Modeling the effect of substrate stoichiometry on microbial carbon use efficiency and soil C cycling

Microorganisms degrade soil organic matter (SOM) and apportion newly acquired substrates into enzyme production, biomass growth, and respiration. The fraction of acquired substrate that is released into the atmosphere as heterotrophic respiration is determined by the microbial carbon use efficiency (CUE), commonly defined as the fraction of carbon uptake that is allocated to microbial growth and enzyme production. Despite recent demonstrations that changes in CUE can greatly affect predictions of global soil C stocks, most models do not incorporate process-level representation of CUE or how it varies with substrate stoichiometry. Here we introduce coupled C and N cycling into a prognostic CUE model that uses the dynamic energy budget theory to predict CUE at each time step. We solve this model over a range of substrate C:N to simulate the effects of N addition on CUE, and test the model against previously published measurements of CUE after nutrient enrichment with a range of substrates. We find that CUE declines with microbial N limitation due to C overflow and acquisition strategies that favor N immobilization. We also demonstrate that including an intracellular reserve pool in the model alleviates decreases in CUE by allowing excess C to be stored during periods of N limitation. Consistent with previous studies, we find that predictions of soil C stocks are highly sensitive to CUE. Furthermore, we show that interactive effects between substrate inputs and temperature result in a wide range of possible CUE values under global change scenarios.