Depolymerization of Organic Matter As a Bottleneck for C Cycling.

Enzyme activities play an undisputable role in soil carbon cycling. They catalyse the breakdown of organic substrates into small compounds before they can further be assimilated and respired or transformed into microbial metabolites. To account for this major process, a new generation of models of C dynamics has emerged during the past decade, reporting enzyme properties and decomposer physiology. However, these models are built on the premise that organic matter (OM) can be represented as one single entity or is divided only into a few pools, while organic matter exists as a continuum of many different forms. The key question thus still remains: how does enzyme functional diversity regulate the progressive depolymerization of OM and its turnover?

We challenge this question with a new model based on substrate accessibility to the decomposers and to their enzymes. This model integrates the fact that a given substrate exists in many different forms, depending on its stage of polymerization or its interactions with other organic or mineral phases of the soil matrix. The model reports how the level of polymerization of the substrate evolves under enzyme action until a threshold value where the substrate becomes accessible for microbe uptake.

Our simulations reveal how the lignolytic activity regulates the action of cellulolytic enzymes during the decomposition of woody litter. We also show how the breakdown of highly diverse plant and microbe substrates needs the successive action of different decomposer communities. We finally demonstrate that enzymes action strongly controls the amount of C sequestered in soil and its biochemistry.