4.1. Variations of the isotopic composition in different water
The δ18O and δ2H in precipitation initially enriched and then depleted significantly from May to October. Some scholars have concluded that δ18O and δ2H in precipitation are consequence of the effects of temperature and precipitation amount. The consequence produce a dominant factor for plant and soil waters, regarding the isotopic inheritance from precipitation to plant water, and their studies conducted in Ordos Plateau, where located in monsoon regions(Liu et al., 2021). There is a clear difference between their conclusions and our results. Precipitation amount had little influence to δ18O and δ2H in precipitation, temperature explained 49% to δ18O, whereas, 42% to δ2H. It may be caused by air trajectory, which influence δ18O and δ2H in precipitation (Wu et al., 2016) by influencing moisture source. Our research sites located in south of Qilian Mountains, which situated the intersection of the alpine regions, the northwest regions, and the monsoon regions (Yang et al., 2019). They had more frequency of transition times than Ordos Plateau, where influenced by internal circulation and monsoon circulation (Gou et al., 2011), influencing the moisture sources depleted atmospheric water vapor.
Plant water trends responded well to the trends of δ18O and δ2H in precipitation from May to July, but not well from August to October. Several studies have previously documented the consistency of precipitation and plant water (Phillips&Ehleringer, 1995; Meinzer et al., 2006; Sprenger et al., 2016; Plavcová et al., 2018). Our different results may be due to that precipitation intensity exerts little effect on the soil water as almost all rainfall returns to the atmosphere via evapotranspiration (Dai et al., 2019a). Soil water is the primary source of plant water, soil moisture meets the needs of plant growth from May to July. However, during the rapid growth of plants (August and Septermber), fierce water competition stimulates plants to use water in each soil layer in a balanced manner. This is an adaptation mechanism established to promote growth until October. Furthermore, soil water isotopic compositions resulting from soil water evaporative differences influenced the plant water uptake pattern of plants (Rothfuss&Javaux, 2017).
The δ18O, δ2H and SWC displayed larger variability in shallow soil depths, compared with deeper soil depths from May to October for the both sites. Groundwater isotopes were consistent in the deeper soil and remained relatively stable. This result is consistent with previous studies of arid and semi-arid regions (Fischer et al., 2017; Wang et al., 2019; Guo, et al., 2020). Both δ18O and δ2H values were enriched in June and July, depleted in August in the shallow soil layers. The isotopically enriched soil water of shallow depths matched well with the lower SWC, in contrast to the depleted to the higher, the results probably attributable to less precipitation and intensive evaporation (Gazis&Feng, 2004) in July, and rainfall recharge with negative isotopic values (Wang, et al., 2019) in August. The results indicate that precipitation recharge, evaporation, and antecedent moisture all influence soil water isotopic compositions (Brooks et al., 2015).