Anthropogenic activities introduce nitrogen (N) into terrestrial ecosystems, inducing changes in quantity and quality of plants. However, little is known about how do nitrogen (N)-mediated such changes alter functional diversity of green leaf traits? How does N-mediated changes in leaf quality contribute to the rate of litter decomposition at intra- or inter-species level? How can such N-mediated changes in quantity and quality be scaled up to explain and predict community-level litter decomposition? A common garden litter decomposition was conducted in an alpine meadow on the Tibetan Plateau, where six litter species and litter mixtures from the communities that had experienced eight-year N addition treatment, were subjected to decomposition for 641 days, with three harvests. For litter mixtures, littles were selected randomly from aboveground shoots collected from 0.5 × 0.5 m2 quadrats. We found that N enrichment increased green leaf and litter N concentrations, and caused small and idiosyncratic variations in phosphorus and carbon concentrations. Plasticity in leaf nutrient concentrations induced little variation in litter decomposition in the six species. Meanwhile, N enrichment induced a reduction in leaf trait diversity and community-level litter decomposition. Correspondingly, positive relationship between decomposition rate (k) and leaf trait diversity were observed. N enrichment reduced dissimilarity of leaf traits, which could relate a reduction of litter decomposition at community level. By integrating intra- and interspecific trait variation with the abundances of coexisting species, these findings can help predict ecosystem function with N enrichment.