Decadal increases in grassland biomass production from chronic N additions are eliminated by annual hay cutting

Nitrogen (N) inputs generally benefit grassland biomass production. Supporting evidence, however, comes mainly from experiments with high N input rates and from grasslands without utilization. Using a 10-yr field experiment in temperate grassland with and without hay cutting in late growing season, we addressed whether and why the impacts of low-level N addition on biomass production depend on grassland utilization. Low-level N addition significantly increased the decadal cumulative biomass production by 25% under non-haying condition, but did not increase it under haying condition. Such divergence of N impacts was driven by the different responses of dominant species. Hay cutting increased species richness but reduced biomass production because the positive contribution of increased species richness to production could not compensate for the decline in biomass of dominant species. The haying-induced decline in biomass production under N addition was associated with a decrease in individual abundance and mass of dominant species. Our results highlight the biomass production trade-off between the effects on the abundance of dominant species and species richness in haying grasslands under low-level N addition, and provide empirical evidence for the importance of species abundance in understanding the relationship between biodiversity and ecosystem function.