Long-term increase in the ratio of broadleaved trees promotes soil microbial diversity and stabilizes bacterial but not fungal networks in mixed conifer–broadleaved plantations

Mixing monoculture conifer plantations with broadleaved trees has been shown to improve soil microbial diversity and alleviate microbial nutrient limitation. However, how different conifer-broadleaved mixing ratios regulate soil microbial diversity and network stability remains poorly understood. Here, we investigated the effects of three long-term (> 40 years) mixing ratios of coniferous and broadleaved trees (2: 1, 1: 1, and 1: 2) on soil microbial communities across the Inner Mongolia Plateau. We found that increasing the proportion of broadleaved trees significantly increased soil microbial α diversity and led to clear shifts in microbial community composition. Notably, bacterial and fungal communities exhibited contrasting network responses: bacterial network stability increased with higher broadleaved tree proportions, whereas fungal network stability declined. In addition, the relative abundance of the fungal phylum Ascomycota increased significantly with increasing broadleaved tree ratios, while bacterial community composition remained relatively stable at the phylum level. Structural equation modelings (SEMs) further revealed that bacterial community responses were jointly driven by biotic factors (key species abundance and rare species richness) and abiotic factors, whereas fungal communities were predominantly regulated by abiotic factors, particularly soil organic carbon (SOC). Together, these results demonstrate that conifer-broadleaved mixing ratios exert distinct controls over bacterial and fungal communities through different ecological pathways, with important implications for soil microbial stability and forest ecosystem functioning.