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

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 associ-
ated 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.
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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 com-
munity 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 neces-
sarily 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, inter-
ruption 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 consider-
able 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 bioindica-
tors, 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 socio-
economic 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 devel-
oping (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 indi-
rectly change soil structure or properties and thus lead to changes in agro-
ecosystem 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
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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 sustain-
ability 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 taxo-
nomic 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
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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 under-
standing of the function of macrofauna in soil processes. It has been sug-
gested 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 asso-

ciated 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:
1. 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 civ-
ilization, 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.
2. 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
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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.
3. 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 func-
tion 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 distri-
bution of organic matter in soils.
ORGANIC CARBON AND ITS DISTRIBUTION IN SOILS
Organic materials act as binding agents within the soil, holding individ-
ual particles together. A review of the role of organic matter in aggregate sta-
bility 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 introduc-
tion 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) sug-
gest 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 condi-
tions. Well-incorporated organic materials add to the stability of soils by
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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 mate-
rial 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 ha
Ϫ1
) in the absence of other soil ani-
mals. 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 docu-

mented. Working with forest soils in the Netherlands, Van Der Drift (1963)
recorded litter disappearance rates of up to 4200 kg ha
Ϫ1
in a year. Similar
work by Raw (1962) estimated that the earthworm species Lumbricus terrestris
removed about 1.2 t ha
Ϫ1
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 do 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
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CHANGING SOIL BIOLOGICAL HEALTH IN AGROECOSYSTEMS 341
woodlice, are likely to stay closer to the surface. Mesofauna do 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 activi-
ties 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 qual-

ity of plant debris and root material being returned to the soil system. Inputs
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342 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
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 3.13 1.90 N/A N/A
1914 4.33 1.77 1.92 2.21
1944 4.05 1.80 1.92 2.11
1966 4.35 1.90 2.08 2.11
1987 4.64 1.78 1.94 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.
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 chemi-
cals 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 fertil-
ization 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 mat-
ter (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.
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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 glu-
cose, which decompose rapidly, still contribute to the stable organic materi-
als 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 gov-
ern rates of decomposition processes in the short term, over longer time peri-
ods 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 charac-
teristic root patterns, although these can be greatly modified by soil condi-
tions. 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 2 m, whereas roots of
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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
1 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 mate-
rial supplies a continuum of organic materials to the soil. These perennial
systems are not usually cultivated, and this not only allows the plant root sys-
tems 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 propor-
tion 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 plough-
ing. 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 min-
eralization of carbon and high respiration losses. Rapid recovery/reproduc-
tion 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) esti-
mated 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 8 years of the experiment. House et al.
(1984) summarize the effects of cultivation on the distribution of soil organic
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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 .94 1450
23–46 .61 2000
46–69 .47 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 “conven-
tional 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 biologi-
cal health in a given locality. It is possible that intensive monitoring of soil
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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 out-
lined. Studying the changes in the amount and distribution of soil organic
carbon together with data on the main functional groups in the soil can pro-
vide 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 applica-
tion 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 becom-
ing 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 inves-
tigate the impacts of modern cultivation methods on soil faunal populations.
Thus, the measurement of soil biological health in regard to agroecosys-
tem change and sustainability may present researchers with several dilem-
mas. 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., mov-
ing 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 produc-
tive 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 346
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 sys-
tems. Thus, the challenge to agriculturists and soil scientists must be to inves-
tigate 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.
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