Fire Severity Drives Long-Term Divergence of Soil Microbial Communities and Functional Traits in an Alpine Forest

Increasing wildfire severity under climate change may alter the trajectory of belowground recovery in alpine forests, yet its long-term effects on soil microbial communities remain poorly understood. Here, we examined soil physicochemical properties and microbial communities eight years after fire across unburned, low-, moderate-, and high-severity fire sites in an alpine forest of western Sichuan, China. Fire severity was associated with persistent divergence at depths of 0–10 cm, with significant changes in soil organic carbon (SOC), soil water content (SWC), available nitrogen (AN), and microbial biomass. High-severity fire strongly reduced AN and available phosphorus (AP), whereas moderate-severity fire maintained the highest SOC and SWC. Bacterial alpha diversity remained relatively stable, whereas fungal richness declined markedly under high-severity fire, indicating greater fungal sensitivity to fire disturbance. Fire severity explained 57.35% and 87.32% of bacterial and fungal community variation, respectively, while structural equation modeling indicated that microbial responses were primarily mediated by changes in nutrient availability, soil moisture, and microbial biomass. Functionally, microbial communities shifted from resource-acquisition strategies under low-severity fire toward potential stress-tolerant and saprotrophic-dominated strategies under high-severity fire. Moderate-severity fire enhanced bacterial network complexity and increased the abundance of specific mycorrhizal taxa (e.g., Russula), whereas high-severity fire promoted saprotrophic and stress-resistant taxa such as Actinobacteriota and Penicillium, while suppressing mycorrhizal groups. Overall, fire severity acted as a persistent environmental filter that reorganized microbial community structure and functional strategies of the topsoil, suggesting that increasing fire severity may delay belowground recovery and weaken ecosystem resilience in alpine forests.