Background/Question/Methods   Soils harbor large stores of carbon (C), more than in vegetation and the atmosphere combined, and play an important role in the global C budget. The role microbes play in determining the fate of soil organic carbon (SOC) has long been appreciated, but the challenges associated with characterizing interactions between microbes, the extracellular enzymes they produce, and SOC has presented significant hurdles to fully understanding decomposition dynamics. To make accurate predictions about the rate of CO2 release from soils, it is necessary to understand how the SOC landscape, environmental variables such as temperature, and interactions among microbes influence the costs and benefits associated with extracellular enzyme production. We present a theoretical framework that explicitly incorporates the entire spectrum of enzyme- substrate interactions, and the costs and benefits associated with producing extra-cellular enzymes. We describe how the optimal enzyme production strategy can be determined and how it is likely to change in response to varying environmental conditions.

Results/Conclusions   If resources are acquired independently in constant conditions, the optimal enzyme production strategy is to synthesize a single enzyme associated with each resource, regardless of substrate composition. Temporal variability in environmental conditions and substrate variability allows what would otherwise be sub-optimal strategies to persist. The C:N ratio of the enzymes themselves is linearly related to the C:N of microbial demand whereas the metabolic costs of enzyme production influence the C:N of microbial demand in a highly non-linear fashion. Furthermore, a change in the rate of CO2 production can be directly tied to shift in enzyme production, which in turn, reflects a shift in substrate utilization. We also demonstrate how the theoretical framework enables other measurements, such as C and N isotopes, to infer shifts in extracellular enzyme production and substrate utilization.