Testing the scaling effects and mechanisms of N-induced biodiversity loss: evidence from a decade-long grassland experiment

Although extensive studies demonstrate that nitrogen (N) enrichment frequently reduces plant diversity within small quadrats (0.5-4m(2)), only a few studies have evaluated N effects on biodiversity across different spatial scales. We conducted the first experimental test of the scale dependence of N effects on species richness from a 10-year N treatment (1.75-28gNm(-2)year(-1)) in a typical steppe. We used species-area relationship (SAR) to analyse the scale dependence of species loss with power model S=cA(z) (S is species number, A is area, c is intercept, and z is slope). Absolute species loss decreased at sampling area > 8m(2). Proportional species loss (compared to control) decreased and critical threshold (N-crit) for biodiversity losses increased with sampling areas. These scale dependences were quantified as increasing slope (z-value) of SAR with N addition. Through SAR decomposition, we found that this overall positive effect was in response to positive effects of changes to the species abundance distribution over negative effects of overall species richness losses.Synthesis. As nitrogen (N) enrichment typically occurs at scales much larger than individual plots, understanding how N enrichment affects the scaling patterns of biodiversity is necessary for biodiversity conservation and ecosystem management in response to anthropogenic N deposition.