Fig. 1

Illustration of the principal reactions involved in the decomposition of plant residues in the environment. The abiotic reactions are catalysed by transition metals such as iron, and by minerals of geological origin. Light promotes certain abiotic reaction paths. Enzymes secreted into the environment by microbes such as saprotrophic bacteria and fungi catalyse a similar range of reactions, but via different chemical mechanisms and at different rates. A representative pH profile based on [47] is indicated