Forest gaps amplify seasonal turnover and decouple microbial stability from functional potential in a karst forest

Forest gaps represent small-scale disturbances that influence belowground ecological processes, yet most studies adopt a static perspective and rarely consider how their effects are modulated by seasonal climatic variation. We investigated the seasonal dynamics of soil bacterial and fungal communities along a disturbance gradient from forest gaps through gap edges to adjacent closed-canopy soils in a subtropical karst forest, sampling across four seasons. By integrating high-throughput sequencing with ecological null models, niche analyses, and co-occurrence network analyses, we examined how seasonal variation interacted with gap disturbance to regulate microbial community assembly and predicted functional potential. Bacterial temporal turnover was significantly greater in forest gaps than in closed-canopy soils and was accompanied by greater seasonal divergence in community composition. Null-model analyses showed an increase in the relative contribution of deterministic processes to the assembly of both bacterial and fungal communities within gaps. Although niche breadth expanded in both kingdoms, niche organization diverged: fungal communities showed greater niche overlap and a higher proportion of generalists, whereas bacterial communities showed reduced niche overlap. Co-occurrence networks in gap soils were more densely connected but contained fewer keystone taxa and showed a topologically simplified architecture. Correspondingly, predicted functional potential was associated primarily with keystone-taxon structure in closed-canopy soils but with both keystone-taxon and whole-community structure in gap soils. Together, these findings suggest that although forest gaps transiently accelerate microbial turnover and nutrient cycling, the associated network simplification and loss of keystone-associated control over specialized functions may ultimately constrain the capacity of soil microbiomes to support forest regeneration under increasing seasonal variability.