Chronic Atmospheric Nitrogen Deposition Reduces Plant Species Richness and Shifts Community Composition Across a European Temperate Grassland Biodiversity Gradient

This study investigates long-term atmospheric nitrogen deposition effects on plant species richness, functional group composition, and trait diversity across a European temperate grassland N-deposition gradient within the context of plant community ecology and environmental biogeochemistry, an area of growing scientific importance given its implications for critical load policy calibration for European N-deposition regulation and grassland biodiversity conservation management. Using paired historical resurvey design comparing 1990s versus 2018-2020 plant community composition against EMEP-modeled N-deposition accumulation, with multivariate ordination and linear mixed models, we examine reactive nitrogen enrichment promoting nitrophilous grass and tall forb dominance that competitively exclude low-stature stress-tolerant specialist species, reducing overall species richness and forb functional diversity in 184 permanent grassland plots resurveyed in 2018-2020 against 1990s baselines across 8 European countries (Germany, Netherlands, UK, Sweden, Denmark, France, Czech Republic, Switzerland) drawn from semi-natural grassland sites with 20-50+ year management continuity across an N-deposition gradient from Atlantic maritime to continental European climate zones. Results indicate that cumulative N-deposition accounts for 48.4% of variance in species richness decline across plots; each additional 100 kg N/ha cumulative deposition is associated with a 2.84 species/m2 loss, with forb richness declining 3.2x faster than grass richness (p < 0.001), with 48.4% variance explained; 2.84 species/m2 per 100 kg N/ha cumulative deposition as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to plant community ecology and environmental biogeochemistry and carry actionable implications for the design of programs and policies targeting critical load policy calibration for European N-deposition regulation and grassland biodiversity conservation management.