Global patterns of vertical distribution of soil microbial biomass carbon

Small changes in soil organic carbon (SOC) may have great influences on the climate-carbon cycling feedback. However, there are large uncertainties in predicting the dynamics of SOC in soil profile at the global scale, especially the role of soil microbial biomass in regulating the vertical distribution of SOC. Here, we developed a global database of soil microbial biomass carbon (SMBC), soil microbial quotient (SMQ, the ratio of SMBC to SOC), and SOC from 312 soil profiles, as well as climate, ecosystem type, and edaphic factors associated with these soil profiles. We assessed the global pattern of vertical distributions of SMBC and SMQ and the contributions of climate, ecosystem type, and edaphic factors to their vertical patterns. Our results showed that SMBC and SMQ decreased exponentially with depth, especially in the top 0-40 cm soil. SOC also decreased exponentially with depth but in different magnitudes compared to SMBC and SMQ. Edaphic factors (e.g., soil clay content and C/N ratio) were the most important controls for the vertical distributions of SMBC and SMQ, probably by mediating the preservation of substrates and nutrient supply for microbial growth in soils. Mean annual temperature and ecosystem types (i.e. forests, grasslands, and croplands) exerted weak influences on SMBC and SMQ. Overall, our data synthesis provides quantitative information of how SMBC, SMQ, and SOC changed along soil profiles globally and identifies important factors that influence their vertical distributions. The findings can help improve the prediction of C cycling in the terrestrial ecosystem by integrating the contributions of soil microbial roles in Earth system carbon models.