Plant C:N ratio predicts community composition changes under N addition in an alpine meadow

Plant functional traits play important roles in determining plant responses to environmental change and further shaping community composition, but the role of plant stoichiometry remains poorly understood in regulating community responses to continually increased nitrogen (N) deposition. In this paper, we used a 5-year manipulative experiment with six N addition rates (N-rate) to explore how the N response of different plant functional groups including forbs, grasses, sedges and legumes links to above-ground plant carbon-to-N ratio (C:N, related to N use strategies) in an alpine meadow. We found that plant C:N explained 45.8% and 42.6% of N response variation among functional groups in plant above-ground net primary productivity (ANPP) and abundance. Over increasing N-rate, functional groups with higher C:N (grasses and sedges) tended to grow better and have greater abundance, yet those with lower C:N (forbs and legumes) were at risk of loss, subsequently mitigating the N-caused decline in community-level plant C:N. This was mainly due to that functional groups with higher C:N not only possessed higher N use efficiency but also had greater competitiveness for above-ground light due to higher plant height. In addition, we also found that plant C:N differences among functional groups remained relatively constant over N-rate, suggesting that the ability of this trait to indicate functional group-specific N responses was stable. Synthesis. Our results show that the N responses of different herbaceous plants can be well explained by plant C:N. Thus, we suggest the prediction of plant community composition changes under N deposition would be greatly improved by considering this easily measured stoichiometric trait.