Nitrogen addition drives local extinction of legumes in meadow steppe

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 yr−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 yr−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 N₂-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.