Altered rainfall patterns reshape hillslope water balance and amplify carbon and nitrogen losses in a periglacial grassland

Global warming is shifting rainfall patterns towards fewer but more intense events, leading to longer intervals between rainfall events. However, the effects of these changes on hydrological processes and elemental transport in the highly vulnerable periglacial ecosystems remain largely unexplored. This study used a rainfall manipulation experiment to examine how altering rainfall intervals with fixed seasonal total precipitation affects hydrological processes and carbon, nitrogen, and phosphorus export in a periglacial grassland on the Tibetan Plateau. Four treatments were applied on a uniform slope (∼11°): natural rainfall (CK), 3-day interval (P1), 7-day interval (P2), and 11-day interval (P3). Our results revealed that altered rainfall patterns reshape hillslope water balance. Specifically, P1 increased surface runoff (SR), soil water storage (SWS) and soil evaporation while reducing deep percolation (DP), because frequent small rainfall events enhanced soil water retention in the 0-30 cm layer while limiting infiltration into deeper layers. In contrast, P2 and P3 significantly increased SR while reducing SWS, resulting in a non-significant effect on DP. Moreover, distinct thresholds were identified for the generation of SR and DP at 1.8 mm and 1 mm rainfall, respectively. Additionally, altered rainfall patterns significantly increased the losses of dissolved carbon, total nitrogen and nitrate in SR but did not affect ammonium and phosphate. Notably, nitrate losses exhibited a nonlinear response to changing rainfall patterns, which peaked at a 7-day interval instead of 11-day interval with the highest single rainfall amount. This suggests that the extreme rainfall events may not affect the dissolved nitrogen exports in the periglacial grasslands as it requires not only a large single rainfall amount but also high rainfall frequency. Therefore, future studies should consider the interactions between rainfall frequency and single event size to disentangle the responses of hydrological and biogeochemical processes to climate extremes in these fragile periglacial ecosystems, thus supporting sustainable watershed management for the Asian Water Tower.