Similar effects of seasonal nitrogen enrichment on plant species–area relationship in a temperate grassland

1. Nitrogen (N) enrichment often decreases plant species richness in local communities, thus altering plant species–area relationships (SAR). However, whether N enrichment affects the SAR across global grasslands has not yet been well explored. Additionally, whether and how seasonal N enrichment alters the SAR remains unknown.2. This study employed data from global grasslands (meta-analysis) in combination with a seasonal field N addition experiment during the first 6 consecutive years (2014–2020) in a temperate grassland in northern China.3. We found that N addition did not alter the SAR slopes across global grasslands and our experimental grassland, but significantly reduced the intercepts (plant species richness at the 1 m2 plot scale). There was no significant difference among seasonal N treatments. The SAR intercepts were negatively correlated with increasing experimental years in our temperate grassland, without significant differences among the three seasonal N addition treatments, indicating a cumulative effect on the reduction in plant species richness at 1 m2 with N-enriched conditions.4. These results suggest that N enrichment affects the SAR in grassland ecosystems by reducing the SAR intercepts. Therefore, biodiversity conservation activities should be applied urgently in localities facing increasing atmospheric N deposition.