The Effect of Phyllostachys edulis Expansion into Subtropical Machilus thunbergii Forests on Soil Microbial Community Diversity

Phyllostachys edulis (P. edulis) expansion threatens the structure and function of subtropical forests. Yet, how it impacts soil microbial diversity and network topology has not been fully explored. Herein, a typical transect of P. edulis expansion into Machilus thunbergii (M. thunbergii) forests was selected. Soil samples (0–10 cm) were collected along the transect from three typical vegetation types: P. edulis forest (BF), mixed P. edulis and M. thunbergii forest (BBLF), and M. thunbergii forest (BLF). Subsequently, soil physicochemical properties and microbial diversity were analyzed to elucidate the mechanisms by which P. edulis encroachment affects soil microbial diversity and its network topological characteristics. Results showed that P. edulis expansion increased soil pH and total phosphorus (TP), yet decreased soil water content (SWC), total carbon (TC), and total nitrogen (TN). Additionally, P. edulis expansion enhanced the Shannon and Simpson diversity indices of soil bacteria, while fungal diversity showed a V-shaped pattern, with the lowest in BBLF. Moreover, microbial community composition shifted significantly, with the relative abundances of Proteobacteria, Actinobacteria, Chloroflexi, Basidiomycota, and Mortierellomycota increasing, whereas those of Acidobacteria, Firmicutes, Verrucomicrobia, and Ascomycota were decreasing. Network analysis revealed that P. edulis expansion shifted bacterial networks from cooperative-dominated interactions to a balance of competition and cooperation, while fungal networks formed core-taxa-dominated structures. Overall, our findings clarify that P. edulis expansion reshapes soil nutrient conditions to regulate microbial diversity, composition, and interaction networks, providing critical insights into the belowground ecological effects of P. edulis invasion into subtropical forests.