Grazing and fertilization influence plant species richness via direct and indirect pathways in an alpine meadow of the eastern Tibetan Plateau

Grazing and fertilization influence plant species richness through multiple pathways, but the exact pathways and their relative contributions are unclear. Here, we report on a 3-year field experiment on an alpine meadow in which we manipulated grazing and fertilization simultaneously to assess their direct and indirect effects on plant species richness. Results indicated that both grazing and fertilization significantly influenced plant species richness via above- and belowground pathways. Specifically, the direct effect of grazing (-0.30, effect size) on species richness was negative, mainly due to selective feeding. This was comparable, in magnitude, to a positive indirect effect of grazing (0.36) on species richness, due to decreased aboveground biomass and increased belowground biomass. The direct effect of fertilization (-0.34) on species richness was negative, which was attributed to the changes of soil pH and available nitrogen; this effect was double the indirect effect (-0.16), which largely stemmed from the increased aboveground biomass rather than changes in belowground biomass. No significant interaction of grazing and fertilization on species richness was observed, but the effect of fertilization on species richness was masked by the effect of grazing in the combined treatment. It was concluded that moderate nitrogen addition (<51.0 kg N ha(-1) year(-1)) could increase local pasture productivity while also sustaining species richness stability.