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 istransformed to soil organic C and CO2. Stabilization of a portion of the litter C yields materialthat resists further transformation; destabilization yields material that is more susceptible tomicrobial respiration. Stability of the organic C is viewed as resulting from three general sets ofcharacteristics. Recalcitrance comprises, molecular-level characteristics of organic substances,including elemental composition, presence of functional groups, and molecular conformation, thatinfluence their degradation by microbes and enzymes. Interactions refers to the inter-molecularinteractions between organics and either inorganic substances or other organic substances that alterthe rate of degradation of those organics or synthesis of new organics. Accessibility refers to thelocation of organic substances with respect to microbes and enzymes. Mechanisms by which thesethree 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 dynamicswould 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) moreattention to effects of inter-molecular interactions (especially Al complexation) on enzyme activityand substrate degradation; (3) increased effort to experimentally manipulate soils, preferablyacross a range of soil types; (4) study of stabilization and destabilization mechanisms underconditions 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 effectsof other mechanisms operating simultaneously.