Niche Conservatism and Community Assembly Reveal Microbial Community Divergent Succession Between Litter and Topsoil

Natural restoration is an effective approach for restoring degraded ecosystems, yet the successional patterns and assembly mechanisms of aboveground (litter layer) and belowground (topsoil) microbial communities remain poorly understood. We applied the niche conservatism framework to investigate niche partitioning, successional patterns and community assembly processes of microbial communities in the litter and topsoil layers during long-term vegetation restoration in southwestern China. The results showed that, during vegetation succession, the potential source communities of microbial communities in the litter layer gradually shifted from being dominated by the topsoil to being dominated by the litter. Fungal communities had a significantly higher proportion of external immigrants (> 80%) than bacteria (> 40%) and archaea (< 20%). During succession, bacterial and fungal communities in the litter and topsoil layers underwent niche differentiation, displaying a divergent succession pattern, while archaeal communities showed niche overlap, following a convergent pattern driven by stochastic processes. Additionally, the dispersal rate (m) and β-diversity turnover rate (slope) of bacterial and fungal species in the litter were significantly lower than in the topsoil, with community assembly being more influenced by deterministic processes in the litter. This study reveals that higher habitat specialisation in the litter imposes stronger filtering effects on the colonisation of most microbial groups, particularly fungal communities, highlighting the role of strategy differentiation in shaping microbial communities.