Stabilization and destabilization of soil organic matter: mechanisms and controls

We present a conceptual model of the processes by which plant leaf and root litter is transformed to soil organic C and CO2. Stabilization of a portion of the litter C yields material that resists further transformation; destabilization yields material that is more susceptible to microbial respiration. Stability of the organic C is viewed as resulting from three general sets of characteristics. Recalcitrance comprises molecular-level characteristics of organic substances, including elemental composition, presence of functional groups, and molecular conformation, that influence their degradation by microbes and enzymes. Interactions refers to the inter-molecular interactions between organics and either inorganic substances or other organic substances that alter the rate of degradation of those organics or synthesis of new organics. Accessibility refers to the location of organic substances with respect to microbes and enzymes. Mechanisms by which these three characteristics change through time are reviewed along with controls on those mechanisms. This review suggests that the following changes in the study of soil organic matter dynamics would speed progress: (1) increased effort to incorporate results into budgets for whole soil (e.g., converting to a kg/ha basis) so that the relative importance of processes can be judged; (2) more attention to effects of inter-molecular interactions (especially Al complexation) on enzyme activity and substrate degradation; (3) increased effort to experimentally manipulate soils, preferably across a range of soil types; (4) study of stabilization and destabilization mechanisms under conditions that are well defined yet more relevant to soil environments than those used previously; and (5) experiments better designed to isolate mechanisms so results are not confounded by effects of other mechanisms operating simultaneously.