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Global change will lead to increases in regional precipitation and nitrogen (N) deposition in the semi-arid grasslands of northern China. We investigated the responses of vegetation carbon (C) pools to simulated precipitation and N deposition increases through field experiments in a typical steppe in Inner Mongolia. The treatments included NH4NO3 addition at concentrations of 0 (CK), 5 (LN, low nitrogen), 10 (middle nitrogen, MN), and 20 (HN, high nitrogen) (g m(-2) a(-1)) with and without water. After three consecutive years of treatment, from 2010 to 2012, water addition did not significantly change the size of the total vegetation C pools, but it significantly decreased the ratio of root:shoot (R:S) (P = 0.05) relative to controls. By contrast, N addition significantly increased the total vegetation C pools. The C pools in the LN, MN and HN treatments increased by 22, 39 and 44 %, respectively. MN produced the largest effect among the N concentrations, although differences between N-added treatments were not significant (P > 0.05). N addition significantly reduced the ratio of root:shoot (R:S) (P = 0.03). However, there were no significant interactive effects of water and N addition on the vegetation C pools.