Abstract: The relative limitation of microbes by soil organic carbon (SOC), nitrogen (N) and phosphorus (P) is linked with soil microbial activities, so how change of plant species diversity (PSD) affects microbial resource limitation would partly determine its impacts on SOC dynamics and nutrient cycling. However, the responses of microbial resource limitation to increasing PSD have poorly explored. Here, 45 plots covering a natural PSD gradient were used to investigate the effects of PSD on microbial resource limitation in a subtropical forest. Extracellular enzymatic stoichiometry along with a laboratory N and P addition experiment were used to determine microbial resource limitation. Contents of microbial biomass C, N and P significantly increased, but C:P and N:P ratios in microbial biomass were unchanged as PSD increased. Soil microbes were generally co-limited by C and P, but not by N across the 45 plots. Increasing PSD did not alter microbial N limitation, alleviated microbial C limitation and aggravated microbial P limitation. The alleviated microbial C limitation or aggravated microbial P limitation was attributed to increased soil C availability but decreased P availability, which resulted in stimulated soil C:P and N:P ratios and in turn greater C:P and N:P imbalance between soil and microbial biomass under higher PSD. Our results highlight the divergent effects of increasing PSD on microbial resource limitation. Considering that microbial C and P limitations are widespread, the patterns observed in the current study should be applicable broadly.

Key words: microbial carbon limitation, microbial phosphorus limitation, plant species diversity, extracellular enzymatic stoichiometry, soil nutrient availability