Resource co-limitation of community biomass but not structure of an alpine grassland

Anthropogenic environmental changes are influencing the structure and function of many ecological communities, but their underlying mechanisms are often poorly understood. We conducted a seven-year field experiment to explore the ecological consequences of nitrogen (N) and phosphorous (P) enrichment in a high-altitude Tibetan alpine grassland. The enrichment of both N and P, but not either alone, resulted in increased plant above- and below-ground biomass. By contrast, N, but not P, enrichment reduced species richness, and altered plant phylogenetic diversity and structure. The observed decline in species richness under N addition was driven by the loss of species characterized by high leaf nitrogen content, and associated with higher soil manganese level and greater belowground competition. Our study highlights resource co-limitation of community biomass but not structure of the study grassland, while also identifying soil metal toxicity and belowground competition as important mechanisms driving community changes after nutrient amendment.