Disentangling the effects of microbial functional diversity and carbon availability on soil organic carbon decomposition

The factors governing stability of soil organic carbon vary from chemical characteristics to physical occlusion from either biotic (such as plant roots, soil fauna and microorganisms) or abiotic agents (such as water). By mediating the decomposition potential, microbial community diversity and structure may play an important role in the fate of soil organic carbon. In this theoretical study, we aim to understand the role of the microbial community diversity and composition in soil organic carbon storage and decomposition. 

We constructed a model describing a microbial process network incorporating diverse organic matter compounds and microbial groups. The microbial groups varied from each other with respect to their affinity to depolymerise, take-up, metabolise and assimilate organic compounds. We allowed for adaptation of microbial communities to available carbon, and competition among microbial groups.  We tested this process network with functionally diverse microbial communities which were subjected to varying carbon availability. This framework allowed us to explore organic carbon decomposition rates and their temporal evolution under different conditions of microbial diversity and carbon availability, as well as the tendency of a soil microbial system to store carbon. 

We found that the microbial community functional diversity is a good predictor of organic carbon decomposition rates. This result suggests that an  organic carbon decomposition rate modifier could be defined based on functional diversity and then included in soil carbon models. Furthermore, we observed that organic carbon decomposition by functionally similar communities in carbon poor conditions slowed down after approximately half of the initial carbon was consumed. In the same conditions, functionally diverse communities with a higher number of biotic agents allowed a more complete decomposition. However, with increasing initial carbon availability, the functional diversity of the microbial community ceased to play a role in soil carbon storage. These results link microbial community diversity and carbon availability to decomposition potential and thus organic carbon stability in soils.