1. Global environmental changes are altering ecosystem stability, sometimes by altering biodiversity. For example, by driving grassland plant species loss, nitrogen (N) addition can reduce ecosystem stability. In other cases, however, N addition may alter productivity and ecosystem stability, by increasing the dominance of particularly productive or stable species. 2. We examined how N addition affected plant diversity, productivity, and the temporal stability of productivity in an 8-year grassland experiment. We found that N addition enhanced productivity and decreased ecosystem stability throughout the experimental period, even though it reduced species richness in the first four years, but increased richness during the subsequent years. 3. During the early years, N addition decreased ecosystem stability by synchronizing species fluctuations and population stability of the dominant species. During later years, N addition did not increase stability, even though it increased species richness; instead, N addition continued to decrease ecosystem stability by decreasing species dominance and stability of the dominant species, without changing the identity of the dominant species. 4. Synthesis. Our results indicate that N addition decreased ecosystem stability via mechanisms that were both dependent and independent of plant diversity, and that the mechanisms involved shifted over time. N addition impacted on ecosystem functioning through species richness and the effects on species dominance and the stability of most dominant species, highlighting the join effect of multiple biotic drivers in regulating ecosystem stability.