Microbiome reorganization and extracellular polymeric substances are associated with greater soil nutrient retention in mixed-species forests

Mixed-species forests are increasingly used to improve soil nutrient retention and water purification services that are often weakened in pure plantations. However, the microbial processes underlying these improvements remain insufficiently understood. In this study, we investigated soil bacterial and fungal communities, their assembly processes, and extracellular polymeric substances (EPS) across pure coniferous forests, pure broadleaved forests, and mixed forests in a subtropical watershed. The results showed that total organic carbon was a major environmental factor associated with variation in microbial community structure. Compared with CPF and BPF, MF exhibited a stronger contribution of stochastic processes, particularly dispersal limitation, to microbial community assembly. MF soils also contained a higher proportion of transient taxa and showed co-occurrence networks with lower connectivity, a higher proportion of negative correlations, and more keystone taxa. These patterns suggest that mixed forests support a more spatially heterogeneous and dynamically assembled microbiome. In addition, soil EPS concentrations were higher in MF and were significantly associated with dissolved organic carbon, nitrate nitrogen, and shifts in bacterial and fungal community structure. Overall, the study suggests that mixed-species forests are associated with changes in microbial assembly and EPS accumulation, which may help explain their greater soil nutrient retention capacity.