Anthropogenic warming and land‐use change are expected to accelerate global soil organic carbon (SOC) losses and change plant species composition and richness. However, how changes in plant composition and species richness mediate SOC responses to climate warming and land‐use change remains poorly understood. Using data from a 7‐year warming and clipping field experiment in an alpine meadow on the Qinghai–Tibetan Plateau, we examined the direct effects of warming and clipping on SOC storage versus their indirect effects mediated by plant functional type and species richness. We found that warming significantly increased SOC storage by 8.1% and clipping decreased it by 6.4%, which was closely correlated with the corresponding response of below‐ground net primary productivity (BNPP). We also found a negative correlation between SOC storage and species richness, which was ascribed to the increased BNPP via enhancing the dominance of grasses and decreasing species richness under warming. The lower SOC storage under clipping was caused by the clipping‐induced decrease in BNPP via weakening the dominance of grasses and increasing species richness. Our findings highlight that the SOC storage in this alpine meadow under climate warming and clipping was primarily governed by BNPP changes, which was mediated by changes in the dominance of grasses and species richness. Overall, our study demonstrates that shifting to the dominance of grasses and changing species richness would benefit soil C sequestration under climate warming, but this positive effect would be dampened by grazing or hay harvest. Read the free Plain Language Summary for this article on the Journal blog.

1. 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.2. 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.3. 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.