In both natural and agroecosystems, soil organisms are responsible, to a varying degree (depending on the system), for performing vital functions in the soil ecosystem (Barrios, 2007). These are carried out in a range of scales from microns to meters to kilometres.
Many of the functions performed in soil and, often controlled by the myriad of organisms, are focussed on “hot spots” of activity where biological activity may be concentrated, for example at the root/soil interface (the ”rhizosphere”) or locations where there are pools of organic matter such as areas of decaying plant material (the “detritusphere”) or, the linings of earthworm burrows (the “drilosphere”) which are covered in earthworm secretions and so are nutrient rich whilst other functions may occur in the non-rhiozosphere soil. The functions 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 (click to open table).
To make it easier to understand the complexity of the interactions between the soil biota and reduce their functions to manageable levels, soil functions can be divided into specific tasks (Barrios, 2007) with microorganisms lying at the heart of many of the functions (click to open table). From these a direct link to ecosystem services may be made (Decaens et al., 2006).
Another way is to divide the soil biota on the basis of body size or social grouping such as roots, ecosystem engineers, litter transformers, phytophages and parasites, micro-predators and microflora (click to open table). This approach can take into account the potential top-down regulatory controls of larger organisms (e.g. the ecosystem engineers) over smaller ones. For example at the top level, termites, ants and earthworms acting as ecosysetem engieers, 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. At a lower level, microflora act upon organic matter and nutrient cycles, root and rhizosphere processes and plant production (with both positive and negative effects). Of course it must be remembered that the interaction may also work the way i.e. a "bottom-up" approach where the activity of microorganisms, for example, may regulate those at a higher level through feeding and release of nutrients.
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