Soil depth affects bacterial, but not fungal community structure and assembly in Robinia pseudoacacia plantations

Forest soil microbial communities play an important role in nutrient cycling and overall ecosystem functioning; however, their responses to variations in soil depth and forest age (chronosequence) remain insufficiently understood. Therefore, it is necessary to investigate how microbial community assembly varies across soil depths and forest ages to enhance our understanding of microbial diversity and its role in forest ecosystem functioning. In this study, we used 16S rRNA gene and ITS sequencing to characterize bacterial and fungal community traits in both topsoil (0–20 cm) and subsoil (70–100 cm) layers in Robinia pseudoacacia plantations ranging in age from 10 to 50 years old. The results revealed that soil depth significantly influenced bacterial diversity, whereas fungal diversity remained largely unaffected. Bacterial diversity was significantly higher in the topsoil compared to the subsoil (P < 0.05), while fungal diversity did not differ significantly between the two soil layers (P > 0.05). The composition of bacterial and fungal communities was significantly influenced by both soil depth and forest age. Community assembly processes for both groups were predominantly governed by deterministic factors, specifically homogeneous selection. However, with increasing forest age, β-nearest taxon index (βNTI) for bacterial communities significantly decreased in both soil layers, whereas βNTI for fungal communities increased in the topsoil. Soil carbon (C) composition, specifically soil organic C (SOC) and particulate organic C (POC), emerged as the main factors regulating variation in bacterial and fungal assembly processes across the chronosequence of R. pseudoacacia plantations. Network analysis revealed that bacterial network structures in the subsoil were more complex than those in the topsoil. Furthermore, our study highlights that SOC, POC, easily oxidizable organic C (EOC), and total nitrogen (TN) were identified as key environmental factors influencing microbial community composition, co-occurrence network patterns, and assembly processes across soil layers. Our study underscores the critical role of soil C composition in shaping forest soil microbial communities. This study provides empirical evidence that vertical heterogeneity in C availability mediates depth-dependent microbial assembly during forest succession, offering mechanistic insights into strategies for enhancing subsoil C sequestration in ecologically fragile areas.