Abstract: Changes in nitrogen (N) deposition and precipitation regime are widely recognized as key drivers of the structure and functioning of terrestrial ecosystems. Precipitation in semi-arid regions show a trend toward increasing precipitation intensity, however, how ecosystem stability responds to the combined changes in precipitation regime (specifically its intensity and frequency) and N addition, as well as the underlying mechanisms, remains poorly understood. This study focused on a temperate grassland in Inner Mongolia, using 8-year data from a field experiment to investigate the effects of N addition (0, 10, and 40 g N m^-2 yr^-1) and changes in precipitation regime (addition of 80 mm with two different regime: 10 mm × 8 events and 20 mm × 4 events) on ecosystem stability. The results showed that N addition reduced ecosystem stability by decreasing forb abundance and species asynchrony, mediated by alterations in soil pH. In contrast, increased precipitation alleviated the negative impact of N addition on ecosystem stability. This mitigating effect was more pronounced under higher precipitation intensity (i.e., lower precipitation frequency), mainly driven by precipitation-induced increases in soil moisture and soil pH. Improved soil moisture and pH supported stability through species richness and asynchrony, whereas the stabilizing role of forb abundance was precipitation-regime dependent. Our findings emphasize that shifts in precipitation regimes are indispensable for accurately predicting the responses of grassland ecosystem stability to N deposition.

Key words: nitrogen deposition, precipitation regime, temporal stability, forbs, grassland