Tree species mixing enhances the diversity–function relationship in subtropical Cunninghamia lanceolata plantations

Forest biodiversity enhances ecosystem functionality and underpins sustainable forest management by improving soil nutrient cycling. As a representative sustainable management practice, tree species mixing (TSM) increases this functionality by regulating plant-soil nutrient interactions. This study compared the effects of TSM management on stand features, plant diversity, and soil microbial properties across different developmental stages of Cunninghamia lanceolata plantations. The results demonstrated that TSM management significantly enhanced the overall functional efficiency of the ecosystem. Specifically, TSM management improved stand features and reduced competition intensity among trees, which increased α-diversity of each vegetation layer while decreasing its β-diversity. Furthermore, TSM management increased litter layer thickness and soil available phosphorus content, with the magnitude of these effects varying across different management stages. Concurrently, although there was a reduction in α-diversity of bacteria (Chao1: −7.3%; Shannon: −2.7%), soil core microbial community exhibited an enrichment of oligotrophic bacteria (Acidibacter: +29.1%) and an increase in core fungal taxa, a shift that enhanced the decomposition of organic matter (litter thickness: +27.8%) and the transformation of nutrients (available nitrogen (N): +32.6%). Structural equation modeling (SEM) further confirmed that TSM management primarily drives soil carbon accumulation through the “tree diversity–core bacterial community–microbial biomass” pathway. In summary, this study reveals that TSM management promotes forest plant diversity and improves litter and soil conditions at the cost of reducing α-diversity and increasing the soil core bacterial community, ultimately leading to enhanced overall ecosystem functional efficiency. This finding provides important guidance for optimizing the structure, function, and resilience of degraded Chinese fir plantations, and offers a scientific basis for future decisions on balancing microbial community changes in the context of species diversity conservation and soil fertility restoration.