Variation of soil-plant-atmosphere continuum stable isotope and water source in Qinghai spruce forest of the eastern Qilian Mountains

Understanding the hydrogen and oxygen stable isotope composition and characteristics of different water bodies in soil-plant-atmosphere continuum is of significance for revealing regional hydrological processes and water cycle mechanisms. In this study, we analyzed the stable isotopic composition, relationship and indicative significance of precipitation, soil water (0~100 cm depth) and xylem water of Qinghai spruce (Picea crassifolia) forest in the eastern Qilian Mountains, and explored the circulation process among different water bodies. The results show that the stable isotopes of precipitation vary greatly during the entire observation period. The values of δ²H and δ¹⁸O in the precipitation in the warm season are richer than those in the cold season, and the slope and intercept of local meteoric water line (LMWL, δ²H = 6.79δ¹⁸O+7.13) are both smaller than global meteoric water line (GMWL, δ²H=8.17δ¹⁸O+10.56). The stable isotopes of soil water at different depths underwent different degrees of evaporative fractionation, and the δ¹⁸O and δ²H of shallow soil water varied greatly, while the deep soil water tended to be similar. The topsoil (0~10 cm) can respond quickly to precipitation, and the response of the deep soil has a time lag. In the whole growing season, 0~30 cm and 60~100 cm soil water are the main water sources of Qinghai spruce. The water source of Qinghai spruce was from all soil layers in May and September, mainly from the shallow soil layer (0~30 cm) in August and October, and mainly from the deep soil layer (60~100 cm) in June and July.