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Climate and land-use change are some of the most profound threats to the biodiversity and functioning of the Earth’s ecosystems. However, potential synergistic effects of these drivers through biodiversity change on ecosystem functioning remain unclear. Here we examined how aridity and land-use (overgrazing and haying) affect above-ground biomass and soil organic carbon (SOC) through changes in plant species richness across 701 grassland sites in China. We found that aridity and grazing reduced SOC through decreasing plant species richness, but did not significantly affect above-ground biomass. Notably, we observed strong negative synergistic effects of aridity and grazing, suggesting that soil carbon content was particularly threatened by grazing in arid environments. By contrast, haying reduced above-ground biomass and had no significant effect on SOC, although it increased plant species richness. Plant species richness had greater positive effects on SOC than on above-ground biomass, and its effects became stronger in more arid regions. Together, the results demonstrate that aridity and overgrazing threaten soil carbon content via their detrimental effects on plant diversity, and that detrimental overgrazing effects are particularly strong under arid conditions. However, the study also indicates that certain management types like haying or less intensive grazing can maintain or enhance plant diversity and soil carbon content, and that the beneficial effects of plant diversity are particularly important in arid environments.
•Aridity and grazing reduced SOC through decreasing plant species richness.•Haying reduced above-ground biomass and increased species richness.•Species richness had greater positive effects on SOC than on above-ground biomass.•Species richness effects became stronger in more arid regions.