Foliar fungal diseases respond differently to nitrogen and phosphorus additions in Tibetan alpine meadows

The effects of nitrogen addition on fungal diseases in agroecosystems have been widely documented, while largely unknown is how phosphorus fertilization will affect foliar fungal diseases in natural ecosystems. Here, we implemented a factorial experiment with nitrogen and phosphorus addition in an alpine meadow of the Qinghai-Tibetan Plateau to test the effects of nutrition fertilization on foliar fungal diseases severity (i.e., % of the leaf area that was covered by fungal lesions) at plant species-, family- and community-levels, respectively, and also among different disease types (biotroph vs. necrotroph). At plant species-level, nitrogen and phosphorus addition increased diseases severity for two and five plant species respectively. At plant family-level, nitrogen addition increased pathogen load on Poaceae plants, while phosphorus fertilization increased pathogen load on Scrophulariaceae. Moreover, we found that addition of both nitrogen (p = .034; coefficient = 0.436) and phosphorus (p = .012; coefficient = 0.622) increased pathogen load of biotrophic pathogens, but only nitrogen addition (p = .010; coefficient = 1.769) increased pathogen load induced by necrotrophic pathogens. At plant community-level, nitrogen addition (p = .009; coefficient = 0.218) alone, rather than phosphorus addition or the interaction of nitrogen and phosphorus addition, increased community pathogen load. We concluded that nitrogen addition affects foliar fungal diseases more than phosphorus at the community-level, and fungal diseases caused by necrotrophic pathogens were more sensitive to nitrogen addition.