As a key factor that affects various ecosystem processes, plant nitrogen (N) concentration is very important to forecast the responses of ecosystem to global climate change, e.g., altered precipitation regime and enhanced N deposition. Grassland, one of the most sensitive ecosystems to climate change, has been paid insufficient attentions in terms of the intra- and inter-annual dynamics of plant N concentration. In this study, a five-year manipulative field experiment was conducted by employing N addition (10 g N m^-2 a^-1) and water addition (80 mm a^-1 in terms of five intensities, i.e., 2 mm×40 times, 5 mm×16 times, 10 mm×8 times, 20 mm×4 times, and 40 mm×2 times) in a typical steppe in Inner Mongolia temperate grassland. We analyzed the effects of water and nitrogen additions on the intra- and inter-annual dynamics of plant nitrogen concentration. We found that water addition of 10 mm×8 times and 40 mm×2 times decreased ecosystem plant N concentration in the dry years, while that of 20 mm×4 times increased ecosystem plant N concentration, but these increases or decreases were not significant statistically. Under water additions of other lower intensity (2 mm×40 times and 5 mm×16 times), neither increase nor decrease patterns were clearly observed across years. Plant N concentration of two dominant species, Stipa krylovii and Artemisia frigida, was both lower in the early and late growing season, while it was higher in the middle of the growing season. Water addition decreased plant N concentration of these two dominant species throughout the growing season, but without significance. The N addition enhanced ecosystem N concentration and plant N concentration of two dominant species across the growing season. However, this enhancement could be down regulated by water addition, and this down regulation due to water addition increased with the duration of experiments. The N and water additions both increased inter-annual variability of ecosystem N concentration, while the effects they exerted on intra-annual variability of plant N concentration of two dominant species dependent on the annual precipitation amount. Our work has important implications for model improvement and an advanced understanding of grassland-response to N deposition and the altered precipitation regime.