Greater impacts of reduced than oxidized nitrogen on grassland plant species losses

Increasing atmospheric nitrogen (N) deposition is a major threat to terrestrial vegetation biodiversity. The reactive N deposition includes reduced (NH x ) and oxidized (NO y ) forms, which play different roles in a variety of biological processes. Whether NH x and NO y affect the vegetation biodiversity differently or equivalently has been paid little attention despite decades of research on biodiversity changes in response to N deposition. Combining a meta-analysis and an N gradient experiment, we found that reduced N (i.e., NH 4 + ) addition resulted in a significant species loss of forbs. By contrast, oxidized N (NO 3 - ) addition showed a much weaker impact on species richness. The greater impact of reduced than oxidized N on the species loss could be due to the susceptibility of forbs to NH 4 + toxicity and nitrification-induced acidification, which weakened their competition for light and other resources compared with grasses. These results provide direct evidence that reduced N exerts greater impacts on grassland plant species losses than oxidated N, emphasizing that the ratio of NH x to NO y , in addition to the total N deposition amount, should be considered as an important predictor of grassland biodiversity losses.