Early Season Precipitation Addition Offsets Warming Effect on Carbon and Water Use Efficiency in Qinghai–Xizang Plateau

Carbon and water fluxes are critical indicators of grassland ecosystem function and are sensitive to climate change. However, how seasonal precipitation changes interact with warming to affect alpine grassland carbon and water use efficiency (CUE and WUE) remains unclear. We conducted a 2-year in situ warming and early/mid-growing season precipitation addition experiment on the Qinghai–Xizang Plateau, China. The results showed that compared with ambient conditions, warming reduced ecosystem respiration by 20.0%, net ecosystem exchange by 154.8%, and gross ecosystem productivity by 49.4%, resulting in a 157.0% decrease in CUE and a 57.8% reduction in WUE. Warming increased the dominance of legumes but decreased the dominance of grasses. Under warming conditions, the addition of precipitation increased during both the early and mid- growing seasons decreased the dominance of legumes. Under warming, the addition of precipitation increased CUE in the early growing season in both 2020 and 2021, and increased WUE in 2020. Warming combined with the addition of mid-growing season precipitation increased CUE in 2020. The soil temperature was negatively correlated with CUE during the early growing season, but not in the mid-growing season. The results of structural equation modeling showed that warming and precipitation addition affect both CUE and WUE by regulating grass productivity during the early growing season, whereas they affect CUE and WUE by regulating legume productivity during the mid-growing season. Our findings emphasize that plant community composition is an important factor regulating the seasonal variations in CUE and WUE under environmental changes.