Aiming at an improved understanding of the conditional nature of,oil organic matter stability, we present an overview of (1) biotic strategies and (2) ecological processes by which decomposer organisms gain access to, or are prevented from metabolising soil organic resources. The biotic strategies discussed comprise well-known activities, such as the release of exo-enzymes, the mechanical crushing of organic residues, the bioturbation of soil mass, and the fixation of carbon in the living biomass. The ecological processes described have received less attention regarding their importance in prolonging the persistence of soil organic matter. Model calculations illustrate that cell energy de-land forces micro-organisms to operate at low decomposition rates, and that diffusion losses inhibit microbial growth and impede the formation of new microbial colonies. The specialisation of decomposer organisms towards particular microhabitats and substrates gives rise to refuges where organic resources are temporarily not accessed. We derive four stability criteria, by which we classify organic matter pools: passive versus active stabilisation, and partial versus absolute refuge. Two case studies confirm that in temperate soils a dominant quantity of organic material resides in the passive stabilisation/partial refuge status and persists in spite of being accessible and decomposable. We conclude that soil organic matter is stabilised by a complex of mechanisms that constrain decomposition rates, several of which are not based on substrate quality or soil conditions, but on the biology of the decomposing soil organisms. (c) 2004 Elsevier B.V. All rights reserved.
