Interplanting Phoebe bournei modifies soil microbial community characteristics in Cunninghamia lanceolata monocultures

The prolonged monoculture of Cunninghamia lanceolata depletes soil fertility, making the introduction of broadleaf trees—Phoebe bournei indispensable for soil restoration—yet its impact on the belowground ecological environment remains inadequately explored. Here, we investigated how uneven-aged interplanting of P. bournei in subtropical C. lanceolata stands alters microbial community characteristics and correlates with major edaphic variables across 0–60 cm soil profile. Interplanting P. bournei improved soil aeration, water conservation, and nutrient availability, driving significant shifts in bacterial and fungal β-diversity and partial but significant changes in community composition. It also enhanced ecological drift and reduced dispersal limitation, strengthening homogeneous selection in bacterial communities (0–40 cm) while promoting drift in fungal communities (0–60 cm). These structural and assembly changes suggest potential functional alterations in organic-matter degradation and nutrient cycling. Interplanting P. bournei led to marked improvements in soil microbial ecology, as evidenced by higher abundances of Acidobacteria and Ascomycota, indicative of enhanced complex-carbon degradation; elevated bacterial cellulolytic capacity and accelerated organic-matter turnover; promotion of saprotroph–symbiotroph nutrition; and reduced subsoil pathotroph abundance, which may indirectly support C. lanceolata health. Lastly, soil nutrient elements (e.g., soil organic carbon, total phosphorus) and enzyme activities (e.g., Cellobiohydrolase, Sucrase) were identified as key drivers of microbial community structure and functional potential, highlighting their critical roles in shaping soil microbial ecosystems. In summary, uneven-aged interplanting P. bournei in C. lanceolata plantations optimized soil ecosystem functions, offering a sustainable strategy to enhance forest productivity and improve soil health.