Synergistic effects of nitrogen and CO₂ enrichment on alpine grassland biomass and community structure

Global environmental change is altering the Earth's ecosystems. However, much research has focused on ecosystem-level responses, and we know substantially less about community-level responses to global change stressors. Here we conducted a 6-yr field experiment in a high-altitude (4600 m asl) alpine grassland on the Tibetan Plateau to explore the effects of nitrogen (N) addition and rising atmospheric CO₂ concentration on plant communities. Our results showed that N and CO₂ enrichment had synergistic effects on alpine grassland communities. Adding nitrogen or CO₂ alone did not alter total community biomass, species diversity or community composition, whereas adding both resources together increased community biomass, reduced species diversity and altered community composition. The observed decline in species diversity under simultaneous N and CO₂ enrichment was associated with greater community biomass and lower soil water content, and driven by the loss of species characterised simultaneously by tall stature and small specific leaf area. Our findings point to the co-limitation of alpine plant community biomass and structure by nitrogen and CO₂ , emphasising the need for future studies to consider multiple aspects of global environmental change together to gain a more complete understanding of their ecological consequences.