Abstract
Nutrient enrichment has the potential to influence biodiversity and primary productivity, but the general patterns and potential mechanisms are still elusive. Using a five-year field nutrient addition experiment, we evaluated response patterns and potential mechanisms of plant diversity and productivity to nitrogen (N) enrichment, phosphorus (P) enrichment, and nitrogen (N) plus phosphorus (P) enrichment. The results showed that N or P addition alone significantly decrease species richness by 25.9% and 23.5%. N plus P addition was stronger than N or P addition alone by 51.9% decline of species richness. Plant community composition under P addition was similar to control but separated in the N and N plus P addition plots from the control plots. N addition remarkably increased aboveground net primary productivity (ANPP) by 93.3% by increasing grass biomass, whereas P addition had no effect on ANPP. However, P addition stimulated the positive effect of N addition on ANPP, leading to 127.8% enhancement of ANPP under N plus P addition. N addition decreased species richness by increasing community height and decreasing soil pH, while P addition decreased species richness by increasing community height and soil available phosphorus. The increased community height, community weighted means (CWM) of leaf N content and leaf area explained the enhancement of ANPP under N addition. Our findings are crucial for understanding of the effects of N and P enrichment on ANPP and species richness in the semi-arid grassland, and will provide scientific support for optimizing the management and utilization of semiarid grassland.