Statistical analyses
Before statistical analysis, we examined whether the data conformed to a normal distribution (Shapiro–Wilk test) and tested for homogeneity of variance (Levene’s test). We conducted data analysis according to the following four steps. First, a Two-way Analysis of variance (ANOVA) was used to examine the effects of N addition, altered precipitation, and their interaction on the following: soil abiotic parameters (soil temperature, moisture, pH, NH4+-N, and NO3-N); plant properties (aboveground and belowground biomass); N2O flux (emission peak); and the functional gene abundance related to nitrification (AOA, AOB) and denitrification (nirS , nirK , andnos Z). Second, a repeated-measures ANOVA was performed to assess the effects of treatments on N2O flux during the growing season. Significant differences of the above-mentioned parameters were assessed using post-hoc tests (Duncan’s test at P < 0.05). Third, linear regression analysis was used to evaluate the relationships among N2O flux and soil abiotic factors and plant properties. Finally, a Pearson correlation analysis was used to investigate the correlation between functional gene abundances and N2O flux. These statistical analyses were carried out in SPSS version 21.0 (SPSS, Chicago, IL, U.S.A.) and were visualized using Sigmaplot 12.5 software (Systat Software Corporation, U.S.A.).