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., NH₄⁺) addition resulted in a significant species loss of forbs. By contrast, oxidized N (NO₃^-) 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 NH₄⁺ 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.