OOS 41-10
Substrate limitation in microbial decomposition models
Recent developments in soil organic carbon (SOC) decomposition include the explicit incorporation of enzyme and microbial dynamics. A characteristic of these models is a feedback between substrate and consumers which is absent in traditional first order decay models. Under sufficient large substrate, this new feedback allows an unconstrained growth of microbial biomass. A second phenomenon is a strong sensitivity to microbial carbon use efficiency and microbial turnover under warming. Here, we take a look at these physiological models by retaining the basic enzyme dynamics, and consider mechanisms where the marginal rate of decomposition decreases as the microbial biomass and the production of extracellular enzymes increase. We present two mechanisms that lead to a marginal return: a finite amount of sites where enzyme-substrate reactions can take place and a case where microbes feed on extracellular enzymes. Finally, we relax the condition that the microbial productivity of enzymes scales with microbial biomass.
Results/Conclusions
The models that incorporate a decreasing marginal return with increasing microbial biomass and enzyme production are structurally different from previous microbial models. Microbial biomass growth is curbed which reduces the feedback between substrate and microbial biomass. Moreover, at a constant substrate concentration, microbial biomass does not grow indefinitely, but now reaches a finite equilibrium. In presence of a decreasing marginal return, increased enzyme production does not lead necessarily to enhanced microbial growth. Because there is a respiratory cost associated with enzyme productivity, a decreasing marginal return allows for consideration of microbial resource optimization. Overall, microbial models with decreasing marginal return result in stronger substrate limitation and are qualitatively similar to the traditional first order decay formulation widely used in decomposition models such as CENTURY.