Increasing atmospheric nitrogen (N) deposition leads to ecosystem N-P imbalances worldwide, with important consequences on ecosystem services by altering plant N:P ratios. While the direct and positive effects of N inputs on plant N:P ratio at the species level is well established, it remains unclear how the N-induced changes in species composition contribute to the community-level alterations of N:P ratio, which are more closely related with ecosystem properties and functioning. Based on a field experiment with six N addition levels ranging from 0 to 50 g N m(-2) yr(-1) in a temperate semiarid grassland, we report more sensitive responses of community-level plant N:P ratio when considering changes in species composition than the expectation based only on arithmetic means of different species. Such exacerbation of plant N-P imbalances was caused by the fact that N inputs stimulated the dominance of two species with higher N:P stoichiometry flexibility. By linking species-specific variation of stoichiometry flexibility and the changes in species dominance with community-level variations of N:P ratio, our results uncover a new mechanism underlying the N-induced N-P imbalances in plant communities, which should be considered in biogeochemical modeling.