Unimodal response of N2O flux to changing rainfall amount and frequency in a wet meadow in the Tibetan Plateau

The rainfall amount and the frequency of extreme rainfall events have been predicted to increase in the Qinghai-Tibetan Plateau during the growing season. These changes will likely affect ecosystem processes, including those that control nitrogen (N) cycling and storage; however, the direction of the changes remains unclear. In this study, we experimentally altered the amount and frequency of precipitation events during the growing season (May through October) at an alpine wetland in the Qinghai-Tibetan Plateau. The treatments included ambient rain (CK) plus 25 mm of extra water for each irrigation event but with different irrigation frequency, i.e., weekly (DF1), biweekly (DF2), every three weeks (DF3) and every four weeks (DF4). During the growing season, the N2O flux showed a large seasonal variation. Compared with the treatment of natural rainfall events, the increase in the rainfall amount promoted the N2O emission flux. As the frequency of rainfall events increased, the aboveground biomass increased significantly from 85.82 g·m−2 to 245.79 g·m−2, and the accumulated N2O emissions first increased and then decreased; we observed the peak of the N2O flux at the DF2 rainfall frequency. We also found the significant linear relationships between soil N2O flux, nitrate nitrogen, and microbial biomass nitrogen content. Furthermore, among the soil microbial factors, higher rainfall frequency (DF1) significantly reduced the relative abundance of the original dominant species (Alphaproteobacteria) in the studied wetland soil. Our results indicate that alpine wetlands are highly sensitive to increased precipitation variability and high frequencies of extreme rainfall events could significantly reduce N2O emission. Future precipitation patterns could weaken the contribution rate of wetland N2O emission to the global warming.