Microbial Biomass and Extracellular Enzyme Patterns Are Associated with Soil Organic Carbon Variation Along a Plant Species Diversity Gradient in a Subtropical Forest

Plant species diversity can enhance soil organic carbon (SOC) storage, but the microbial processes associated with this relationship remain incompletely understood in subtropical forests. We surveyed 20 natural forest plots spanning a Shannon–Wiener diversity gradient and quantified SOC, soil nutrients, microbial biomass carbon, nitrogen, and phosphorus (MBC, MBN, and MBP), and extracellular enzyme activities. Based on linear regression predictions across the observed diversity gradient, SOC increased by 154.4%, total nitrogen by 67.0%, and dissolved organic carbon by 153.6%, whereas total phosphorus and available phosphorus decreased by 42.1% and 82.5%, respectively. SOC-normalized β-glucosidase (BG) activity decreased by 63.4%, while acid phosphatase (AP) activity increased by 359.2%, revealing contrasting patterns of extracellular enzyme activities associated with carbon and phosphorus acquisition. In contrast, leucine aminopeptidase (LAP) and β-N-acetylglucosaminidase (NAG) activities showed no significant relationships with plant diversity. The BG/AP ratio declined significantly along the diversity gradient (R2 = 0.76, p < 0.001). SOC was positively associated with MBC (R2 = 0.70), MBN (R2 = 0.68), and MBP (R2 = 0.47). These findings suggest that microbial biomass and extracellular enzyme patterns are closely associated with SOC variation along a natural plant diversity gradient, while declining phosphorus availability may influence belowground nutrient cycling. Further experimental studies are needed to clarify the microbial mechanisms underlying these associations.