Afforestation is an effective approach to rehabilitating degraded ecosystems, but its implementation often causes standing forest soil microbial communities to fluctuate. It is unknown how changes deep and forest ages (chronosequence) affect soil microbial diversity and assembly, hindering our ability to understand the sustainability of rehabilitated ecosystems. To address this issue, we focused on acquiring biophysical data from a chronosequence of Robinia pseudoacacia plantations of 10 to 50 years old in the exploration of soil microbial (bacterial and fungal) community dynamics associated with variations in species diversity and assembly. Results showed that soil depth effects bacterial, but not fungal diversity, the bacterial diversity was significantly higher in the topsoil than subsoil, while fungal diversity did not differ between the two soil layers. The composition of both bacterial and fungal communities (at the phylum level) was significantly influenced by soil layer and forest age. The assembly processes of soil bacterial and fungal communities were predominantly driven by deterministic processes (homogeneous selection). However, with the increase of forest age, the βNTI values for bacterial communities significantly decreased in both soil layer, but the βNTI values of fungal communities significantly increased in the topsoil. Soil carbon composition (i.e., particulate organic carbon, easily oxidizable organic carbon) were the main factors regulating the variation of soil bacterial and fungal assembly processes among different forest ages. Network analysis showed that the network structure of bacteria in the subsoil were more complex than in the topsoil, whereas no significant structural differences were observed in the fungal networks between the two soil layers. Additionally, study highlights that soil organic carbon (SOC), particulate organic carbon (POC), easily oxidizable organic carbon (EOC), and total nitrogen (TN) are key factors influencing microbial community composition, co-occurrence network, and assembly processes in different soil layers. Our study demonstrated the importance of soil carbon composition in the assembly of forest soil microbial community. These findings offer valuable insights for future research on deep soil microbial diversity and forest ecosystem succession, particularly in ecologically fragile regions.