Fire severity and soil chemical legacies shape early post-fire bacterial and fungal responses in a subtropical Pinus massoniana forest

Wildfire strongly alters soil microbial communities, yet the severity-dependent soil factors associated with early post-fire microbial responses remain incompletely resolved, particularly the relative roles of soil physicochemical legacies, fire severity, and recovery time. We investigated soil bacterial and fungal communities across four fire-severity classes (unburned, low, moderate, and high severity) at two recovery stages (one month and one year after fire) in a subtropical Pinus massoniana forest. High-throughput sequencing was combined with analyses of microbial α and β-diversity, predicted functional guild composition, co-occurrence network topology, and structural equation modeling to separate direct fire effects from indirect effects mediated by soil physicochemical properties. Fire severity explained 23% and 14% of the variation in bacterial and fungal community composition, respectively ( P = 0.001). At 1 MAF, HS soils had lower relative abundances of Acidobacteriota and Ascomycota but higher Actinobacteriota and Basidiomycota than UB, and several severity-associated differences persisted at 1 YAF. At 1 MAF, HS soils had 59% lower SOC and 47% lower TN but 46% higher TP than UB; by 1 YAF, the SOC and TN differences had narrowed to 15% and 14%, TP was similar to UB. Predicted nitrification and aerobic ammonia-oxidation groups were approximately 40% lower in HS than LS at 1 MAF; this difference narrowed but remained approximately 20% lower at 1 YAF. Network analyses indicated early post-fire expansion of the combined bacteria-fungi association network under high-severity fire, followed by reduced connectivity and substantial restructuring at 1 YAF. Structural equation models indicated that persistent soil chemical legacies were strongly associated with predicted microbial functional composition, with stronger effects than soil physical properties. Fire-induced soil chemical legacies were associated with divergent short-term bacterial and fungal responses: bacteria showed greater compositional plasticity, but predicted nitrogen-cycling groups remained lower under high-severity fire, whereas fungi exhibited slower, vegetation-dependent recovery of putative symbiotic guilds. These findings provide a mechanistic basis for predicting microbial resilience and emphasize soil nutrient restoration as a key leverage point for post-fire ecosystem management.