Abstract: Natural secondary forests constitute the main body of forests in the mountainous areas of Beijing. An in-depth investigation on the distribution patterns and key influencing factors of soil microbial communities across different stand types and soil depths, as well as revealing how soil bacterial community diversity and cooccurrence network assembly respond to stand structure and environmental factors, is of great significance for understanding the functional maintenance and succession dynamics of warm-temperate Quercus mongolica forests. In this study, typical regional Q. mongolica natural secondary forests were selected as the research object. Soil samples from different depths (0-20 and 20-40 cm) were collected from three stand types (pure Q. mongolica natural secondary forest; mixed Q. mongolica, Populus davidiana, and Betula platyphylla natural secondary forest; and mixed Q. mongolica and Pinus tabuliformis natural secondary forest). Stand structure indicators, soil physicochemical properties, and microbial communities were measured. The results showed that: (1) Soil bacterial communities exhibited distinct stratification with soil depth. The alpha diversity indices of bacterial communities in the upper soil (0-20 cm) were generally higher than those in the lower layer (20-40 cm), with significant difference observed in the coniferous-broadleaved mixed forest (P<0.05). Among different stand types, the upper soil of the coniferous-broadleaved mixed forest had the highest bacterial alpha diversity indices (Shannon, Chao1, and Sobs indices), while the pure forest had the lowest. (2) Soil organic matter content and nitrogen content (TN and AN) were the core factors regulating the alpha diversity of bacterial communities. In the upper soil, these factors were significantly correlated with species richness (Chao1 and Sobs) (P<0.05), whereas in the lower soil, they were significantly correlated with species diversity (Shannon) (P<0.05). (3) Mixed forests constructed more complex bacterial co-occurrence networks than pure forests, exhibiting higher numbers of nodes and edges, as well as greater modularity. The mixed forest soils were enriched with more key functional taxa belonging to the phyla Acidobacteria and Actinobacteria, which acted as hub nodes and effectively enhanced the aggregation effect and ecological functioning. (4) Across all stand types, the topological parameters of the bacterial co-occurrence networks in the upper soil layer were higher than those in the lower soil layer, indicating that the upper layer is the primary carrier for the ecological functions of bacterial communities. Variations in stand types and soil factors significantly affected the topological structure of the bacterial co-occurrence networks. This study reveals the diversity and co-occurrence network assembly patterns of soil bacterial communities in different types of warm-temperate Q. mongolica secondary forests. These findings are of great significance for understanding the synergistic enhancement of ecosystem multifunctionality and provide scientific support for the maintenance and sustainable management of forest ecosystems.

Key words: soil microbial diversity, stand type, co-occurrence network, environmental factor, community structure