SOIL ECOLOGY IN SUSTAINABLE AGRICULTURAL SYSTEMS - CHAPTER 7 (end) pptx

Swift INTRODUCTION Hans Jenny described soil development as a function over time of theinteraction of climate, parent material, topography, and biota Jenny, 1941.While this paradigm was

CHAPTER 7

Biological Management of Soil Fertility as

a Component of Sustainable Agriculture:

Perspectives and Prospects with Particular Reference to Tropical Regions

M J Swift

INTRODUCTION

Hans Jenny described soil development as a function over time of theinteraction of climate, parent material, topography, and biota (Jenny, 1941).While this paradigm was devised to account for the outcome of long-termprocesses, Jenny, in accord with other soil scientists, also recognized theimportance of the biota to the more immediate properties of soil fertility.Despite this recognition of the significance of biological processes, soil biology

as a discipline has historically played a relatively small role in the development

of soil fertility management practices

The role of science is to provide a predictive understanding of naturalphenomena Armed with this understanding, and within the limits of certaintythat science can set, humans gain the potential to manage their physical andbiological environment with insight and sensitivity In ecology, with the study

of the biological world at the scales of population, community, ecosystem,and landscape, the capacity for prediction has been limited in comparison withthat in chemistry or physics or in other areas of biology such as physiologyand genetics Thus science-based soil management has until recently largelytreated the soil as a physicochemical system The biological components ofsoil management models are largely restricted to the physiological response

of the plant to soil conditions During the last two decades, however, priorities

in soil management have shifted and led to the development of new approaches

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that can be gathered together under the title of “biological management of soilfertility.”

Biological management of soil fertility implies the harnessing of the logical resources of the ecosystem, particularly those of the soil itself, for themanipulation of soil fertility We should be clear at the outset that this is notthe same as organic farming (NRC, 1989) It does not eschew the use ofinorganic inputs, but rather focuses on increasing the efficiency of their use

bio-by biological means Emergence from the circumstances of nutritional ciency and poverty that characterize the lives of many of the small-scalefarmers of the world can only be achieved where there are sufficient resources

defi-to raise food productivity With very few exceptions this will only be possible

by the provision of external sources of some nutrient elements Biological andphysicochemical management should thus both be regarded as essential com-ponents of an integrated approach to soil fertility management

This shift in emphasis has been summarized recently by the formulation

of a new paradigm for soil fertility research, which asserts that we should,

“rely on biological processes by adapting germplasm to adverse soil tions, enhancing soil biological activity and optimizing nutrient cycling tominimize external inputs and maximize the efficiency of their use” (Sanchez,1994)

condi-Increased interest in a biological approach to soil fertility management is

of course predicated on a significant maturing of the discipline that has takenplace over the last two decades and that has been summarized in a range ofreviews (Swift et al., 1979; Tinsley and Darbyshire, 1994; Fitter, 1985;Pankhurst et al., 1994; Woomer and Swift, 1994) It has also been driven by

a variety of other causes First, many farmers in developing countries, incontrast with those of the industrialized world, still have only limited access

to inorganic fertilizers (FAO, 1993) This skewed distribution, and the highcost of fertilizer in most parts of the world, emphasizes the need for increasingthe efficiency of their use, and many researchers see a combination of inorganicwith organic sources of nutrient as the best route for this Second, it is nowapparent that many of the great gains in production made in the green revo-lution by use of high-yielding varieties with high inputs of inorganic fertilizercannot be maintained indefinitely Among the causes attributed to yielddeclines under long-term cultivation are changes in soil fertility associatedwith loss of organic matter and the accompanying decline in soil physical andchemical properties

The third driving force for a more ecological approach to soil managementhas come from the sustainable development agenda in which central concernwith the maintenance of yield is closely associated with desires to conservenatural resources, including a greater value accorded to maintenance of biodi-versity Forced to an extreme, sustainability may be seen as mutually incom-patible with increased agricultural productivity It has been interpreted by manyagricultural research scientists, however, as signifying increased efficiency inresource use, including the need to utilize all available resources within eco-

nomic limits that are realizable in the long term as well as profitable in theshort term (Lynam and Herdt, 1989; Spencer and Swift, 1992).

Last, but by no means of least importance, the ecological approach to soilfertility management has been favored by the change in farming systemsresearch to a more participatory and circumstance-related method of develop-ing responses to farmer’s constraints This approach necessitates a more sen-sitive awareness of environment variation and its importance in regulatingecosystem function (Swift et al., 1994a)

This chapter will address the issue of biological management from theparticular standpoint of research for increased productivity and sustainability

in small-scale farming systems in the tropics, with particular reference toAfrica The urgent need for research to improve agricultural production in thisregion is seen from the way in which a variety of indicators in Africanagriculture all point in the same direction Per capita food production isdeclining, associated with high rates of soil nutrient depletion, while fertilizeruse is increasing at only a very slow rate and fertilizer use efficiency remainswell below that found in the rest of the world (FAO, 1993; Stangel et al., 1994)

BIOLOGICAL MANAGEMENT OF SOIL FERTILITY

Soil Populations and Processes

The central paradigm for the biological management of soil fertility is “toutilize farmer’s management practices to influence soil biological populationsand processes in such a way as to achieve desirable effects on soil fertility”(Swift et al., 1994a) Biological populations and processes influence soilfertility in a variety of ways each of which can have an ameliorating effect onthe main soil-based constraints to productivity:

1 Symbionts such as rhizobia and mycorrhiza increase the efficiency of ent acquisition by plants

nutri-2 A wide range of fungi, bacteria, and animals participate in the processes ofdecomposition, mineralization, and nutrient immobilization and thereforeinfluence the efficiency of nutrient cycles

3 Soil organisms mediate both the synthesis and decomposition of soil organicmatter (SOM) and therefore influence cation exchange capacity; the soil N,

S, and P reserve; soil acidity and toxicity; and soil water-holding capacity

4 The burrowing and particle transport activities of soil fauna, and soil particleaggregation by fungi and bacteria, influence soil structure and soil waterregimes

Specific examples of the ways in which these functions are performed andregulated are given in the other papers in this volume and in the reviewsreferred to earlier

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Soil Management and Biological Processes

An understanding of the biological processes of soil is of no practical valueexcept in the context of the regulatory influence of management practices Therange of management practices that a farmer can employ to regulate soilfertility is limited (Table 1) Most of these practices are not unique to biologicalmanagement but common to farming practice the world over They also have

a history as long as that of agriculture itself The most sophisticated expression

of these practices is often to be found in so-called traditional agriculturalsystems such as shifting cultivation or valley-bottom rice production.All of the practices listed in the first column of Table 1 influence soilbiological populations and processes in a number of ways (column three) Themost direct means of biological management are those associated with the use

of biological inputs such as N-fixing bacteria, mycorrhiza, or soil fauna as ameans of enhancing the endemic biological activities These procedures arethe focus of much contemporary research (e.g., see reviews by Giller andWilson, 1991, and Pankhurst et al., 1994), but build on practices that farmershave utilized for many centuries Direct management is also achieved by theuse of organic matter inputs — in effect a means of selectively feeding theheterotrophic biological populations of soil — a practice of very ancient originbut at times eschewed in modern agriculture Equally direct but usually unin-tentional effects are also achieved by the use of pesticides, which may killparticular groups of soil organisms that are involved in processes of signifi-cance to soil fertility Management techniques such as tillage and fertilizationalso influence the activity of the biota indirectly by altering the physical andchemical environment of the soil

Despite the fact that these relationships between management practicesand soil biological activities have been known throughout most of the history

of soil science, very little attempt has been made until recently to scientificallymanage soil populations and processes The capacity to do so rests, as prefaced

in the introduction, in the ability to predict the outcome of the effects ofmanagement practice on soil biological activity and hence the impact on soilfertility Successful biological management can only be said to be achievedwhen this interactive chain can be predictably followed through from input tooutcome

The success of biological management practices thus rests on two ditions that must be satisfied: the availability of a management practice that

precon-is practically and economically acceptable to the farmer and the demonstration

by the scientist that the practice leads to enhanced soil fertility In the followingsections these two aspects are considered (in reverse order) in relation to theuse of organic inputs as a means of biological management

Table 1 Farmers’ Management Practices for Influencing Soil Fertility Through Manipulation of Biological Processes

Management practice Constraints to use Biological processes influenced Soil fertility effects

Competition with fertilizer Competition with indigenous biota

N-fixation Nutrient uptake by mycorrhiza

Increased N-aquisition Increased efficiency of uptake of P and other nutrients

Increased efficiency of H2O uptake Increased heavy metal tolerance

Fauna burrowing Decomposition

Soil structure/porosity Stimulation of nutrient release Organic matter inputs

Land set aside Livestock and fodder availability

— Cost

— All: Labor availability Opportunity cost of other uses

Decomposition SOM synthesis

Increased short-term nutrient availability

Increased nutrient storage/exchange Soil physical structure improved Soil water regimes improved Acidity/toxicity diminished Soil fauna/microflora growth Macropore formation improved

(macrofauna) Soil aggregation improved (microflora)

Inorganic fertilizer inputs Cost

Tillage (Intensive tillage) (Intensive tillage)

incorporation

Short-term nutrient availability increased

and particle size reduction

Root growth in tilled layer promoted

diminished

Nutrient losses increased Long-term nutrient storage diminished

Environmental and health impact

Nontarget organism populations diminished or eradicated

Destabilization of nutrient cycles Loss of soil structure

Table 1 Farmers’ Management Practices for Influencing Soil Fertility Through Manipulation of Biological Processes (continued)

Management practice Constraints to use Biological processes influenced Soil fertility effects

MANAGEMENT OF ORGANIC INPUTS

Regulation of Nutrient Dynamics by Resource Quality Control

Biological management of soil fertility depends on the manipulation oforganic inputs to the soil more than on any other practice The basic scientificpremise for this practice is that the organic matter supplies energy and nutrients

to the soil biota to stimulate their activities and therefore promote soil fertility.Organic inputs are processed by a complex web of soil organisms (Figure1), but biological management is based on the assumption that the outcomesare relatively consistent These outcomes are multiple and include the gener-ation of inorganic nutrients such as NH4, NO3, PO4, and SO4 from organiccompounds of these elements; the synthesis of soil organic matter (SOM) fromprecursor compounds in the organic inputs; and modification of the physicalstructure of the soil as a result of stimulation of biological activities (Table 1,line 2) Quantitative variation in these outcomes occurs as result of the variety

of potential organic inputs (Table 1), the diversity of organisms involved inthe conversion processes, and the complexity of the interactions between them

Figure 1 A crop residue (detritus)-based soil food web from a calcareous soil under

arable cropping in The Netherlands Under conventional tillage the based food web is dominant; under reduced tillage the fungal and earthworm paths are more prominent (Verhoef and Brussaard, 1990) (From De Ruiter

bacteria-et al., 1993 J Appl Ecol., 30:95–106 © 1993 with permission from Blackwell

Science Ltd., Oxford.)

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The biological activities are regulated by the quantity and quality of the OMadded, the environmental conditions under which the processing takes place(including climate and soil type), and the status of the soil community (Figure2a and b) Increasing insight into the ways in which these factors regulate soilbiological activity constitutes the main progress in achieving a predictivecapacity for biological management of soil fertility Most attention has beengiven to the use of organic mulches to supply nutrients such as nitrogen and

to the possibilities of utilizing “resource quality control” (Swift, 1984; 1987)

as a tool for managing nutrient availability

Resource quality refers to the regulatory effect of the chemical composition

of an organic resource on its rate and pattern of decomposition and nutrientrelease (Figure 2) (Swift et al., 1979) Early studies of decomposition processesdemonstrated the importance of N concentration and C:N ratio as determinants

of the N supplying capability of plant residues (Iritani and Arnold, 1960;Russell, 1961) Studies in natural ecosystems subsequently demonstrated thatother indices such as the lignin concentration or the lignin:N ratio provided abetter predictor of N release patterns (Melillo et al., 1982; Melillo and Aber,1984) Berg and McClaugherty (1987) suggested that N is not released from

(a) Figure 2a Regulation of decomposition and mineralization processes Decomposition of

residues of cowpea (Vigna unguiculata) in an Alfisol in Nigeria Factors of the

environment (surface vs buried), resource quality (leaves — high quality vs.

stems — low quality), and the decomposer community (C = coarse-mesh litter bags giving access to all soil organisms; F = fine-mesh litter bags, access

to microorganisms only) are all shown to significantly affect the extent of decomposition (After Ingram and Swift, 1989.)

forest litters until decomposition of lignin commences, and others have alsostressed the importance of the more recalcitrant components as regulators ofdecomposition rate and nutrient release (Feller, 1979; Gueye and Ganry, 1978).Polyphenols have also been implicated as regulators of N release in materialsthat, despite being nutrient-rich and low in lignin, are slow to release N (Vallisand Jones, 1973) It has been proposed that the polyphenols interact with N

to form stable polymers that are resistant to breakdown and therefore delay Nrelease (Martin and Haider, 1980; Stevenson, 1986)

These theories have been used to evaluate organic residues as sources andsuppliers of nutrients within cropping systems For example, Tian et al (1992a,b) investigated the patterns of decomposition and nutrient release of a range

of crop residues and agroforestry inputs under field conditions in a humidtropical environment They found that decomposition and N-release werestrongly correlated with N, lignin, and polyphenol concentrations (Table 2).The patterns of release of P, Ca, and Mg were similar to that of N The rates

(b)

Figure 2b Hierarchical regulation of decomposition and nutrient release P =

physico-chemical factors setting the broadest limits to the extent of decomposition (i.e., conversion of state R1 to state R2), these limits being successively fine-

tuned by resource quality (Q) and the composition of the decomposer munity (O) (After Swift, Heal, and Anderson, 1979.)

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of decomposition were also closely related to the patterns of accumulation ofinorganic N in soil in incubation experiments (Figure 3).

This type of research, which has recently been reviewed in detail by Myers

et al (1994), lays a strong scientific foundation for practices of organic mattermanagement (Sanchez et al., 1989; Swift and Palm, 1995) Palm and Sanchez(1991), Fox et al (1992), and Tian et al (1992a) have all utilized informationfrom decomposition experiments to derive predictive equations relating nutri-ent availability in soil to the chemical composition of applied litters or residues.One example of such a relationship is shown in Figure 4 While the appropriateequation may differ according to the materials chosen, it is clear that suchrelationships give a good prediction for the outcome of the application oforganic residues to soil

The same basic principles of decomposition regulation summarized inFigure 2b are also incorporated in a number of simulation models such as theRothamsted Model (Jenkinson et al., 1987) and CENTURY (Parton et al.,1989), which have also been shown to have a high predictive power Theseresults give us confidence that we are developing tools that will enable organicinputs to be managed with a much higher degree of sensitivity and predict-ability in the future

Environmental Regulation of Organic Matter Management

Organic matter chemistry — resource quality — is not the only factorneeded to make reliable predictions of the effects of organic matter inputs InFigure 2b the physicochemical environment is pictured as exerting controlover biological processes from a higher level in the hierarchy than resourcequality Much of this regulation such as that deriving from the climate liesoutside the control of the farmer There are practices, however, that can bemanipulated so as to “environmentally tune” the influence of organic inputs

on soil biological processes These include the location and timing of theapplication of organic matter It is well established, for instance, that incorpo-

Table 2 Chemical Composition, Decomposition Rate, and

N-Release Rate for Various Agricultural Residues

Material

Lignin (%)

From Tian, G et al., 1992b Soil Biol Biochem., 24:1051–1060.

© 1992 with kind permission from Elsevier Science Ltd.,

Kidling-ton OX5 1GB, UK.

ration of residues in the soil, as opposed to surface mulching, can acceleratedecomposition processes significantly and thus alter the dynamics of nutrientavailability (Figure 5, a and b) The breaking up of the residues during tillagemay be a factor in this effect, as well as the transfer of the organic matter intothe more favorable environment below the soil surface.

The paradigm in Figure 2b also asserts that the pattern of decomposition

is influenced by the nature of the soil community Evidence for this as asignificant effect is more tenuous and its relevance to soil management morecontroversial It is well established that cultivation changes the composition,

Figure 3 Cumulative mineralization of inorganic nitrogen in soil mixed with residues of

Gliricidia sepium (G), Leucaena leucocephala (L), or Acioa barterii (A)

com-pared with soil alone (C) Compare these effects with the data for

decompo-sition and mineralization rates in Table 2 (Redrawn from Tian, G et al., 1992.

Soil Biol Biochem., 24:1051–1060 © 1992 with kind permission from Elsevier

Science Ltd., Kidlington OX5 1GB, UK.)

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