Environmental Filtering and Microbial Interactions Drive Soil Bacterial and Fungal Community Assembly in Subtropical Forests

ABSTRACT Soil microbial communities are pivotal to forest ecosystem functions, yet the mechanisms shaping their assembly across complex environments remain poorly understood. This study examined bacterial and fungal communities within topsoil and subsoil under subtropical coniferous forest (CF), mixed conifer–broadleaf forest (MF), and broadleaf forest (BF) along altitudinal gradients in Dianjiang County, Chongqing, China. Using high‐throughput sequencing, network analysis, and null models, we evaluated their diversity, composition, interaction patterns, and assembly processes and identified key influencing factors. Our results demonstrated that the interactive effects of altitude and forest type significantly influenced microbial community composition. Bacterial assembly was mostly controlled by deterministic processes (mainly homogeneous selection), whereas fungal assembly was mainly governed by stochastic processes (particularly undominated processes). At low elevations, the transition from CF to BF shifted bacterial assembly processes from stochastic to deterministic dominance across soil layers. Microbial network complexity was lowest in MF and decreased with increasing elevation. Local microhabitat factors exerted stronger impacts on microbial communities than plant diversity, with the drivers varying between topsoil and subsoil. Microbial interactions have a more pronounced influence on bacterial and fungal assembly. Altitude also played a central role in shaping bacterial assembly by regulating climate and soil properties, while fungal assembly depended more on interspecific interactions. These findings highlight the pivotal importance of species interactions and environmental conditions in structuring soil microbial community assembly in subtropical forests.