 0.05).

without ATC nitrification inhibitor (Juma and Paul, 1983) to study the effect of the nitrification inhibitor on N mineralization during 2 weeks of incubation at 28 1°C and - 34 kPa soil moisture tension in the laboratory and on NH, released during a 10-day incubation of fumigated soil. It was found that although the nitrification inhibitor did not affect the mineral N released during 2 weeks of incubation, the amounts of NH, N released in fumigated soils were higher in the inhibitor-treated samples. The extractability ratios (ratio of atom percentage "N excess of extracted N to atom percentage ''N excess of total N) were higher for the samples treated with the nitrification inhibitor compared to those treated with fertilizer alone. Juma and Paul (1983) made a detailed study of the effect of ATC on immobilization of I5N-labeled aqueous ammonia and urea N and found that ATC caused a greater immobilization of fertilizer "N (see Table V) and also increased the rate of release of "N-labeled microbial biomass following fumigation and incubation for 12 weeks (Table XIV). Aulakh and Rennie (1984) found that immobilization of fall-applied labeled urea and KNO, was minimal under fallow conditions (7%) but ranged from 1521% and from 2626% of the applied N as KNO, and urea, respectively, in wheat-stubble fields. Nitrapyrin did not affect the immobilization of fertilizer N, and the amounts of fertilizer N recovered in the organic and in the inorganic N pools were similar in urea and urea plus nitrapyrin treatments 8 months after fertilizer application (Table XV). Other studies have suggested an interesting pathway of nitrite incor-

*

293

EFFECTS O F NITRIFICATION INHIBITORS Table XIV

Effect of ATC Nitrification Inhibitor on Decay of Microbial Biomass in Loam Soil in a 12-Week Laboratory Incubation"

Treatment

"NH, released on fumigation and incubation (ng/g soil)

"N in biomass (ng IsN/g soil)

Decay rate constant'

(weeks)

NH,OH I b NH,OH 2' NH,OH plus ATC Urea plus ATC

18 14 38 35

60 47 I27 1 I7

0.028 0.026 0.020 0.026

24.7 27.2 33.9 26.2

tlnd

~

"From Juma and Paul (1983). "Similar treatments incubated at separate times. 'Decay rate constant expressed as net decaylweek, setting the initial pool sizes to 100%. dHalf-lives for biomass "N.

poration into the organic nitrogen fraction via nitrite self-decomposition and fixation on organic matter in a humic-rich acidic forest soil (pH, 4.5; organic matter, 46%) (Boudot and Chone, 1985). Nitrapyrin application not only reduced the loss of nitrite via chemodenitrifcation (Nelson, 1982) but also decreased the incorporation of nitrite into the organic N fraction (Boudot and Chone, 1985). In later studies, Azhar et al. (1986a) reported that nitrite formed from ammonium oxidation in grassland soil (pH, 6.5; organic C, 4.09%) was incorporated into the organic matter fraction following the pathway suggested by Boudot and Chone (1985). Nitrapyrin application checked the fixation of nitrite into organic matter. It is Table XV Recovery of Fall-Applied "N-Labeled Urea in May 1981 in the Soil Profile (kgN/ha) to 30-cm Depth of Baline Lake Clay Loam (Typic Udic Haploborolls)" Treatment" Urea Urea plus nitrapyrin

Organic N 12.2 a (24.4)d 14.5 a (28.9)

(NH,

+ NO, + NO,) N 32.4 a (64.7) 32.7 a (65.3)

Fixed NH, N'

Total N'

0.7 a

45.3 b (90.5) 48.3 b (96.6)

(1.4 1.2 b (2.3)

"From Aulakh and Rennie (1984). "Urea was applied at the rate of 50 kg N/ha and nitrapyrin at a concentration of 1% of active ingredient per weight of fertilizer N on 27 September 1980. 'In each column, the values differ significantly ( p < 0.05) when not followed by the same letter. dValues in parentheses represent the percentage recovery of fertilizer N.

294

K. L. SAHRAWAT

important to note that this mechanism of nitrite fixation in organic matter has been reported in soils in which nitrification occurred and nitrite accumulated only in small amounts (Azhar et al., 1986b,c). It has been proposed that nitrite formed reacted with phenols, forming nitro- and nitrosophenols. Nitrosophenols tautomerized to form quinone oxime, which could be reduced or oxidized chemically or enzymatically ultimately to form gaseous products of nitrogen. Results from these studies suggest an interesting pathway such that nitrification could lead to incorporation of mineral N (NO,) into organic N. Nitrapyrin has been found to block this pathway by checking NO, accumulation in soils. It should be made clear here that nitrification inhibitors increase immobilization of N by increasing the persistence of NH,. Also, nitrification inhibitors check NO, accumulation in soils and thus block fixation of NO, into organic matter. These two examples are simply two different aspects of the N immobilization process. Nitrite accumulation and its fixation into organic matter occurs under specific soil conditions (Chalk and Smith, 1983), whereas immobilization of mineral N is a more general process, but both are influenced by nitrification inhibitors. B. DENITRIFICATION It has been reported that nitrification inhibitors can inhibit denitrification is soils. For example, Mitsui et al. (1964) showed that nitrapyrin, dicyandiamide, and sodium azide retarded denitrification of nitrate N in wetland rice soils. Similarly, Henninger and Bollag (1976) found that sulfathiazole (ST), potassium azide, and phenylmercuric acetate (PMA) inhibited denitrification by soil microorganisms, but they could not confirm the inhibitory effect of nitrapyrin on denitirification. Other compounds, such as AM (2-amino-4-chloro- 6-methyl pyrimidine), ATC, and anilines also had no effect on denitrification. Some pesticides and nonspecific inhibitors of nitrification may also retard the denitrification process in soil (e.g., see Hauck, 1980, 1983; Goring and Laskowski, 1982). Yeomans and Bremner (1985a,b) found that none of the several herbicides, fungicides, and insecticides tested had any significant effect on denitrifcation of nitrate when added at 10 m a g soil concentration. Some of them had small effects when added at 50 mg/kg soil concentration. These results suggest that commonly used pesticides will have little effect on denitrification when added at normal rates. McElhannon and Mills (1981) investigated the effect of nitrapyrin on denitrification of nitrate in a field planted to sweet corn in a 2-year study. It was found that nitrapyrin reduced the loss of nitrate by denitrification in situations in which a readily oxidizable carbon substrate was available, for example, in the rhizosphere of a living plant, and when nitrapyrin was


295

applied to the nitrogen fertilizer band rather than by broadcast application. Contrary to these findings, Notton et a l . , (1979) found that nitrapyrin stimulated denitrification of nitrate, particularly in the presence of carbon sources such as root debris or acetone in sand culture used for growing turnip, cauliflower, and radish plants. Acetylene, which is an effective inhibitor of nitrification (Walter et al., 1979; Sahrawat et al., 1987),also inhibits nitrous oxide reductase enzyme, which converts N,O to N, (Federova et al., 1973; Yoshinari and Knowles, 1976; Yoshinari et al., 1977), and, consequently, the gaseous product of denitrification is released largely as N,O. In fact, the acetylene block technique is used to measure denitrification loss in soils by measuring N 2 0 emissions on a short-term basis (Yoshinari et al., 1977; Ryden and Rolston, 1983; Keeney, 1986). Bremner and Yeomans (1986) evaluated the effects of 28 nitrification inhibitors on denitrification of nitrate in soil by determining their influence on the amounts of nitrate lost and the amounts of nitrite, nitrous oxide (N,O), and N, produced when soil samples were incubated anaerobically after treatment with nitrate. The inhibitors evaluated included nitrapyrin (N- Serve); etridiazole (Dwell); potassium azide; 2-amino-4-chloro-6methyl pyrimidine; sulfathiazole(ST);4-amino-l,2,4-triazole;2,4-diamino6-trichloromethyl-s-triazine; potassium ethylxanthate; sodium diethyldithiocarbamate; phenylmercuric acetate (PMA); caffeic acid; and dicyandiamide. It was found that only potassium azide of the nitrification inhibitors studied retarded denitrification of nitrate when added at the rate of 10 m a g soil. Some results of this study are given in Table XVI. When added at the rate of 50 mg/kg soil, only potassium azide and 2,4-diamino6-trichloromethyl-s-triazine of the compounds tested inhibited denitrification. The other inhibitors either had no appreciable effect on denitrification or enhanced it when added at the rate of 10 or 50 mg/kg soil. The inhibitory effects of nitrapyrin and etridiazole (Dwell) on denitrification reported earlier (Mitsui et al., 1964; Mills and McElhannon, 1983, 1984; Mills et al., 1976; McElhannon and Mills, 1981; Mills, 1984) could not be confirmed because these compounds had no effect on denitrification when added at the rate of 10 mg/kg soil and enhanced denitrification when they were added at the rate of 50 or 100 mg/kg soil (Bremner and Yeomans, 1986). c . NITROUS OXIDE EMISSION VIA NITRIFICATION AND DENITRIFIC ATION It is generally believed that nitrous oxide (N,O) in soils is produced only through denitrification (CAST, 1976) but other research has clearly established that N,O is also produced during nitrification of ammonium

296

K. L. SAHRAWAT Table XVI

Effects of Some Nitrification Inhibitors on Denitrification of Nitrate in Soil"-b N produced (mg/kg soil) NO3 N lost (mg/kg soil) N 2 0 N

Nitrification inhibitor

N2 N

(NOz + N 2 0 + Nz) N ~

None Nitrapyrin (N-serve) Potassium azide 2-Amino-4-chloro-6-methyl pyrimidine (AM) 2-Mercaptobenzothiazole Sulfathiazole (ST) Etridiazole (Dwell) Potassium ethylxanthate Thiourea 4-Amino-] ,2,4-triazole (ATC) Sodium diethyldithiocarbamate Phenylmercuric acetate (PMA) Dicyandiamide (DCD) 2.4-Diamino-6-trichlorornethyls-triazine (CL- 1580) Caffeic acid

I 09 109 88 108

34 36 I 32

74 72 87 75

108 108 88 107

I10 108 I09 107 109 109 110 I16 I08 I08

38 39 33 26 35 39 38 20 39 31

72 68 75 81 74 70 73 78 69 76

I10 107 108 I07 109

109

33

74

107

109

Ill I17 108 I 07

"From Bremner and Yeomans (1986). "Thirty-gram samples of Canisteo soil (Typic Haplaquoll) were incubated at 30°C with 15 ml water under He atmosphere after treatment with 9 mg nitrate N as KNO, and 0 . 3 mg of the inhibitor (10 mg/kg soil) specified.

(Bremner and Blackmer, 1978; Freney et af., 1978, 1979; Goodroad and Keeney, 1984;Aulakh et al., 1984; Sahrawat et al., 1985). The mechanism of N,O production via nitrification is not clearly understood. The production of N,O via denitrification of nitrate and nitrification of ammonium can be represented as follows: Nitrate reductase

- *

NO3

NH,

NHzOH

Nitrite reductase

Nitrous oxide reductase

'

NOz

I

- '

NzO

Unidentified(H,N,O,?) compound

NO,

(1)

Nz

NO,

(2)

N2O

Because nitrification inhibitors retard oxidation of ammonium to nitrite, it is not surprising that they also retard N,O emissions through nitrification of ammonium. Bremner and Blackmer (1978) showed that nitrapyrin

297


greatly reduced emission of N,O from soils during nitrification of ammonium (Table XVII). Acetylene (C,H,), which retards nitrification of ammonium, also greatly reduces emissions of N,O from soils during nitrification of ammonium (Table XVIII) (Bremner and Blackmer, 1979; Aulakh et al., 1984). Smith and Chalk (1978, 1980) studied the effect of nitrapyrin addition on evolution of N 2 0 , N,, nitric oxide (NO), and nitrogen dioxide (NO,) gases from a calcareous soil treated with ammonia. Nitrapyrin largely reduced the gaseous loss of N, and oxides of N including N,O from soil. Nitrite accumulation occurred in soil treated with ammonia but was prevented by nitrapyrin (see Table X). It is recognized that nitrification inhibitors such as nitrapyrin check accumulation of nitrite N in soils and thus are likely to reduce N,O emissions via chemodenitrification or microbial denitrification of nitrite N indirectly (e.g., see Bremner and Blackmer, 1980; Nelson, 1982; Hauck, 1983; Chalk and Smith, 1983). Freney et al. (1979) found that N 2 0 emitted from soils, apparently via nitrification, at water contents ranging from air-dry t o field capacity was inhibited by HgCI, and toluene. Field studies have shown that nitrapyrin added at field rates of application reduced N,O emissions induced by fertilization of soils with urea and anhydrous ammonia (Table XIX) (Bremner et a / . 1981; Aulakh et al., 1984). In a field study of N,O emission from Australian soils, it was found that under fallow conditions, nitrapyrin significantly reduced anhydrous ammonia-induced loss of N,O only from a calcareous soil (pH, 8.5, organic C, 1.3%) but not from another soil (pH, 7.5; organic C, 2.0%). The inhibitor

Table XVII

Effect of Nitrapyrin on Emission of N 2 0from a Clay Loam Soil (pH 7.8; organic C 4.4%) Incubated under Aerobic Conditions after Treatment with Different Forms of NEVb

Form o f N added None None Ammonium [(NHJ2S041 Ammonium Urea Urea Nitrate (KN03) Nitrate

Nitrapyrin added ( p d g soil)

Amount of N,O N evolved in 20 days (pg/g soil) 4

4 148 10 122 4 6 4

"From Bremner and Blackmer (1978). "Different forms of N were added at a rate of 100 mg/kg soil and incubated at 60% WHC moisture and 30°C.

298

K. L. SAHRAWAT Table XVIII Effects of Acetylene on Nitrification and N20 Production in a Clay Loam Soil (pH 8.1, organic C 4.2%) Treated with Ammonium under Aerobic Conditions""

Treatment ~

~~~

~~~

Ammonium added (wdg soil)

C2H2added (%, v/v)

(NO2 + NO,) N produced in 12 days (mg/kg soil)

N 20 N evolved in 12 days (ng/g soil)

0 0 100 100

0 0.1 0 0.1

11 I50

15-150

Pore sizeb (pm) 1-15

Adelanto loam > Pachappa loam. Jackson (1963) concluded that high-clay soils (Adelanto loam and Pine silty clay) showed greater change in soil water diffusivity due to compaction than low-clay soils like Pachappa loam. Using data from the literature, Libardi et al. (1982) modified an existing equation (Miller and Bresler, 1977) to describe the effect of compaction (bulk density) on the soil-water dflusivity function. The modified equation is of the form Di ( 8 , pb)

(ai - 0 . 4 6 4 ~ exp ~ ) ~(pe)

(3) where Di is the soil-water diffusivity of soil i at a relative water content 8 and a bulk density Ph; 0: and p are constants, and ai is the slope of wetting front to t' function at a known bulk density. Brutsaerts (1979) suggested values of and 8 for 0: and p, respectively. Based on Eq. (3), Libardi et al. (1982) concluded that the smaller the Di, the larger is the effect of bulk density on soil-water diffusivity. This is the case for =

0:


32 1

PINE SILTY CLAY

'ce 3.0

- 3.00 0

o 1.37 1.45

0

O Oo

a 0.6Y 0.4

z :

i5 0.3 K

0

o o of

-

0.2-

I 0.1-5 .08

0.b

0

-

e o

a.

3.0

2.01.0

I

0.4: (b!

-

0.30.2 -

0

o

1.34 1.40

0.4 0.3 0.2

b

* A b

-

0.1.08

I

I

0.1 .08

0.5

RELATIVE WATER CONTENT,

@=

0.6

0.7 0.8 0.9 1.0

(e-ei) (Qs-Qi)

FIG.7. Soil-water diffusivity versus relative water content of Adelanto loam, Pachappa loam, and Pine silty clay at three bulk densities. (Adapted from Jackson, 1963.)

fine-textured soils with a large number of micropores. These soils, already slow in water flow, are most affected by compaction. This conclusion was similar to the findings of Jackson (1963).

4. Sorptivity Sorptivity, So, is a measure of the uptake of water by soil without gravitational effects (Philip, 1957). Sorptivity values depend upon the structure and the antecedent water content of the soil. Walker and Chong (1986) measured sorptivity from horizontal cumulative infiltration in laboratorycompacted soil columns of Alford silt loam (Typic Hapludalfs). Compaction treatments covered six levels of applied stresses (38,65,92, 138,321, and 458 kPa) at four soil water contents (10, 15, 20, and 25% kg/kg). Figures 8 and 9 show the changes in void ratio and sorptivity as influenced by different levels of applied stress and antecedent soil water contents. Starting from a dry condition, both void ratio and sorptivity increased with increasing soil water content, reached a peak, and then decreased. The shape of curves in Figs. 8 and 9 is similar to a typical water content-density curve obtained in a Proctor test. Because void ratio versus water content (Fig. 8) and sorptivity versus water content (Fig. 9) curves were similar in shape, sorptivity was linearly related to void ratio (Fig. 10). Sorptivity increased linearly with an increase in the void ratio of the soil for all six levels of applied stress and four levels of antecedent water contents. Sorptivity can be a useful index that measures the combined effect of applied stress and water content on pore geometry, provided a relationship like that of Fig. 10 holds true for other soil types.

322

SATISH C. GUPTA ET AL. 1.6

1.4 1.5

Pa

E

65

2E 1.3 -

d 1.2 QK Q

9

1.1 -

1.0

-

0 5 10 15 20 25 30 SOIL WATER CONTENT, % kg I kg FIG. 9. Sorptivity as a function of soil-water content at various levels of applied static pressure. (Adapted from Walker and Chong, 1986.)

-

323

COMPACTION EFFECTS ON SOIL STRUCTURE 30 (v

-v-. -ul.

E

-

cn"

20

d

9

G I-

2

10

li

$ 0 0.75

0.95

1.15

1.35

1.55

1.75

1.95

VOID RATIO, e , m3/m3

FIG. 10. Relationship between sorptivity and void ratio. (Adapted from Walker and Chong, 1986.)

Since soil structure may change (swell or shrink) when soils become wet, hydraulic properties may not reflect the structure of soil in its original form. Fluids like air, which does not interact with the soil matrix, are thus better in describing the pore geometry of soils in its original condition. Two properties that describe the pore geometry of soil without the complications of fluid interaction with the soil matrix are gas diffusion and air permeability.

5. Gas D n o s u ' i It has been well recognized that the main mechanism of gaseous exchange between soil and atmosphere is molecular diffusion (Currie, 1984). The coefficient of mutual diffusion for a pair of gases interdiffusing within air-filed pores of the soil is conveniently expressed as a fraction of Do, the coefficient of diffusion in the absence of impeding solids. The ratio DID,is, within broad limits, independent of the nature of diffusing gases, and it is a measure of pore continuity and tortuosity (Currie, 1984). In addition to being affected by pore geometry, DID, is strongly influenced by the amount of air or water content. According to Currie (1961), the decrease in DID, with increase in water content is large for a simple pore system, i.e., one with a unimodal pore size distribution, such as sand or gravel. In a compound pore system (bimodal size distribution), such as a packing of soil crumbs or a tilled soil,

324

SATISH C . GUPTA ET A L .

the decrease in DID, is small as the intracrumb pores are wetting but becomes large, as in the simple system, when intercrumb pores are wetted. Figure 11 shows a hypothetical relationship of DID, to air content for a packing of soil crumbs (Currie, 1983). Curves 1-3 and 4 represent the diffusion versus air content relationship of intra- and intercrumb pores, respectively. Figure 11 shows that as the crumbs wet to saturation (Curves 1-3), the decrease in diffusion is small. However, as more of the continuous intercrumb pore system wets, diffusion decreases in proportion to the fourth power of the remaining air-filled pore space (Curve 4). Currie (1984) studied the diffusion of hydrogen through air in I-2-mm aggregates of Batcombe clay loam that had been subjected to various levels of compaction. Figure 12 shows the change in diffusion coeffkient at five levels of compaction. The two-part relationship between diffusion coefficient and air content was similar over a bulk density range from 0.86 to 1.29 Mg/m3, but the two parts became less distinct with increased compaction (Fig. 12). Also, the range of air content over which the diffusion coefficient is exponentially related to air content decreased with an increase in the level of compaction. This reflected a shift in the proportion of large (interaggregate) to small (intraaggregate) pores during compaction. 6 . Air Permeability

Air permeability is the ability of soil to allow convective transport of air in response to a total pressure gradient. Like hydraulic conductivity or permeability, air permeability reflects the size and continuity of airfilled pores. Bowen (1966) studied the effect of vertical applied stresses (0, 7, and - 1

0

lntercrumb

0.1

+-I

0.2 0.3 (cm3 cm-3)

Crumb+

0.4

0.5

0.6

E

Frc. 11. Generalized reiationship of DID,, versus air content (E) in packing of soil crumbs. (After Cume, 1983.)

325


0.3

-

80.2 -

\

P

0.1

1 0.99 2 1.06 3 1.12 4 1.20 5 1.29

-

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

AIR CONTENT, E (cm3 cm-3)

FIG. 12. Relationship of DIDo versus air content of 1-2-mm soil aggregates at five levels of compaction. (Adapted from Currie, 1984.)

35 kPa) on air permeability of Ruston loamy fine sand. Figure 13 shows relatively little influence of compaction on air permeability when the soil was near air-dry water content. However, air permeability changed significantly due to compaction when soil water was 6% or more. Bowen (1966) suggested that higher air permeability in soils compacted at water contents of 6% or more was due to lower bulk density of these soils; in other words, to larger pores.

40

?

30

X

g 20 t d

zw

0

10 0

0

2

4

6

8

10

12

MOISTURE CONTENT, % 9g-l

FIG. 13. Air permeabilitiesversus moisture content for a Ruston fine loamy sand. (Adapted from Bowen, I%.)

326

SATISH C. GUPTA ET AL.

B. SOILMATRIX Effects of soil compaction on soil solid-solid interaction have been characterized in terms of aggregate size distribution, aggregate density, and wet and dry aggregate stabilities. 1 . Aggregate Size Distribution

Voorhees et al. (1979) studied the effect of field traffic (compaction)on aggregate size distribution and random roughness following tillage. Figure 14 shows the aggregate size distribution following tillage in wheel-tracked and nontracked areas of a Nicollet silty clay loam. Wheel traffic from all field operations was restricted to the same wheel path for 5 years. The field equipment weights, including tractor weight, ranged from 3700 to 7300 kg. Data in Fig. 14 show an increase in the proportion of large aggregates when compacted Nicollet silty clay loam soil was subsequently tilled. In the same experiment, Voorhees et al. (1979) also found that the density of clods taken from tilled wheel-tracked areas was higher than from nontracked areas (Table 111). Mechanical crushing strength of 50mm clods from tilled wheel-tracked area increased from 13 to 56 kPa. In a study on response of winter cover crops to soil compaction, Flocker et al. (1958) measured the changes in the physical condition of a Yo10 fine sandy loam seedbed at three levels of compaction (Table IV). Compaction treatments included (a) severely compacted plots, on which a jeep weighing about 908 kg was driven across the field, followed by a tractor with 182

--_--_ -

NONTRACKED

WHEEL- TRACKED

AGGREGATE DIAMETER, mm

FIG.14. Aggregate size distribution of subsequently tilled nontracked and wheel-tracked Nicollet silty clay loam, after planting, May 1975. (After Voorhees et a/., 1979.)


327

Table Ill Clod Density as Affected by Wheel Traffic" Clod density (Mdm') Date sampled

Untracked

Wheel-tracked

After planting, May 1975 After planting, May 1977

1.56 1.44

I .72 1.66

"Adapted from Voorhees et al. (1979).

kg of water in each rear tire, two trips with a cultipacker pulled by the same tractor, and finally, by a jeep carrying a load of 363 kg of sand; (b) moderately compacted plots, on which a tractor was driven across the field once; and (c) lightly compacted plots, on which no traffk other than subsequent tillage operations was allowed. The seedbed was prepared by double-disking all plots once and harrowing with a spike-toothed harrow twice. Bulk density, clod population, clod density, and clod shear strength increased as the level of compaction increased (see Table IV).

2. Aggregate Stability Power and Skidmore (1984) studied the effect of compaction on wet and dry aggregate stability. Compaction treatments included an application of vertical stress of 2.45 MPa in the laboratory to undisturbed surface samples that had been brought to a soil matric potential (Jl,,,) of -33 or - 100 kPa. Surface samples corresponded to cultivated and uncultivated fields of Reading silt loam (Typic Argiudols). Power and Skidmore (1984) defined dry aggregate stability as the energy needed to crush the compacted sample between two parallel plates. Wet stability is defined as the amount Table IV Physical Properties of Yolo Fine Sandy Loam as Influenced by Compaction Treatment" Bulk density (Mdm3) Compaction treatment Light Moderate Severe

0-3 cm

3-6 cm

Clod population (kg)

1.22 1.39 I .56

1.29 1.42 1.54

8.4 21.8 43.7

"Adapted from Flocker et a!. (1958).

Clod density (Mdm')

Clod shear strength (kPa)

I .49

0.49 0.75 0.87

1.50 1.64

328

SATISH C. GUPTA ET AL. Table V

Dry and Wet Aggregate Stabilities of Uncompressed and Compressed Readings Silt Loam"

Compressed (2.45 MPa) Treatments

Uncompressed

Soil-water pressure -33 kPa - lOOkPa

Dry aggregate stability,

Cultivated Uncultivated Cultivated Uncultivated

(J/m2) 7.18 33.54 12.52 40.25 Wet aggregate stability. (kdkg) 0.48 0.30 0.86 0.85

30.46 38.97

0.28 0.75

"Adapted from Powers and Skidmore (1984).

of soil left on a 0.25-mm sieve (60-mesh) after a sample has been lowered and raised through a distance of 27mm, 25 times per minute, in a tank of water. Table V shows that dry aggregate stability of Reading silt loam increased as a result of compaction for both cultivated and uncultivated samples. Differences in soil-water content (Jim = - 33 and - 100 kPa) at the time of compaction had a minimal effect on dry aggregate stability. Power and Skidmore (1984) attributed the increase in dry aggregate stability of compacted soils to an increase in bonding of particles because these particles were forced into closer proximity during compaction. Wet aggregate stability of Reading silt loam was slightly lower or unchanged due to compaction (Table V). This indicated that bonds between particles created during compaction were not stable to wet sieving. Power and Skidmore (1984) also concluded that compaction of samples breaks bonds formed during natural aggregation. These bonds are more resistant to the disruptive action of differential swelling and entraped air exploding off the water-submerged aggregates.

111. MECHANISMS OF SOIL STRUCTURE CHANGES DURING COMPACTION In the previous section, we quantified parameters that describe the effects of compaction on pore geometry or soil matrix. In this section, we will review possible mechanisms that control changes in soil structure (pore geometry or soil solid-solid interactions) during compaction.


329

Day and Holmgren (1952) microscopically examined the nature of changes occuring in moist 1-2-mm aggregates after application of a mechanical stress. Figure 15 shows photomicrographs of compressed samples of Yolo silty clay loam. Yolo soil with a 25% water content (corresponding to the lower plastic limit) when slowly compressed to a terminal stress of 49 kPa showed distinct aggregate boundaries with inter-aggregate spaces contributing to an appreciable proportion of the total volume. However, the aggregates are crowded together rather than (as normally) found in a loose pack and appear to be somewhat deformed because many of the interaggregate contacts are line segments rather than points (Fig. 15a). At a later stage of compression (Fig. 15b), the interaggregate pore spaces remaining at 49 kPa had almost completely disappeared at 148 kPa. Although traces of aggregate boundaries are visible in Fig. 15b, each aggregate has come into contact with adjacent aggregates over its entire periphery. This indicated that plastic flow has occurred extensively. The effect of water content on the state of compaction is shown in Fig. 1%; In the case, Yolo (I-2-mm) aggregates containing 15% water were subjected to an applied mechanical stress of 148 kPa. At this water content, plastic deformation, which was prominent at the lower plastic limit (25%), was much reduced. It can be seen from Fig. 15c that a number of aggregates are braced against the others, giving mechanical strength against further compression. These *observationsindicate that plastic flow occurred in the contact zone between aggregates, but not extensively. During compaction, the forces exerted upon an individual soil aggregate by surrounding aggregates produce a complicated force system. Theoretically, these forces can be resolved into direct and shear stresses. Plastic flow will occur only if shearing stress exceeds the shearing strength of aggregates. With these theoretical bases, Day and Holmgren (1954) suggested the following mechanisms for the changes in soil structure upon application of mechanical load. At the beginning of compaction, contact

FIG. 15. Photomicrographs of compressed samples of Yolo silty clay loam at the following combinations of water content and applied stresses: (a) W = 25%. ua = 49 kPa, (b) W = 25%. u, = 148 kPa. and (c) W = 15%. u= = 148 kPa. (Adapted from Day and Holmgren, 1952.)

330

SATISH C. GUPTA ET AL.

area between the aggregates is small. Due to large localized stresses in the inter-aggregate contact zones, shear may occur in the vicinity of the contact points, even though the shearing strength may be relatively high. With further compaction, the contact area between aggregates increases. However, the flattening process is self-degenerating because it causes more uniform distribution of load. Eventually the plastic flow ceases when the shearing stress falls below the shearing strength. If shear stress due to applied load is greater than shear strength, there will be a plastic deformation, and the pores between aggregates will disappear. Since shear stress versus shearing strength of soil controls the plastic deformation in soils, mechnically applied loads and water content at the time of compaction have strong influence on soil structural changes. Larson and Gupta (1980) interpreted the mechanism of soil structural changes during compaction from pore water pressure measurements. The experimental system consisted of a uniaxial compression of 60 g of moist soil (-

D [ ATABASEj

\1/ r &

\

Line s e l e c t i o n

/

1 PRIMARY TRIALS

I

Small sward p l o t s

Adaptation

Productivity Selecti2criteria

Persistance

I

1

I .

6

\

/

4 I

U t i l i z a t i o n trials

SECONDARY TRIALS

L.r/& \

Adaptation

Productivity

Persistance

Cultivar selection

I

\L REGISTRATION

RELEASE

COMMERCIALIZATION

FIG.3. Evaluation procedures for potential new cultivars.

# \

-3

-J \

BREEDING ANNUAL Medicago SPECIES

413

whose end use was precisely defined. Crawford (1983) adopted the attitude with annual medics that specific environments have specific demands and detailed three main stages as they were affected by the limitations imposed in semiarid environments, in which success is not forthcoming in each and every year of experimentation. He emphasized the importance of detailed meteorological records as an aid to interpretation of the data gathered, and these, together with differing edaphic characteristics, help to explain the adaptation of certain species to their environments. The stages used in evaluation in the South Australian program are ( I ) nursery rows; (2) swards for dry matter and seed production; and (3) swards for tolerance to grazing and potential animal production.

I . Nursery Rows The nursery stage serves four main purposes: (1) to establish uniformity of the accession; (2) to assess potential growth and development in the nursery environment; (3) to isolate segregants, or physical contaminants at collection, or heterogeneous allelomorphs: and (4)to produce seed for further evaluation. Uniformity is assessed by observation or measurement of 17 morphological and 12 agronomic characteristics (Crawford, 1983). The agronomic data act as a guide to potential production and help suggest geographical regions to which lines may be adapted as influenced by factors such as time of flowering and maturity and ability to regenerate in subsequent years. As the annual species of Medicago are self-pollinated, selections isolated in the nursery stage can be harvested separately to constitute a new line and further grown to establish homogeneity. High seed yield is not only an important agronomic characteristic per se but ensures adequate supplies of seed to extend the most promising accessions to the next stage of evaluation.

2. Swards f o r Dry Mutter and Seed Production The number of accessions that can be promoted from nursery rows to the sward evaluation stage is generally limited. It must be established, therefore, whether 1. All accessions should be grown in all major environments or only in some specific environments. 2. All accessions of the most appropriate species should be grown in the most appropriate environments.

414

E. J. CRAWFORD E T A L .

3. Only some accessions with the most appropriate characteristics should be grown in specific environments. Major environments are established in terms of climatic and edaphic limitations and the most appropriate species can then be selected from past experience. The relationship between soil type at the site of collection as recorded in passport data and the most likely area of adaptation in the proposed new environment is established. Local experience in South Australia confirms the superior adaptation of M . littoralis and M . tornata to light-textured soils, the latter being more tolerant of slight acidity (pH 5.0-6.5). Medicago truncatula is better adapted to loamy soils with higher levels of available lime (Crawford, unpublished data). Availability of resources and relative priorities largely determine the number of and extent to which accessions can be evaluated at this stage. Earlier evaluation programs, when the most appropriate species had yet to be determined, demanded a broader base on which to make judgements. When, however, a single or a reduced group of species have shown good adaptation, more accessions of fewer species became the logical approach. By this stage of the program Rhizobium requirement should have been established. Although commercial attempts at broad-spectrum bacterial culture development have been effective in the past in a relatively narrow group of associated species, Brockwell and Hely (1966) showed that the long-establised strain U45 is ineffective for N fixation on M. rugosa. This led to the development of strain W118 and, more latterly, strain CC169 specifically for ‘Paragosa’ gama medic. However, none of these strains are tolerant of low soil acidity, and it was not until Howieson and Ewing (1986) used Rhizobium meliloti isolated from acid soils (pH 5.0) in Sardinia, Italy, that species such as M. murex and M . polymorpha could be successfully grown on acidic soils in Western Australia. The observed persistence of rhizobia into the second year shows that there is potential to extend some annual medic species into soils hitherto found to be too acidic for medic. These are often in rainfall regions too low for subterranean clover. An important aspect of the development of medic cultivars in the ley farming system is the quantification of symbiotically fixed nitrogen by the medic stands and the subsequent availability of this nitrogen to cereal crops in the rotation. The measurement of N fixation has only become feasible with the advent of ”N diffusion techniques (McAuliffe et al., 1985; Bergerson and Turner, 1983), and this has now lead to a limited evaluation program for the measurement of N fixation at the post-release stage. Cultivars of Medicago are now being compared for their ability to fix nitrogen over a range of available soil nitrogen, and their potential for increasing soil fertility is being assessed. In a recent experiment performed

BREEDING ANNUAL Medicago SPECIES

415

at the Northfield Research Centre, ‘Paraggio’barrel medic fixed over 90% of its total seasonal nitrogen requirements under conditions of low available soil nitrogen (

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