Atmospheric nitrogen (N) deposition, climatic variables and anthropogenic management affect grassland-ecosystem stability by driving changes in plant community and loss of plant diversity. Determination of their influences on plant composition and diversity is essential for predicting ecosystem response to future global changes and for making policies of sustainable grassland management. We evaluated the relative contributions of atmospheric N deposition, climatic variables and grassland management to plant diversity and N accumulation by temporally monitoring the changes in plant community and N status in a temperate grassland ecosystem. Ambient low-level N deposition combined with long-term grassland enclosure led to increases in above-ground biomass and abundance of grasses and sedges, and decreases in those of forbs. The dominant species of perennial forbs and grasses also displayed divergence in their biomass and abundance over time. Nitrogen deposition made greater contributions to the divergent changes in perennial forbs and grasses. However, air temperature and precipitation fluctuations mainly contributed to the annual fluctuations in annual and biennial species. Long-term enclosure aggravated the cumulative effects of soil inorganic N on plant-diversity loss while mowing had opposite effects on the N accumulation. Synthesis and applications. Ambient low-level N deposition can drive loss of plant diversity due to N accumulation in grassland ecosystem under long-time enclosure. Therefore, appropriate grassland management, such as rationally grazing and/or mowing, may be effective practices in terms of maintenance of plant biodiversity under the scenario of enhanced atmospheric N deposition.