High Site Index Drives the Soil Microbial Network Complexity and Function in Chinese Fir Mixed Plantations After Near‐Natural Transformation

The site index (SI), an important indicator of forest health, has been well‐documented due to its impact on tree growth. Soil microorganisms, as decomposers that control nutrient cycling in forest ecosystems, usually respond significantly to changes in site index. However, most of the existing studies have focused on the direct effects of SI on tree growth, while the response mechanism of the microbial community network and its function has rarely been explored. In view of this, this study investigated the co‐occurrence network and function of soil microbial communities under different site index (SI‐14.96, SI‐15.70, and SI‐16.90) and soil depths (0–20 cm, 20–40 cm, and 40–60 cm) within a mixed Chinese fir plantation. The increase of site index significantly improved the soil physical and chemical properties of the Chinese fir plantation, including total phosphorus, total nitrogen, exchangeable magnesium, and soil water content. Dominant bacterial communities included Acidobacteria, Chloroflexi, and Proteobacteria, while Ascomycota and Basidiomycota dominated the fungal community. The variation in bacterial community structure was mainly driven by soil depth (R2 = 37.33%), while the fungal community structure was influenced primarily by the site index (R2 = 20.80%). Soil phosphorus, organic carbon, and soil water content drove microbial community variation. In the relatively high site index, the topological properties of the bacterial and fungal co‐occurrence network, including nodes, edges, and the average clustering coefficient, reached the highest, showing the highest network complexity, and the keystone taxa were more abundant in the surface soil. Functional annotation analysis further indicated that bacterial functions related to nitrogen cycling and arbuscular mycorrhizal fungi were both significantly highest at SI‐16.90. In general, a relatively high site index (SI‐16.90) for Chinese fir plantations can improve the complexity of the soil microbial network, enhance the abundance of keystone taxa, and optimize the nitrogen cycle and the function of arbuscular mycorrhizal fungi. These findings are of great significance to the shaping of soil microbial diversity and ecological functions and provide a practical basis for improving soil ecology with a high site index in forest management.