Greater impacts of reduced than oxidized nitrogen enrichment on plant diversity losses in a temperate grassland

Abstract Background Increasing atmospheric nitrogen (N) deposition is a major threat to plant diversity globally. Recent observations show that the reduced-to-oxidized (NH x /NO y ) ratio of N deposition has been changing spatially and temporally. How and to what extent different N forms (i.e., NH x and NO y ) influence grassland plant species loss are still unclear. Methods We employed a field manipulative experiment by using three N forms [i.e., Ca(NO 3 ) 2 , NH 4 NO 3, and (NH 4 ) 2 SO 4 ] with six N addition levels (0, 4, 8, 16, 24, 32 g N m −2 year −1 ) in a temperate grassland and conducted a greenhouse experiment culturing four plant species corresponding different plant functional groups under Ca(NO 3 ) 2 or (NH 4 ) 2 SO 4 addition. Results Results from our field experiment showed that the plant species loss rate was greater under NH 4 + –N than NO 3 − –N enrichment. Plant species loss was driven by light asymmetry under NO 3 − –N enrichment, while it was co-driven by light asymmetry and soil acidification under NH 4 NO 3 enrichment. Under NH 4 + –N enrichment, light asymmetry, pH decrease, NH 4 + toxicity, and metal toxicity jointly affected species loss. The greenhouse experiment provided direct evidence that legumes and forbs are more physiologically susceptible to NH 4 + -induced toxicity than grasses. Conclusions Our results emphasize that N forms play a vital role in affecting grassland plant diversity. This suggests that regions with higher NH x enrichment may experience more severe plant diversity losses as N deposition continues to increase. Therefore, appropriate measures should be adopted to mitigate species losses.