Nitrogen (N) deposition greatly influences ecosystem processes through the alteration of plant nutrition; however, there is limited understanding about the effects of phosphorus (P) inputs, especially within the backdrop of N deposition. Here we investigated the plant stoichiometric responses to P addition under both ambient conditions and with N addition in a temperate grassland in Inner Mongolia over a two-year period. The effects of P addition on foliar nutrition and stoichiometric ratios were highly dependent on the presence of N supply, in that P addition showed no significant impacts on foliar N or P concentrations (or pools in community biomass) or N:P under ambient N conditions, but enhanced foliar N in dominant species and foliar P in almost all species (and pools in community biomass) with N addition. These results may be explained by P addition significantly changing the soil properties (soil pH and available P) when applied in combination with N additions. Moreover, there was no biomass response to nutrient additions. Our results suggest that N deposition may enhance the absorption of additional P by plants in semi-arid grasslands, and that P limitation with increased N deposition might be less important for limiting plant growth than previously anticipated.