Mineral-associated organic carbon predicts the variations in microbial biomass and specific enzyme activities in a subtropical forest

Tropical and subtropical forests are critical in global carbon budgets. Although separating soil organic carbon (SOC) into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) advances our understanding of SOC sequestration, how SOC and its fractions (POC and MAOC) interact with soil microbial traits in tropical and subtropical forests remains unclear, which is critical for improving soil C projections. Here we sampled soils beneath six stands planted with different tree species in a humid subtropical forest to assess the interactions between microbial traits (biomass and specific enzyme activities) and SOC and its fractions. We found that mineral-associated organic carbon (MAOC) accounted for 78–86% of SOC and co-varied strongly with SOC, representing the largest resource pool. There were significant relationships between microbial (especially bacteria) biomass and specific enzyme activities and SOC (especially MAOC). The results of linear mixed models indicated that SOC and particularly MAOC could significantly predict the variations in microbial (especially bacteria) biomass and specific enzyme activities. However, microbial traits failed in predicting the variation in SOC and particularly the MAOC. There were significant negative relationships between microbial biomass and specific C- and N-acquisition enzyme activities and these relationships disappeared when controlling SOC or MAOC but not when controlling particulate organic carbon (POC), suggesting that there is a tradeoff of microbial investment between biomass growth and resource acquisition, and this tradeoff is controlled by SOC, particularly the MAOC. Overall, our findings provide strong evidence for the interactions between microbial traits and soil C fractions and thus highlight that MAOC plays a critical role in regulating microbial biomass, specific enzyme activities, and their tradeoff, whereas microbial biomass and specific enzyme activity may not be important determinants of MAOC in subtropical forests.