Soil organism play an essential part in the functioning of soil and carry out a range of soil processes (Barrios, 2007).
To summarise the nutrient and energy flow interactions which occur within an ecosystem or between different ecosystems, a “food web diagram (see opposite)” may be used. Such diagrams can encompass a wide range of scales, from large mammals through to soil organisms such as fungi and bacteria and often include minerals such as organic and inorganic forms of nitrogen.
The activity at one scale may affect the process occurring at another. For example, mites or collembola feeding on bacteria and fungi at the scale of a few millimetres have effects on microbial processes and their communities within a small zone of a few centimeters (Anderson, 1995). On the other hand, the feeding and burrowing behaviour of earthworms may create pores and burrows of a few millimeters in diameter that affect soil structure and hydrological processes at the scale of metres, while feeding activities of termite and ant colonies may affect soil physical and chemical processes over the scale of hectares (Swift et al., 1996). The activity of smaller organisms is therefore expressed against the background of the effects of larger organisms, which are in turn, expressed against the backdrop of climate, plant community and soil properties. In this hierarchical system, higher levels constrain activity at lower levels of spatio-temporal organization, through top-down controls (Lavelle et al., 1997). 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).
The stability of the “food web” is thought to be linked to the abundance of the different functional groups of organisms (De Ruiter et al,. 1995) although it is unclear as to how many species are needed to sustain the ecosystem processes required for a particular ecosystem service (Barrios, 2007).
Many of the elements contained with the “food web” are critical to its correct functioning. However, sometimes knowledge of a particular part of the web is missing or incomplete so a “black box” approach is often adopted which contains the elements within which very little is known.
One starting point for the soil food web is to look at the energy requirements of the organisms involved. The basis of nearly every food web are autotrophic organisms. These convert the energy from solar radiation into biomass via photosynthesis or analogous processes. Examples of these are the plants and the cyanobacteria, the latter of which inhabit the first few millimetres to centimetres of soil and co-incidentally also fix atmospheric nitrogen. These traits help in the formation of soil crusts in bare soils and thus limit erosion. Similarly, their nitrogen fixing abilities can aid soil fertility and in some cases, cyanobacteria live in intimate associations with plant roots.
The solar energy fixed in the biomass of plants and autotrophic soil micro-organisms is slowly recycled via a cascade of degraders, which can include the larger soil animals, e.g. earthworms, enchytraeid worms, nematodes and mites all the way to the final degraders that use the intermediate by products produced along the food chain. An example of the organisms that operate at this lowest level are the methane oxidising bacteria, which convert methane, on of the end products of anaerobic decay, back into carbon dioxide. Thus the cycle can start again.
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