Abstract: An experiment comprising a factorial combination of three water and four nitrogen levels (12 treatments), laid out in a split plot design, was conducted to understand the effects of precipitation change and nitrogen deposition on vegetation community structure and biomass in Stipa breviflora desert steppe in Siziwang Banner, Inner Mongolia. The main plot treatments were: CK (natural rainfall), W (30% rainfall addition), R (30% rainfall reduction), while the subplot treatments were 0 (N₀), 30 (N₃₀), 50 (N₅₀), and 100 (N₁₀₀) kg N·ha^-1·a^-1. The results show: 1) The application of water and nitrogen resulted in significant changes in community composition and species importance values. The R treatment had the lowest number of community species, and the importance value of dominant species in community was increased. The addition of nitrogen resulted in a curve of ‘parabola’ shape for number of species present (i.e. the number of community species was higher at medium nitrogen application rates than at N₀ or N₁₀₀). 2) With increased rate of nitrogen addition, species richness, Shannon-wiener diversity and Pielou evenness index were initially increased, but decreased at higher nitrogen application rates. The maximum values were observed in treatments CK-N₃₀, W-N₃₀ and R-N₅₀ in 2016 and 2018, and the lowest value of CK-N₃₀, W-N₃₀, R-N₅₀ in the drought year (2017), respectively. The main effects of water, nitrogen and water-nitrogen interaction all have significant effects on the above three plant diversity indexes (P<0.05). 3) Water and nitrogen both stimulated aboveground biomass increase. Water, nitrogen as well as water and nitrogen interaction have significant effects on aboveground biomass (P<0.05). The proportion of perennial grasses in the total aboveground biomass was increased by short-term nitrogen addition, while the percentage of perennial weeds was decreased.

Key words: nitrogen deposition, precipitation variation, desert steppe, community structure, biomass