The complete destruction of the community of soil organisms—for example, due to erosion—results in obvious loss of soil ecosystem functions. Far less is known about the consequences of the loss of soil biodiversity for the sustainable delivery of ecosystem goods and services (Wall et al. 2001). Results from empirical studies on the relation between soil biodiversity and ecosystem functions range from positive to neutral or even negative (Mikola et al. 2002). However, it is obvious that soil biodiversity may act as a source of insurance, thereby making systems more stable. For example, when soil biodiversity is reduced by one stress factor or event, the soil community may be less able to recover from a repeated or second stress. Such accumulation of stresses may well result in the loss of stability of soil ecosystem processes (Griffiths et al. 2000).
Belowground processes that involve decomposition of organic matter, transformations of nutrients, and the supply of nutrients from the soil for plant growth are driven in the first instance by the activities of bacteria and fungi (Wardle 2002). Biotic drivers of microbially driven processes in soils include plants, their herbivores and pathogens, and other soil animals. Plant species differ in the quantity and quality of litter (dead organic matter that is sufficiently intact to be recognized) and rhizosphere materials that they return to the soil. This in turn governs the composition, growth, and activity of the microflora, and, hence, the rates of soil process (Hooper & Vitousek 1998). Aboveground herbivores can strongly influence soil organisms through a number of mechanisms by which they alter the quantity and quality of resources entering the soil (Bardgett & Wardle 2003). Soil animals have important effects on microbial activity at a range of scales (Lavelle 1997): by consuming microbes directly, by transforming litter and thus altering its physical structure, by creating biogenic structures, or by entering into mutualisms with microbes either in the "external rumen" or in the gut cavity (Lavelle 1997).
Some groups of soil organisms have a direct relationship with plant roots (Wardle et al. 2004). Soil pathogens and root herbivores are notorious for causing yield reductions of arable crops, re-sowing failures in grassland, disease in orchards that requires replanting, and die-back in production forest. Root pathogens and herbivores can also have strong impacts on the species composition of natural vegetation and the rate of changes therein (Brown & Gange 1989; van der Putten 2003). Root pathogens and herbivores contribute to primary and secondary plant succession (Brown & Gange 1992; van der Putten et al. 1993; De Deyn et al. 2003) and plant species diversity (Bever 1994; Packer & Clay 2000). Plant species that escape from soil pathogens may become invasive in new territories (Klironomos 2002; Reinhart et al. 2003). Root symbionts, such as mycorrhizal fungi and nitrogen fixing microorganisms (e.g., Azobacter, Rhizobium, and Cyanobacter), are important for plant nutrition in unmanaged systems and in non-tilled, low-input arable systems (Smith & Read 1997).
The various biotic and abiotic drivers in soil systems do not operate independently of one another, and interactions among them are important determinants of soil processes. For example, a recent litter exclusion study found that soil microarthropods play an important role in decomposition in a tropical forest but not in a temperate forest, where the abiotic drivers of microbial activity emerged as being of greater importance (Gonzalez & Seastedt 2001). Similarly, substrate fertility determines the role of microbially driven processes in supplying nutrients: in fertile conditions, plants produce litter of high quality from which ammonium (NH4+ is readily released by microbes and taken up by plants, while in infertile conditions plants produce poor-quality litter protected by polyphenols from which nitrogen is not readily mineralized. In these infertile systems, plants often bypass the mineralization process entirely by taking up organic nitrogen directly (e.g., Northup et al. 1995).
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