Of the total terrestrial net primary productivity of 62.6 Pg C yr-1, forests are responsible for 52 percent (ca. 32.6 Pg C yr-1) (IPCC 2001; Saugier et al. 2001; Gower 2002). When combined with the large land surface area covered by forests (boreal, temperate, and tropical), savannas, and shrubland systems (ca. 72X106 km2), it is clear that nearly 50 percent of the organic input into soils are from forest litter and plant roots. The role of the forest soil biota is immediately highlighted when we realize that the majority of the world's forests rely heavily on internal nutrient cycling to maintain productivity (see, for example, Miller et al. 1979) and, in the total absence of soil biota, these systems would become extremely nutrient limited.
Not only is nutrient cycling controlled by the soil biota but several major sources of nutrient input to forests (e.g., mineral weathering and N fixation) are also mediated or strongly affected by soil biological activity. Thus, factors that impair soil biological activity (e.g., acidification, heavy metal pollution) have negative consequences for both nutrient inputs and subsequent cycling. There is also an increasing appreciation that our understanding of forest nutrient cycling and the high inorganic N concentrations currently found in many of the soils of the major developed regions of the world are atypical of unmanaged, pristine forests (Perakis & Hedin 2002), where more tightly coupled nutrient cycling, frequently involving intimate links between primary producers and soil biota, may dominate (Ohlund & Nasholm 2001). At the most basic level, forest soil biota are essential for the maintenance of the forest, but these organisms also have very direct effects on the ecosystem services provided within the forest.
Although examples exist that show where soil biodiversity is critical in maintaining an ecosystem service (e.g., specific edible fungal fruiting bodies, inoculation or manipulation of wood stump saprotrophs to control pathogens), in general we are ignorant of the extent to which soil diversity is important in maintaining services. The observation that increased inorganic N additions to forest soils can greatly impair methane oxidation rates (Wang & Ineson 2003) is an example of how soil organisms are clearly controlling an important ecosystem service, but direct information of the diversity of the species involved is proving extremely difficult to extract (Bull et al. 2000).
In unmanaged humid tropical forests it is rare that any specific identifiable group of organisms have a role in soil processes. Litter breakdown is accomplished by many disparate groups of organisms such as crabs, millipedes, cockroaches, and so on. Within these groups there is always some marginal overlap in feeding-niche parameters. Similarly, soil-feeding termites (which exhibit particularly high species diversity in African forests) produce significant methane fluxes and play an important role in P cycling and soil organic matter turnover. However, all these processes are also mediated by other animals and microbes, and strongly buffered by the biophysical pools of soil organic matter and ion exchange sites (Anderson 1994). Rate determinants of key processes, such as hydro-logic pathways and erosion, are affected by the combined activities of microbes, animals, and plant roots on aggregate stability and soil structure (Silver et al. 2000).
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