Nitrogen addition drives local extinction of legumes in meadow steppe

Abstract Nitrogen (N) enrichment poses a critical threat to legume diversity through three interlinked mechanisms: stimulating growth of non‐leguminous competitors, enhancing community canopy coverage and inducing light limitation. Canopy management practices such as mowing may partially mitigate these impacts by reducing species dominance and restoring light availability. Through a decadal field experiment in a meadow steppe, we systematically investigated the interactive effects of chronic N addition and mowing regimes on legume biomass dynamics and species persistence. Our findings demonstrate a time‐dependent relationship between N enrichment and legume species richness. Most legume species, particularly rare and subordinate ones, were eliminated at intermediate‐to‐late stages when N addition rates exceeded 10 g N m −2 year −1 . This collapse correlated with the progressive suppression of legumes by biomass accumulation in the dominant non‐legume species, Leymus chinensis . Beyond this ecological threshold, only Thermopsis lanceolata persisted in the community, though its biomass ultimately declined with increasing N inputs and associated metal element uptake. Greenhouse experiments revealed this species' survival strategy involves rapid downregulation of N fixation capacity—a potentially critical functional trait distinguishing it from other legumes in the study site. Mowing partially counteracted N‐induced biodiversity loss at sub‐threshold enrichment levels (i.e. < 10 g N m −2 year −1 ) by weakening the competitive dominance of nitrophilous plants and maintaining light penetration. However, this management intervention proved ineffective against legume extirpation under prolonged N exposure or supra‐threshold addition rates. Our mechanistic analysis identified three cascading drivers of legume extinction: (1) competitive exclusion by N‐responsive species, (2) light limitation from canopy closure and (3) phytotoxic effects of metal element accumulation. Synthesis . This study predicts an ecosystem regime shift toward simplified N 2 ‐fixing associations and dominance of non‐symbiotic plant species in temperate grasslands under chronic N deposition. Such compositional changes could fundamentally alter terrestrial N cycling patterns, with cascading consequences for ecosystem functioning. While mowing provides limited buffering capacity at moderate N loads, our results emphasize the imperative for stricter atmospheric N emission controls to preserve legume biodiversity and associated ecological services.