Moisture-driven microbial regime shifts mediate nutrient dynamics in reservoir riparian zones

Reservoir-regulated hydrological regimes generate riparian ecotones through cyclic inundation and exposure, establishing transient biogeochemical hotspots that modulate elemental fluxes across land-water continua. Although functionally specialized microbiomes drive critical nutrient transformations within these ecotones, their resilience thresholds confront escalating pressures from hydrological extremes, which may induce catastrophic state transitions disrupting biogeochemical cycles. Despite their importance, the underlying mechanisms regulating microbial community dynamics, state transitions, and their ecological consequences under fluctuating hydrological regimes remain poorly understood. This study employs alternative stable state theory to investigate hysteretic responses of microbial communities along moisture gradients, coupled with mechanistic evaluation of their mediation on nutrient cycling. Our findings reveal bistability in microbial assemblages, demonstrating regime shifts with hysteresis under moisture stress. Through potential landscape analysis and soil multifunctionality assessment, we identified two distinct ecological states: under low moisture conditions (<15 %), microbial communities maintain structural stability with slow nitrogen cycling and carbon metabolism, dominated by drought-adapted taxa (e.g., Actinobacteriota); while under high moisture conditions (>30 %), they transition to moisture-dependent states characterized by Desulfobacterota and Bacteroidota dominance, exhibiting enhanced carbon metabolism and denitrification capacity. Moisture stress significantly reduced α-diversity while increasing dispersal limitation and network complexity, suggesting enhanced niche differentiation under arid conditions. These contrasting states fulfill complementary ecosystem functions: the low-moisture state preserves soil carbon and nitrogen, whereas the high-moisture state achieves nitrate removal, effectively mitigating eutrophication risks. By mapping microbial collapse and recovery trajectories along moisture gradients, this study provides a mechanistic understanding of riparian ecosystem resilience and offers actionable insights for the predictive management of reservoir riparian zones.