1. Changes in soil carbon (C) sequestration in grassland ecosystems have important impacts on the global C cycle. As such, it is important that researchers better understand the underlying mechanisms affecting soil C. Increasing evidence has shown that atmospheric nitrogen (N) deposition can cause dramatic changes in grassland soil C. It remains unclear whether herbivore grazing, a primary means to manage and utilize grassland resources, can regulate the effects of N deposition on soil C, and whether these effects are dependent on plant community diversity.

2. Here, we examined the joint effects of herbivore grazing and N-addition on soil organic C (SOC) stocks in two types of communities with low and high plant diversity, respectively.

3. Our results showed that the effects of N-addition and its combination with herbivore grazing on grassland SOC were inconsistent in the two types of communities. In the low-diversity community, N-addition greatly decreased SOC stocks, while grazing significantly increased it. Additionally, the grazing-induced increase in soil C stocks in presence of N-addition was so great that it completely counteracted the significant decline in SOC induced by N-addition. However, in the high-diversity community, we observed no effects of N-addition on SOC and grazing increased SOC only in the absence of N-addition and had no significant effect in presence of N-addition.

4. Synthesis and applications. Our study suggests that increased N deposition can trigger a remarkable reduction in soil C sequestration in grasslands with low plant diversity, but that herbivore grazing can offset this decline, which may help to mitigate greenhouse gas emissions caused by atmospheric N deposition. As a result, we suggest that moderate herbivore grazing should be considered as an effective grassland management measure for maintaining and improving grassland soil C sequestration as the increasing global change such as elevated atmospheric carbon dioxide, N deposition, and biodiversity losses threat.