Precipitation Regimes Reshape Grazing and Nitrogen Effects on Diversity–Productivity Coupling in Arid Steppes: Mechanisms and Thresholds

Abstract Dryland ecosystem stability is increasingly threatened by amplified precipitation variability, nitrogen ( N ) deposition, and grazing pressure. While these drivers independently alter grassland structure, their interactive effects on the critical diversity‐productivity relationship remain poorly understood, particularly under contrasting precipitation regimes. Through a 3‐year split‐plot experiment (four grazing intensities × N addition) in a Loess Plateau steppe, we examined how interannual precipitation variability modulates the effects of grazing and N on the coupling strength between diversity (species richness, SR) and productivity (peak aboveground biomass, ANPP). Three key findings emerged: First, SR–ANPP coupling was strongly precipitation‐dependent: low grazing with N enhanced their synergy in wet years, whereas drought combined with heavy grazing triggered severe decoupling. Second, functional group asynchrony drove these dynamics: ephemeral forbs amplified SR responses to wet years, whereas drought‐tolerant grasses stabilized ANPP but accelerated SR collapse in dry years. Third, soil pathways mediated the coupling dynamics: precipitation and soil C‐N pools facilitated coordination, while grazing‐induced increases in bulk density exerted counteracting effects. Our CDS (Climate‐Disturbance‐Soil feedback) framework demonstrates that moderate grazing optimizes precipitation responsiveness when combined with strategic N inputs, offering actionable thresholds for maintaining coupled ecosystem functions under climate extremes.