Land-use-driven change in soil labile carbon affects microbial community composition and function

The conversion of natural forests to plantations can lead to a substantial loss of soil organic carbon (SOC) and threaten the function of soil microbial communities. However, the effect of forest conversion on the relationship between SOC dynamics and shifts in the composition and function of the microbial community remain poorly understood, although such information is critical for predicting the long-term effect of land-use change on global SOC dynamics and storage. To fill this knowledge gap, our study investigated the influence of two decades of converting natural forest to a tea plantation on SOC storage, microbial community composition, C-degrading enzyme activity and the abundance of C-degrading microbial functional genes in a subtropical region in China. Two decades after converting natural forest to tea plantation, SOC storage decreased by 30%, and labile SOC [including easily mineralized C (Cmin), readily oxidizable C (ROC), dissolved organic C (DOC), and microbial biomass C (MBC)] decreased by 21–71%. Land-use change decreased the relative abundance of copiotrophic bacteria but increased the relative abundance of oligotrophic bacteria and lignin-degrading fungi. This resulted in a decrease in cellulase activity and cbhI gene abundance involved in cellulose degradation and conversely an increase in ligninase activity involved in lignin degradation after the conversion. Such shifts in microbial community composition and function associated with land-use change were mediated by the decrease in the labile C pool rather than the total SOC. Our study provided new insights into not only shifts in microbial communities but also specific C functional genes and related enzymes after forest conversion. These findings highlight that labile SOC, as an important functional soil component for microbial communities, significantly affects SOC dynamics and storage.