Structure and Function in Agroecosystem Design and Management - Chapter 16 docx

920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 335 CHAPTER 16 Changing Soil Biological Health in Agroecosystems Julian Park CONTENTS Introduction 335 Agroecosystem Sustainability and Soil Health 336 Organic Carbon and Its Distribution in Soils 339 Organic Carbon as an Indicator of Biological Health in Agroecosystems 341 The Quality and Quantity of Crop Debris Returned to the Soil 343 The Growth and Turnover of Plant Roots 343 Cultivation 344 Managing Soil Biological Health 345 Acknowledgments 347 References 347 INTRODUCTION In an agricultural context, the complexity surrounding the concept of sustainability and the difficulty of moving from consideration of theoretical definitions to practical action currently provide an important issue for researchers (Fresco and Krooneneberg 1992; Park and Seaton 1995; Moffatt et al., 1999) When examining criteria associated with sustainability, there is support for considering the ecological underpinning of production systems that interact with the natural environment (Lowerance, 1990) This is associated with the view that it is desirable for ecosystems to be able to sustain function and thus maintain a given level of productivity into the future 0-8493-0904-2/01/$0.00+$.50 © 2001 by CRC Press LLC 335 920103_CRC20_0904_CH16 336 1/13/01 11:16 AM Page 336 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT In most agroecosystems, the degree of intervention is usually larger and more frequent than natural disturbance rates, with the primary objective being to maintain productive output Some degradation is both inevitable and acceptable in these systems, with different soil types and climate zones being able to withstand varying levels of intervention (Burke et al., 1995) In ecosystems, such intervention is related to resistance (the ability of a community to avoid displacement in the face of disturbance) and resilience (the speed with which a community returns to its former state after it has been disturbed and displaced) This ability to withstand intervention is similar in nature to the concept of health It is probable that agroecosystems will necessarily exist in a less than “full health” state as defined in a natural ecosystem if they are to remain productive, i.e., a reduction in species diversity, interruption of natural nutrient cycles, and loss of soil structure Further, soil health is increasingly being recognized as an important component of the sustainability of agroecosystems and is an area which is attracting considerable attention (Pankhurst et al., 1995; Park and Cousins, 1995; Doran et al., 1996; de Bruyn, 1997) If it is assumed that a soil health index (Haberern, 1992) can be agreed, then a key question is how farming practices influence soil health and what mechanisms may lead to improved health In this chapter, agroecosystem sustainability is discussed in relation to soil health Although there is considerable interest in soil fauna as bioindicators, I focus here on soil carbon as a holistic (proxy) measure of soil health The distribution of organic carbon in soils is outlined, particularly in relation to the return of plant debris to the soil system and the role of soil fauna in these processes The manner in which farming practices affect the amount and distribution of soil organic carbon (organic matter) is discussed before conclusions are drawn about the possibility of altering soil biological health in productive agroecosystems AGROECOSYSTEM SUSTAINABILITY AND SOIL HEALTH Fresco and Kroonenburg (1992) suggest that in order to be sustainable, land use must display a dynamic response to changing ecological and socioeconomic conditions In this situation, the maintenance of adaptive capacity within a production system becomes important Soil degradation and erosion is a serious problem in many parts of the world, both developed and developing (Pimentel et al., 1987) This can often be related to changes in cropping practice or the intensity of cultivation, both of which either directly or indirectly change soil structure or properties and thus lead to changes in agroecosystem health (Boardman, 1990) An agroecosystem in a poor state of health will be more vulnerable to certain (inappropriate) farming practices at a given moment than one in a better state of health For instance, in terms of an agricultural system, this may mean that there is an increased likelihood of soil erosion, which may reduce the options available for food production at 920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 337 CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 337 some point in the future Assessment frameworks can be envisaged that relate to the concept of sustainability so long as criteria can be put in place to assess possible short- and long-term repercussions of change On the basis of these criteria and knowledge of the current situation, questions need to be asked about the effects of a given change in land use on the options available for food production in the future, and whether this change is following broadly desirable dynamic pathways Park and Seaton (1995) suggest these pathways should maintain and, hopefully, increase the adaptability within a given production system, maintaining a direction which can fulfill both short-term needs (i.e., be economically viable) and long-term objectives (i.e., be sustainable) This will require the maintenance of healthy ecosystems There has been substantial debate surrounding the notion of ecosystem health (Schaeffer et al., 1988; Rapport, 1989; Allen and Hoekstra, 1992; Suter, 1993; Rapport et al., 1998) and, in non-agricultural contexts, Constanza (1992) and Rapport (1989) have proposed using ecosystem health as an end point for environmental assessment and management Ecosystem health is defined by Rapport (1990) as the ability to maintain productivity, to handle stress, and to recover to equilibrium after perturbation Similar principles can be related to agricultural systems The need to maintain production (e.g., resistance to disease or inappropriate management) and to recover productive capacity following a larger disturbance (e.g., resilience following flooding or drought) are central facets of desirable agricultural production systems Furthermore, a measure of the degree of agroecosystem health as a state of a productive unit may be used to monitor sustainable development The success of this approach depends upon finding important variables to measure the state of the system in order to characterize its health from both viability and sustainability perspectives Similar approaches have been utilized to explore the concept of soil health Doran and Parkin (1994) define soil health as the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health Thus, a measure of soil health may change between soil types and be related to both the present state of the soil and the reserve or potential within the soil to respond to change In relation to soil biological health, the functional role of soil organisms near the bottom of the food chain, their numbers, mass, and diversity mean that they may provide an indicator of the state of (agro)ecosystems (Pimentel et al., 1980; Holloway and Stork, 1991; Currie, 1993) Paoletti et al (1991) reviewed the use of soil invertebrates as bioindicators and suggested that much caution and modesty be associated with their development They point out that whatever indicators are chosen, they must give a sufficiently clear response to agroecosystem changes, either in terms of abundance or taxonomic diversity They further suggest that species level identification is much more time consuming—if not impossible This reinforces an earlier statement by Pimentel et al (1980), who suggested the best approach would be to assess 920103_CRC20_0904_CH16 338 1/13/01 11:16 AM Page 338 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT populations and biomass of major groups of biota without attempting to record data on all individual species present in a given ecosystem However, there is still little consensus on how to assess or monitor major groups of biota Pankhurst et al (1995), working in Australia, researched a wide range of soil biological properties with respect to different agricultural practices on two long-term field trials They were able to draw conclusions about the responsiveness of differing biological properties to agricultural management and thus their usefulness as biological indicators De Bruyn (1997) reviews the status of macrofauna as indicators of soil health She believes that the challenge for the future is to shift the emphasis of research towards an understanding of the function of macrofauna in soil processes It has been suggested elsewhere (Park and Cousins, 1995) that the use of body-size spectra may enable the development of simple techniques to provide information about the functioning of soil communities, which can be applied rapidly by local researchers who may not necessarily have a high degree of taxonomic training Doran et al (1996) provide a comprehensive review of soil health and sustainability They believe that the challenge is to develop holistic approaches for assessing soil health that are useful to producers, specialists, and policy makers To explore a more holistic approach, rather than focussing on the function of certain soil groups in relation to soil biological health, it is suggested here that agroecosystem change be explored via changes in carbon structure and processes associated with its distribution through the soil The distribution and flow of carbon in the form of organic material is of critical importance to soil properties The set of processes creating flows through that structure are gravity, wind, water flow, plant growth, animal movement, and human trade flows Changes in land use activity will alter these flows, giving a measurable change within agroecosystems Regular measurement of carbon in the soil system, together with the processes associated with its movement, can provide the basis for monitoring strategies, which will enable decisions to be made about whether the process of change in a given agroecosystem is sustainable Thus, studying the organic carbon structure of soils in parallel with other bioindicators could provide a useful measure of changes in agroecosystems for three reasons: Soil processes are responsive to human intervention Buringh (1984) estimates that on a world basis the soil contains only about three quarters of the organic carbon it did before the spread of civilization, and Doran and Smith (1987) point out that the forests and grasslands of North America declined to between 40 and 60% of their original organic carbon levels following cultivation The processes within the soil are fundamental to plant growth and photosynthesis Perry et al (1989) recognize the importance of the links between the soil and plants that grow on its surface, and how 920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 339 CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 339 this connects with the healthy functioning of the agroecosystem They state that the diversity in the plant community, the microbial community, and the ecosystem as a whole plays a seminal role in buffering against disturbance and in maintaining healthy links between plants and soil The soil itself is the agroecosystem component with the least resilience (Fresco and Kroonenburg, 1992) Thompson (1992) specifically highlights the importance of the soil processes in a short discussion paper on environmental quality objectives He suggests that the first concern must be the protection of the function of the soil—carbon and nutrient cycling and storage, nutrient supply, water supply, filtration and storage, and plant anchorage Further, soil carbon is relatively easy and economic to measure in time and space, responds well to farming practice (although not rapidly), and can be measured without specialist (taxonomic) knowledge Additionally, carbon budgeting and the modeling of carbon and organic matter turnover in soils can provide predictions of the effects of changes in farming practices over time, and a wealth of information already exists on the dynamics and distribution of organic matter in soils ORGANIC CARBON AND ITS DISTRIBUTION IN SOILS Organic materials act as binding agents within the soil, holding individual particles together A review of the role of organic matter in aggregate stability is provided by Tisdall and Oades (1982) The feces and associated digestive products of many soil organisms aid this stability For instance, residues left by earthworms often increase aggregate stability (in Dutch Polders the aggregate stability was increased by 70% following the introduction of earthworms) Wallwork (1976) suggests that the mucus associated with molluscs (which often move well below the soil surface) is a very good soil-binding agent The same principle is true for all soil animals that add saliva to debris as they ingest it The bulk density of soils is usually reduced by the presence of organic materials, and soil organisms such as earthworms increase the pore space within the soil (Edwards and Lofty, 1977) Chen and Avnimelech (1986) suggest that in soil low in organic matter, soil aeration becomes a limiting factor and cannot be simply offset by ensuring adequate nutrients and water Good soil structure is therefore essential Soil erositivity is decreased as the degree of well-incorporated organic matter in the soil increases The exceptions are peat-based or organic soils which may contain very high amounts of organic matter (Ͼ30%) and are therefore susceptible to erosion under certain conditions Well-incorporated organic materials add to the stability of soils by 920103_CRC20_0904_CH16 340 1/13/01 11:16 AM Page 340 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT reducing the direct impact of rain on the soil, increasing aeration, and improving drainage Conversely, compaction of the soil increases water runoff and reduces infiltration Flows of water, at or near the surface, are the precursor of severe rill and gully erosion This incorporation of organic materials is part of a complex process As plant and root material dies, it collects on the soil surface where it starts to decompose under the action of both sunlight and microorganism activity (Zlotin, 1971) In undisturbed soils, this surface litter provides both food and shelter for a range of sizes of animals Soil animals incorporate organic material into the soil where further decomposition takes place Decomposition processes have been discussed elsewhere by Edwards et al., 1970; Dickinson and Pugh 1974; Anderson, 1975; Edwards and Lofty, 1977; Persson and Lohm, 1977; Swift et al., 1979; Hole, 1981; and Giller, 1996 Lee (1985) suggests the disintegration, decomposition, and incorporation of litter results from a combination of solution by percolating rainwater, a minor component of atmospheric oxidation, but most importantly from the “decomposer industry.” Similar observations were made by Russell (1969) who suggests that soil animals are, in fact, the major and often the sole agents for bringing plant leaf litter into the soil so that it becomes accessible to the soil organisms The digging activities of the soil invertebrates cause direct infiltration of surface material through their feeding habits Indirect infiltration occurs through the dragging into the soil of organic fragments as water drains through the vertical pores created by invertebrates Earthworms are often cited as major movers and incorporaters of surface debris Edwards et al (1970) commented that earthworms were capable of consuming nearly all of the litter fall from a forest floor (3000 kg haϪ1) in the absence of other soil animals Although data exist on the disappearance of litter from the soil surface (Van Der Drift, 1963; Edwards et al., 1970; Dickinson and Pugh, 1974; Swift et al., 1979), rate of litter movement through the profile is less well documented Working with forest soils in the Netherlands, Van Der Drift (1963) recorded litter disappearance rates of up to 4200 kg haϪ1 in a year Similar work by Raw (1962) estimated that the earthworm species Lumbricus terrestris removed about 1.2 t haϪ1 dryweight of leaves from the surface in an English apple orchard In undisturbed temperate soils, the main invertebrates working below 20 cm will be earthworms, some of which are known to feed on the surface and defecate underground (Lee, 1985) More recent work by Balesdent et al (1990) studying the incorporation of maize debris suggests that 10–20% of the original plant residue carbon ended up below a depth of 30 cm within a 17-year period Although they not discuss how the carbon arrived in such a position, it can be speculated that movement was either undertaken by soil animals or by water movement through the channels they make (earthworms in particular) Other soil-related animals, such as millipedes, centipedes, and 920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 341 CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 341 woodlice, are likely to stay closer to the surface Mesofauna play a role in the transport of debris, but they are smaller and usually inclined towards predatory or saphrolytic activity within the soil body itself Mixing and transporting plant debris by the soil fauna often enhances conditions for microbial decay The larger soil animals will commute and break up the detrital material For instance the common earthworm pulls leaf material into its burrows to a depth of 10 cm or more They will often emerge at night to feed on surface litter or may be forced to the surface when their burrows become waterlogged Persson and Lohm (1977) recognize that many of the larger soil animals derive their nutrition from the microbial biomass and often ingest plant debris because of the microbes associated with it One of the benefits of such ingestion is that detrital material is shredded and moved during the process, with the possibility that microbial populations may be dispersed by such activity It is extremely difficult to estimate the amount of surface material that enters and moves through the soil as a result of water flows It has already been stressed that this flow is enabled by the burrowing and feeding activities of the larger soil animals In undisturbed moist soils (without surface cracking), the activities of soil animals are likely to be the major facilitator in the incorporation of surface debris ORGANIC CARBON AS AN INDICATOR OF BIOLOGICAL HEALTH IN AGROECOSYSTEMS The dynamics of organic carbon have been shown to be of importance in the cycling of nutrients, maintenance of soil structure, prevention of erosion, and diversity of soil organisms (Nye and Greenland, 1960; Allison, 1973; Doran and Smith, 1987) It is evident that organic carbon plays a vital role in many of the processes within the soil and therefore can provide an indicator of the health of the soil system Agricultural activity affects the amount of organic carbon within the soil, its distribution throughout the profile, and its rate of turnover Although it cannot be argued that soils of low organic carbon status are no longer productive, it can be generally assumed that soils very low in organic matter are more susceptible to erosion, suffer from poor structure, and need a constant input of nutrients if production is to be maintained (Chen and Avnimelech, 1986) Mineral soils of higher organic carbon status are usually better structured and are less likely to be eroded Within agroecosystems, the primary mechanisms by which agriculture influences the dynamics of soil organic matter are by controlling the return of surface debris to the soil, through the crop being grown, and the harvesting method The cropping type and system influences the amount and the quality of plant debris and root material being returned to the soil system Inputs 920103_CRC20_0904_CH16 342 1/13/01 11:16 AM Page 342 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT used in the growth of the crop will influence the quantity of crop produced and thus the return of root and plant material Fertilizers and certain chemicals can have both a direct effect (by increasing the amount of crop grown) and an indirect effect on the movement and rate of decomposition of organic materials in the soil via their effect on the soil community The effect of fertilization can be demonstrated by data from the long-term experiments at Rothamsted Plots that have received higher amounts of nitrogen during the past 150 years have higher levels of soil organic matter in the surface profiles Plots receiving organic fertilization in the form of 35 tonnes of farmyard manure (FYM) directly influence the amount of plant debris entering the soil which explains the large effect its application has had upon soil organic matter (Table 16.1) Fertilizer and pesticide inputs applied during the growing cycle of a crop to boost yield are likely to increase the amount of organic matter returned to the soil within the constraints of that particular cropping system However, the effect of that cropping regime, particularly associated cultivation and export of material at harvest, is likely to have an overriding influence on the dynamics of soil organic matter within that particular agroecosystem For instance, the ploughing of virgin land for arable cropping generally results in a rapid loss of soil organic matter which gradually slows, often reaching a lower, relatively stable state after many years (Lucas et al., 1977; Schlesinger, 1977) Mann (1986) reviewed the changes in soil carbon storage after cultivation and found all soils high in carbon (Ͼ5%) lost at least 20% of this following cultivation There are three primary mechanisms associated with this loss: the quality and quantity of crop debris returned to the soil; the growth and turnover of plant roots; and cultivation Table 16.1 Total Percentage Organic Matter Content of the Top Soil (0–23cm) in the Broadbalk Continuous Wheat Experiment 1865–1987 Treatment FYM N0 N1 N3 Date started 1843 1843 1852 1852 % organic matter 1865 1914 1944 1966 1987 3.13 4.33 4.05 4.35 4.64 1.90 1.77 1.80 1.90 1.78 N/A 1.92 1.92 2.08 1.94 N/A 2.21 2.11 2.11 2.16 N0 ϭ 0, N1 ϭ 48, N3 ϭ 144, kg N per hectare, respectively FYM ϭ 35 tonnes of FYM per hectare, Figures adapted from %N in top soil by assuming a C:N ratio of 10:1, and carbon to organic matter scaling factor of 1.72 Adapted from Glendining and Powlson, 1990 920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 343 CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 343 The Quality and Quantity of Crop Debris Returned to the Soil Campbell et al (1991) suggest that because crop residues are the primary substrata for organic matter replenishment in soils, changes in crops and their management can exert significant influence on soil quality The amount of plant debris returned to the surface of the soil each year is a function of the crop grown, the inputs used upon it, and the amount of biomass taken away at the end of the year The amount of root material and straw returned to the soil depends on how well the crop grows Therefore, high yields of grain will be associated with strong root systems and often more straw and chaff If the straw is baled and taken from the field along with the grain, the organic material returned to the soil is limited to the chaff and the root material In some crops, the roots (or part thereof) are removed (i.e., carrots, potatoes, etc.), and this can limit the return of organic materials still further However, it is not only the amount of organic matter returned that is important, but also its quality, as this affects the rate of decomposition The importance of the quality of the residue is highlighted by Wood and Edwards (1992) who consider that crop rotations, owing to the differences in amount and chemical composition of crop residues, may affect soil organic matter concentration and potential mineralization One measure of residue quality is ratio of C:N (carbon to nitrogen) within the plant material, as it is often the availability of nitrogen which controls the rate of decomposition The rate of decomposition can be further retarded by high amounts of lignin Carbon labeling experiments have shown that even substrates such as glucose, which decompose rapidly, still contribute to the stable organic materials in the soil In fact, a wide range of crops decompose to leave about a third of their initial carbon in the soil after a period of a year (Paul and Van Veen, 1978) This suggests that although the quality of organic material may govern rates of decomposition processes in the short term, over longer time periods it is the quantity of material returned to the soil which provides a more important determinant of soil carbon content The Growth and Turnover of Plant Roots In some agroecosystems the return of surface plant debris is small due to low litterfall, high export, and straw burning In these systems, plant roots provide the major source of organic matter input into the soil (Hansson et al., 1991) Plants vary considerably in rooting pattern and depth, leading to a stratified return of debris Kramer (1983) recognizes that plants have characteristic root patterns, although these can be greatly modified by soil conditions Water tables can considerably affect the depth of rooting, and in some free draining soils rooting can occur to considerable depths For instance, maize (Zea mays) roots can often be found at a depth of m, whereas roots of 920103_CRC20_0904_CH16 344 1/13/01 11:16 AM Page 344 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT lucerne (Medicago sativa) have been recorded at 10 m (Kramer, 1983) Durrant et al (1973), considering root growth in relation to soil moisture of field crops, found that barley and sugar beet were capable of rooting to well in excess of m, whereas potatoes were extracting water from a depth of 0.8 m In growing and penetrating through soils, a large amount of organic material is sloughed off into the soil surrounds, and dead root material is returned by both annual and perennial crops Addition of organic matter to the soil by these mechanisms can be considerable as between 50 and 70% of plant production is likely to be belowground growth (Reichle, 1977; Flitter, 1991) The adoption over a period of time of shallow rooting crops can reduce the amount of deep rooting material entering the soil, the consequence of which could be the gradual loss of organic material in deeper soil horizons Roots below the cultivation layer will improve soil structure in this region, where the formation of vertically orientated pores is a necessity for free drainage and further root development (Goss, 1991) In agricultural terms, perhaps the greatest distinction can be drawn between annual and perennial crops In the latter, roots, root cells, hairs, and tips are constantly being sloughed off and replaced, and this decaying material supplies a continuum of organic materials to the soil These perennial systems are not usually cultivated, and this not only allows the plant root systems to become well established but often aids the formation of a healthy soil community Cultivation On arable soils, annual cultivation is often used to incorporate surface residues, this operation frequently occurring shortly after harvest Incorporation has two main effects on the dynamics of soil organic carbon: it gives very good mixing of debris and soil leading to favorable conditions for microbial decomposition, but conversely this disturbance can kill a proportion of the fauna living in the soil (Madge, 1981) Microorganisms can multiply rapidly to utilize well-incorporated fresh organic matter, and this is evident in the flush of activity following ploughing This food supply may be enhanced because cultivation is likely to expose older organic material in the soil to further attack This can lead to rapid mineralization of carbon and high respiration losses Rapid recovery/reproduction associated with microbial life means that cultivation can increase activity, providing a well-mixed food source within the soil microclimate However, populations of larger soil animals may be kept at a permanently suppressed level due to annual cultivation Edwards and Lofty (1982) estimated changes in the population of earthworms on ploughed, chisel ploughed, and direct drilled soils They found that on direct drilled soils, the populations of the deep burrowing Lumbricidae terrestris and Allolobophora longa increased almost 18-fold over the years of the experiment House et al (1984) summarize the effects of cultivation on the distribution of soil organic 920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 345 CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 345 Table 16.2 Radiocarbon Age of Organic Matter in Soil Collected from Broadbalk, Rothamsted Sampling depth cm Organic carbon % Age in years 0–23 23–46 46–69 94 61 47 1450 2000 3700 After Jenkinson and Raynor, 1977 matter through the soil profile No-till systems create profiles in which the soil organic matter is stratified through the soil, with the bulk of the activity being near the surface These systems maintain the complex biological interactions often seen in nature and are likely to be less leaky in terms of nutrients It is known that organic materials in some deeper soils can be extremely old (Table 16.2) The importance of this deeper soil carbon in longer term agroecosystem processes is not known Indeed, records of rates of change in this subsurface soil carbon in agroecosystems are not well documented A review paper by Hendrix et al (1986) discusses the effects of “conventional and no-tillage agroecosystems” on the detritus food webs in the soil They state that nutrient mobility is generally increased in tilled soils, due partly to the fact that ploughed soils often show increased organic matter decomposition and nutrient mineralization The conclusions of their research clearly have implications within a sustainable systems framework, where the cycling and supply of nutrients is critical to the productivity of the system Within this context, the effects of cultivation can be seen to be unlocking nutrients within the soil and making them available to the growing plant This accelerated decomposition is not confined to the fresh plant material added to the soil, as the older stable humic elements within the soil are also oxidized faster The net effect is that cultivation, although a necessary part of the majority of farming systems, has led to a dramatic depletion of carbon structure within many soils MANAGING SOIL BIOLOGICAL HEALTH Cropping practice has had, and continues to have, a considerable impact on soil carbon levels, their distribution, and rate of mineralization (Burke et al., 1995) Monitoring the flows and distribution of carbon in soil needs to accommodate spatial variation and be undertaken at regular intervals This may mean that national-level monitoring may be a relatively crude process both in time and space, the aim being to provide an indication of areas or regions in which changes in agroecosystem health are occurring rapidly This would enable the targeting of monitoring and research to investigate change processes and to explore farming practices which may improve soil biological health in a given locality It is possible that intensive monitoring of soil 920103_CRC20_0904_CH16 346 1/13/01 11:16 AM Page 346 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT health via a suite of bioindicators is economically viable only at this more localized level In the previous section, some of the main mechanisms associated with the ingression and depletion of organic carbon in agroecosystems were outlined Studying the changes in the amount and distribution of soil organic carbon together with data on the main functional groups in the soil can provide information on the impacts of agroecosystem change on soil biological health In principle, it would seem logical that more environmentally friendly farming systems should improve the biological health of soils However, recent research in the U.K with respect to integrated arable farming systems does not confirm this (Park et al., 1999) For instance, within the constraints of a given farming system or rotation, reducing inputs of fertilizer and crop protection chemicals may well reduce yield, which in turn may lead to a reduction in the return of organic matter to the soil Conversely, the application of crop protection chemicals may have a direct effect on the populations of some soil fauna and may well reduce the diversity of plant material returned to the soil Similarly, more substantive cultivations may be needed if large amounts of debris are to be returned to the soil prior to drilling of the new crop Such material is also often chopped as it leaves the combine harvester This has both an economic and environmental cost in terms of the use of fossil fuels Additionally, some of the one-pass cultivate and drill systems that are becoming increasingly popular in the U.K tend to give the surface soil (in which the majority of organisms live) a thorough mixing Research is required to investigate the impacts of modern cultivation methods on soil faunal populations Thus, the measurement of soil biological health in regard to agroecosystem change and sustainability may present researchers with several dilemmas It is possible to suggest an index against which the current health of soils could be assessed, although this in itself may be problematic However, the cost of a comprehensive monitoring program at a national or international level will necessarily limit either the intensity of sampling or the parameters measured Further, it may be difficult within an agroecosystem context to suggest how the biological health of a given soil could be substantially improved without considerable changes in overall farming system (i.e., moving from a combinable crop rotation to a longer ley-based rotation) For instance, it may be good advice from the point of view of water quality to encourage farmers to reduce the amount of nitrogen they apply However, unless they also change their rotation, cultivation, and management of plant debris, a situation may arise whereby the actual biological health of the soil in a given field may be little altered Whilst relatively intensive annual cropping systems are both productive and maintainable in the short term, over longer periods they alter the movement and distribution of carbon within soils In effect, the modern agroecosystem is typified by systems in which the flow of carbon through the system is large (particularly in human trade flows and soil organism 920103_CRC20_0904_CH16 1/13/01 11:16 AM Page 347 CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 347 respiration), while the stock of residual carbon is gradually depleted Whereas some agroecosystems appear robust enough to withstand these changes, more marginal regions are likely to become prone to wind and water erosion, suffer severe drying in summer, or have shorter periods for cultivation during which physical damage can be minimized These regions in particular require sensitive soil management and are where the exploration of alternative agricultural practices is most urgent, e.g., minimum tillage regimes, the introduction of longer rotations including a period of perennial cropping, use of deeper rooting plants, intercropping practices, maintenance of strategic tree cover, and conversion to organic systems Thus, the challenge to agriculturists and soil scientists must be to investigate the way in which individual and compounded farming practices can influence soil biological health across a range of agroecosystem types as well as to formulate economically viable strategies for monitoring the change to more sustainable agroecosystems ACKNOWLEDGMENTS The author wishes to thank Dr John Finn and Mr Richard Tranter for their help reading and editing the script REFERENCES Allison, F.E., 1973 Soil Organic Matter and Its Role in Crop Production Elsevier, London Allen, T.F.H and Hoekstra, T.W., 1992 Toward a Unified Ecology Columbia University Press, New York Anderson, J.M., 1975 Succession, diversity and trophic relationships of some animals in decomposing leaf litter J Anim Ecol 44:475–495 Balesdent, J., Mariotti, A., and Boisgontier, D., 1990 Effect of tillage on soil organic carbon mineralisation estimated from carbon 13 abundance in maize fields J Soil Sci., 41:487 –596 Boardman, J., 1990 Soil erosion on the South Downs: a review, in Soil Erosion on Agricultural Land Boardman, J., Foster, I.D.L and Dearing, J.A (Eds.) 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