Soil microbial nitrogen-cycling functional groups rather than phosphorus-cycling functional groups regulate plant functional diversity along the successional pathway in subtropical karst forests

While it has been established that soil microbes mediate the element transformation and redistribution among soil, plants, and the atmosphere, the influence of microbe-mediated variations in key soil nutrients on plant communities is not well understood, particularly along the successional pathway in karst ecosystems. Along a gradient of vegetation succession, 30 forest dynamic plots were established in the subtropical karst forests in China to characterize soil microbial nitrogen (N) and phosphorus (P) cycling functional groups, soil nutrients, plant functional traits, plant species diversity, and plant functional diversity. Based on metagenomic sequencing, we characterized soil microbial N- and P-cycling functional groups, and used piecewise structural equation modelling (pSEM) to disentangle the pathways through which soil microbial functional groups affected plant functional diversity. The results showed that soil nutrients were significantly associated with microbial N-cycling functional groups (R 2 = 0.294) and the community-weighted variance (CWV) of plant functional traits (R 2 = 0.168). Standardized effect decomposition showed that functional diversity was positively affected by soil nutrients (indirect effect = 0.24), species diversity (indirect effect = 0.28), and especially CWV (direct effect = 0.45). In addition, the soil microbial N-cycling functional groups, rather than the P-cycling functional groups, indirectly affected CWV of plant functional traits and functional diversity by mediating soil nutrients. Our findings highlighted the important roles of soil microbial functional groups in altering plant functional diversity by mediating nutrient cycling, shedding new light on the successional mechanisms and stress ecology.