The role of nutrients, light, and litter in species loss in an alpine meadow community

The decline in species diversity within nutrient-enriched grasslands is commonly explained by a single hypothesis that often overlooks the potential interconnected roles of soil nutrients, light, and plant productivity. In a 2-year field experiment involving multiple nutrient additions (nitrogen, phosphorus, and potassium; NPK) conducted in an alpine meadow on the eastern Qinghai-Tibet Plateau, we investigated the simultaneous impact of three driving factors (soil nutrients, litter, and light) on species loss. Our findings show that the reduction in species richness can be attributed to belowground soil nutrient enrichment and aboveground light asymmetry. Specifically, the increase in soil nutrients following NPK addition directly contributed to the decline in species richness. Light limitation associated with an increase in the aboveground net primary productivity (ANPP) indirectly accelerates competitive exclusion, leading to species loss. The increased ANPP was primarily influenced by the greater proportion of the grass species Elymus nutans in the NPK-fertilized plots, highlighting the significant role of dominant species in restricting light availability. Contrary to expectations, our results did not support the negative impact of litter on species richness. In summary, our findings indicate that interspecies competition for soil nutrients and light availability are the two primary drivers of species loss in alpine meadow communities. This insight has crucial implications for understanding the effects of nutrient enrichment on biodiversity, ecosystem functioning, and services in alpine meadows on the Qinghai-Tibet Plateau.