Substrate and environmental controls on microbial assimilation of soil organic carbon: A modeling framework

Background/Question/Methods

Microbial assimilation of soil organic carbon is one of the fundamental processes in global carbon cycling and it determines the magnitude of microbial biomass in soils. Mechanistic and mathematical understanding of microbial assimilation of soil organic carbon and its controls is critically important for Earth system model improvements in simulating carbon-climate feedback. Although the efficiency of microbial assimilation of soil organic carbon is broadly considered to be an important parameter, it really comprises two separate components: the one-time assimilation efficiency and the time-dependent maintenance energy. In this study, a new term-microbial active period (the length of microbes being active in one year) is coined, and a simple modeling framework was developed to evaluate the substrate and environmental controls on microbial assimilation of soil organic carbon.

Results/Conclusions

Substrate quality has positive effects on microbial assimilation of soil organic carbon-higher substrate quality (lower C:N ratio) leads to higher ratio of microbial carbon to soil organic carbon and vice versa. Increases in microbial active period from zero stimulate microbial assimilation of soil organic carbon; however, when microbial active period is larger than its optimal threshold, longer microbial active period causes decreases in ratio of microbial biomass carbon in soil organic carbon. The simulated ratios of soil microbial biomass to soil organic carbon are reasonably consistent with a recently compiled global dataset at biome-level. The mathematical mechanism of substrate and environmental controls on microbial assimilation of soil organic carbon developed in this study offers an applicable ways to incorporate microbial controls on carbon cycling into Earth system models for simulating carbon-climate feedbacks.