Soil moisture determines the effects of climate warming on spring phenology in grasslands

Spring vegetation phenology is highly sensitive to global change. However, the effects of ongoing climate warming and N deposition on grassland spring phenology and how abiotic and biotic factors modulate such effects remain unclear. Here, we conducted a factorial field experiment of warming and N addition in three types of grasslands with varying aridity indices (0.20–0.39) in Inner Mongolia, China. Our results showed that warming and the combination of warming plus N addition had similar effects on start of season (SOS). These treatments delayed SOS by 3–5 weeks in low-, moderate and high-aridity grasslands in both wet (2018) and dry (2019) years, except that they advanced SOS by ∼1 week in low-aridity grassland in the wet year. N addition alone slightly advanced or delayed SOS by 2–5 days depending on the grassland type. The magnitude and direction of SOS response to warming and warming plus N addition were closely and linearly correlated to grassland soil moisture, but not to air temperature or soil N availability. Warming-induced reduction in soil moisture drove SOS by affecting species richness and/or functional group composition. By contrast, N addition–induced changes in species richness and functional group composition did not alter the SOS. These combined results indicate that temperature and soil water availability are primary factors regulating SOS in semiarid grasslands. Our findings suggest that annual variation and spatial distribution in soil moisture should be considered in future studies of climate warming and environmental effects on grassland phenology.