Introduction

The bulk of terrestrial net primary production (NPP), along with the bodies and excretions of animals, is returned to the soil as dead organic matter. Some 90% of NPP eventually enters the soil system through dead plants in grasslands; through leaves, roots, and wood in forests; and through organic residue in agricultural fields. Indeed, ecosystems may be viewed as consisting of four functional subsystems: (1) the production subsystem, (2) the consumption subsystem, (3) the decomposition subsystem, and (4) the abiotic subsystem. The decomposition subsystem serves to reduce dead residues to carbon dioxide (CO2) and soil organic matter, and to release nutrient elements for entry into soil food webs, and ultimately for reaccumulation by plants. The decomposition process drives complex belowground food webs, in which chemical forms of nutrient elements become modified. It is responsible for the creation of long- and short-lived organic compounds important in nutrient dynamics, and it fuels the formation of soil structure.

Terrestrial plant growth is highly dependent on the decomposition system, particularly in oligotrophic soils where nutrient stocks are held in litter and soil organic matter, rather than in mineral soil. Het-erotrophic organisms in the soil are ultimately responsible for ensuring the availability of nutrients for primary production (Wardle, 2002). Thus the two subsystems, primary production and decomposition, are dependent upon each other. We emphasize, again, the necessity for evaluation of entire ecosystems when considering their respective parts. The soil subsystem performs crucial functions within terrestrial ecosystems, regardless of how modified the terrestrial ecosystems may be. Decomposition processes in highly modified agricultural systems still involve a significant variety of heterotrophic organisms with characteristic abilities (Wasylik, 1995).

Decomposition per se is the catabolism of organic compounds in plant litter or other organic detritus. As such, decomposition is mainly the result of microbial activities. Few soil animals have the enzymes that would allow them to digest plant litter. Animal nutrition depends upon the action of microbes, either free-living in the soil or specialized in the rhizosphere or in animal guts. However, the term "decomposition" is often used more generally to refer to the breakdown or disappearance of organic litter. In that context, the decomposition of organic residue involves the activities of a variety of soil biota, including both microbes and fauna, which interact together. The term "litter breakdown" has been applied to the interactive process, which results in the disappearance of organic litter.

Continuing interest in decomposition is apparent from the large number of studies of the process that have been published during the past 25 years. More than 1000 such publications have appeared in peer-reviewed journals, and the number would be much larger if symposia or reports on heterotrophs themselves were to be included (Heal et al., 1997). Improved understanding of the decomposition process has accompanied the refinement of methods and conceptual models. The lit-terbag technique (Bocock and Gilbert, 1957; Shanks and Olson, 1961; Edwards and Heath, 1962; Crossley and Hoglund, 1962) has become a major tool in these studies, despite its limitations (Heal et al., 1997). Radioactive tracers (Olson and Crossley, 1962) have been replaced by methods using stable isotopes of carbon and nitrogen (Nadelhoffer and Raich, 1992; Boutton and Yamasaki, 1996). Early models of mass loss (Jenny, 1941; Olson, 1963) defining a decomposition constant, k, are being supplanted by more sophisticated models that consider different constituents of litter (Jenkinson et al., 1987; Parton et al., 1994; Sinsabaugh and Moorhead, 1997).

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