Although not generally visible to the naked eye, soil is one of the most diverse habitats on earth and contains one of the most diverse assemblages of living organisms (Giller et al., 1997). It is one of nature's most complex ecosystems: it contains thousands of different organisms, which interact and contribute to the global cycles that make all life possible - the life support systems. Nowhere in nature are species so densely packed as in soil communities (Hågvar, 1998). For example,

• Over 1000 species of invertebrates may be found in a single m² of a European beech forest (Schaefer and Schauermann, 1990)
• Many of the world’s terrestrial insect species are soil dwellers for at least some stage of their life-cycle (Bater, 1996)
• A single gram of soil may contain millions of individuals and several thousand species of bacteria (Torsvik et al., 1994).
• A typical, healthy soil might contains several species of vertebrate animals, several species of earthworms, 20-30 species of mites, 50-100 species of insects, tens of species of nematodes, hundreds of species of fungi and perhaps thousands of species of bacteria and actinomycetes.

The complex physical and chemical nature of the soil, with a porous structure, immense surface area, and extremely variable supply of organic materials, food, water and chemicals mean that various animal, plant and microbial worlds can co-exist simultaneously and find appropriate niches for their development. This provides a range of habitats for a multitude of fauna and flora ranging from macro- to micro- levels depending on climate, vegetation and physical and chemical characteristics of the given soil. The species numbers, composition and diversity of a given soil depends on many factors including aeration, temperature, acidity, moisture, nutrient content and organic substrate.

Soil biodiversity tends to be greater in forests compared to grasslands and in undisturbed natural lands compared to cultivated fields. However the number and types of organisms vary from one system and environment to another and this is strongly influenced by agricultural practices. It is recognised that soil biodiversity can be used as an indicator of soil quality and stable ecosystems.

The community of soil organisms incorporates plant and animal residues and wastes into the soil and digests them, creating soil humus, the organic constituent so vital to good physical and chemical soil conditions, and recycling carbon and mineral nutrients. This decomposition process includes the release of CO₂ to the atmosphere where it can be recycled through higher plants and the release of essential plant nutrients in inorganic forms that can be absorbed by plant roots or leached from the soil. All soil organisms have important effects not only on soil properties but also on the functioning of the ecosystem.

Soil biodiversity, or the diversity of life in soil at the genetic, organismal and ecological level, is thus comprised of the organisms that spend all, or a portion of, their life cycles within the soil or on its immediate surface (including surface litter and decaying logs).

The easiest and most widely used system for classifying soil organisms is by using body size and dividing them into three main groups: macro-, meso- and micro-biota (Wallwork, 1970; Swift et al., 1979). Unfortunately, as can be seen in Figure 1, the ranges that determine each size group are not exact for all members of each group, often leading to considerable confusion as to whether a particular organism should be considered macro or meso, and so on.

1. Macrobiota, or organisms generally >2 mm in diameter and visible to the naked eye. These include vertebrates (snakes, lizards, mice, rabbits, foxes, badgers, moles and others) that primarily dig within the soil for food or shelter, and invertebrates that live in, feed in or upon the soil, the surface litter and their components (ants, termites, millipedes, centipedes, earthworms, pillbugs and other crustaceans, caterpillars, cicadas, ant-lions, beetle larvae and adults, fly and wasp larvae, earwigs, silverfishes, snails, spiders, harvestmen, scorpions, crickets and cockroaches). Roots, although not generally considered soil organisms, grow mostly within the soil and have wide-ranging, long-lasting effects on both plant and animal populations above- and below-ground, and thus should be included among soil biota.
2. Mesobiotaare organisms generally ranging in size from 0.1 to 2 mm in diameter. These include mainly micro-arthropods, such as pseudo-scorpions, protura, diplura, springtails, mites, small myriapods (pauropoda and symphyla) and the worm-like enchytraeids. Mesobiota have limited burrowing ability and generally live within soil pores, feeding on organic materials, microflora, microfauna and other invertebrates.
3. Microbiota are the smallest organisms (<0.1 mm diam.), extremely abundant, ubiquitous and diverse. The microflora includes algae, bacteria, archaea, cyanobacteria, fungi, yeasts, myxomycetes and actinomycetes that are able to decompose almost any existing natural material. The microfauna includes nematodes, protozoa, turbellarians, tardigrades and rotifers that generally live in the soil water films and feed on microflora, plant roots, other microfauna and sometimes larger organisms (e.g., entomopathogenic nematodes feed on insects and other larger invertebrates).

Soil comunities are so diverse in both size and numbers of species, yet they are still extremely poorly understood and in dire need of further assessment. Research has been limited by their immense diversity and by technical problems. Groups such as viruses, algae, yeasts, myxomycetes, cyanobacteria, rotifers, aphids, gastropods, tardigrades, turbellarians and others, have been little studied and in generally restricted environments . Moreover, there is a large imbalance in the knowledge of tropical versus temperate species (Brussard et al., 1997). For other groups such as the highly diverse beetles (Coleoptera), flies (Diptera), homopterans, hemipterans, cockroaches, snails, pseudo-scorpions and scorpions, many of which have soil-dwelling larval, juvenile or adult stages, there is a poor knowledge of the number and proportion of species associated with soil versus above-ground environments.

Some of the available estimates on the number of species presently described of selected soil biota that have been better studied are given in Table 1. However, that these estimates are still preliminary and much lower than the estimated total number of species within each group. For example, the described number of soil dwelling fungal species ranges from 18-35,000, while the projected number may be >100,000 (Hawksworth, 1991). Other organisms expected to be much more species-rich are the nematodes and mites, with perhaps only 3 and 5%, respectively, of the total species presently described (Walter and Proctor, 1999; Hawksworth and Mound, 1991). The estimates for bacteria and archea species are particularly problematic because of the differences in opinion as to what criteria should be used to define a species, and the present unculturability of many of these organisms (Hawksworth and Kalin-Arroyo, 1995). Besides numbers of species, their biomass is also an important consideration, see a figure from The Soil Biology Primer.

Further information on Habitat, Diversity and Functions of earthworms, termites, soil micro-fauna, rhizosphere, soil algae, soil fungi, soil actinomycetes and soil bacteria

Besides being of interest because of their diversity and the complex relationships they have developed over time between themselves and their environments, soil organisms are also a precious resource important to both human societies and ecosystems. Because of the large range in their sizes and in the functions they perform, the number and type of services they provide to both humans and ecosystems is immensely variable. In both natural and agroecosystems soil biota are responsible, to a varying degree (depending on the system), for performing vital functions in the soil ecosystem. These functions, performed and often controlled by the myriad of organisms in soils, range from physical effects such as the regulation of soil structure and edaphic (in soil) water regimes, to chemical and biological processes such as degradation of pollutants, decomposition, nutrient cycling, greenhouse gas emission, carbon sequestration, plant protection and growth enhancement or suppression (see Table 2).

The activities of soil organisms interact in a complex food web with some subsisting on living plants and animals (herbivores and predators), others on dead plant debris (detritivores), on fungi or on bacteria and others living off but not consuming their hosts (parasites). Plants, mosses and some algae are autotrophs, they play the role of primary producers by using solar energy, water and carbon from atmospheric CO₂ to make organic compounds and living tissues. Other autotrophs obtain energy from the breakdown of soil minerals (oxidation of N, S, Fe and C from carbonate minerals). Soil fauna and most fungi, bacteria and actinomycetes are heterotrophs, they rely on organic materials, directly (primary consumers) or through intermediaries (secondary or tertiary consumers), for carbon and energy needs.

In an attempt to reduce the innate complexity of the soil biota and their functions to manageable levels, various functional groups of the soil biota have been proposed. These groups help illustrate in a simpler manner the functions performed in soil, the organisms that perform them, and which function and biota may be more important in particular ecosystems. Of the different functional group classifications available, perhaps the most useful are those dividing the soil biota into different feeding behaviors and sizes.

The division of soil biota into roots, ecosystem engineers, litter transformers, phytophages and parasites, micro-predators and microflora (see Table 3) is a good example (see Lavelle, 1996), because it also takes into account the potential top-down regulatory controls of larger organisms (e.g., the ecosystem engineers) over smaller ones.

Ecosystem engineers include termites, ants and earthworms, whose bioturbating activities produce structures that can last long periods of time (outlasting the organisms that produced them) and affect soil organic matter dynamics and soil physical processes. Litter transformers include many macro- and micro-arthropods, enchytraeids and other detritus feeders that stimulate the breakdown and decomposition of surface litter and organic matter, producing small, primarily organic fecal pellets. Phytophages and parasites include all organisms that feed upon or destroy plant parts, both above and below-ground. The micropredators are primarily microfauna such as nematodes and protozoa that do not produce any physical structures and survive by predation on microflora and other organisms, thus stimulating mineralization of organic matter and plant nutrient availability. At the lowest level, the microflora act upon organic matter and nutrient cycles, root and rhizosphere processes and plant production (with both positive and negative effects).

The activity of smaller organisms is therefore expressed against the background of the effects of larger organisms, that are in turn, expressed against the backdrop of climate, plant community and soil properties. In this hierarchical system, in which higher levels constrain activity at lower levels of spatio-temporal organization (Figure 2), through top-down controls (Lavelle et al., 1993). Bottom-up control (feedback) also exists, for example, the ecosystem engineers can alter ecosystem performance with effects on their own and other populations (Jones et al., 1994).

Soil Biota and Agriculture

Thus different groups of soil biota clearly influence soil properties and processes, including pedogenesis, nutrient and water cycles and availability, decomposition, aggregation and oporosity, biological processes (see Table 4), However, the identification of their roles in plant production has been difficult (Anderson, 1994). This is often due to scale problems in research, where crop performance is measured at plot scale and over a growing season, which integfrates across (and thus dilutes) the generally smaller scale, shorter term specific effects of soil biota (Lavelle, 2000). In addition it is difficult to determine the various interactions between above and belwo ground biodiversity. Nevertheless, there are examples of both positive and negative effects of some functional groups, particularly microorganisms, phytoparasites/pathogens or rhyzophages, plant roots, and macrofauna on plant production.

Human activities through their different management practices and technologies also exert an important influence on soil biota, their activities and diversity (see Figure 2). Clearing forested or grassland for cultivation drastically affects the soil environment and hence the number and kinds of soil organisms. In general the quantity and quality of plant residues and the number of species of higher plants is greatly reduced thus the range of habitats and foods for soil organisms is significantly reduced. Different types of agricultural practices and systems also greatly influence the soil biota. Through changing the physical and chemical environment the ratio of different organisms and their interactions is significantly altered by agricultural practices, for example, through adding lime, fertilizers and manures, through tillage practices, the use of pesticides and so forth.

The beneficial effects of soil organisms on agricultural productivity and ecological functioning may be affected including:

• organic matter decomposition and soil aggregation;
• breakdown of toxic compounds both metabolic by-products of organisms and agrochemicals;
• inorganic transformations that make available nitrates, sulphates, and phosphates as well as essential elements such as iron and manganese;
• nitrogen fixation into forms usable by higher plants.

Many soil organisms are also detrimental to plant production. For example some moles and rodents may seriously damage crops, snails and slugs are serious pests as well as some ants, aphids and nematodes. As for microflora, bacteria and actinomycetes cause some plant diseases, but most damage is caused by fungi which account for most soil borne crop diseases such as wilts, root rot, clubrot, and blight. Soil organisms may compete for nitrogen with higher plants and, under conditions of poor drainage, soil organisms may compete for limited oxygen.

Although humans generally begin their influence on soil biodiversity with naturally-present communities at a particular site (resulting essentially from ecological and evolutionary forces), they also have the ability to introduce new organisms and, through imposition of different management practices, put selective pressures on the naturally-present or introduced soil biota. This provides the opportunity to manage soil organisms and their activities to enhance soil fertility and crop growth. In theory, probably enough is known to manage these communities, yet considerable basic and applied research is needed to reach appropriate levels of biological husbandry and optimal management of these biological resources (Hendrix et al., 1990).

Agricultural practices can have significant positive and negative impacts. For example, high external-input agriculture can overcome specific soil constraints through the use of inorganic fertilizers, pesticides, and other amendments, in order to meet plant requirements (Sanchez, 1994; 1997). Although these practices have led to considerable increases in overall food production worldwide, they also tend to decrease or disregard the potential benefits of soil biological activities in maintaining soil fertility and enhancing plant production. Furthermore, the misuse or overuse of these practices has led to soil and environmental degradation (i.e., depletion or loss of soil fertility and its physical and biological components, contamination of surface and ground water) and declines in productivity in certain areas of the world. In addition, the vast majority of the world’s farmers do not have access to, or cannot afford, the external inputs necessary to apply the principles and practices of high external input agriculture (Vandermeer et al., 1998).

As presented in the section on management, what is needed is the development and implementation of an integrated approach to agriculture that considers potential impacts on the environment and the soil. Consideration of biological, chemical and physical implications of land use and management practices and of ecological principles will allow agricultural productivity to be sustained in low and high productive environments. Effective soil biological management will provide opportunities for enhancing productivity and for the restoration of degraded soils.

References