4.1. Variations in the isotopic composition of different water
The δ18O and δ2H in precipitation water initially increased and then decreased significantly from May to October. Some scholars have concluded that precipitation δ18O and δ2H produce a dominant factor and are consequence of the effects of temperature and precipitation regarding the isotopic inheritance from precipitation to plant water (Liu et al., 2021). There is a clear difference between our results and their conclusion. This difference could be caused by air trajectory that is a very important factor influencing moisture source (Wu et al., 2016). Transition times between a westerly circulation and the Southeast Asia monsoon supplying isotopically regarding moisture sources depleted atmospheric water vapor (Lihui et al., 2015).
Plant water 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 between precipitation and plant water (Phillips&Ehleringer, 1995; Meinzer et al., 2006; Sprenger et al., 2016; Plavcová et al., 2018). Our result may be due to drought stress in our study area. On the one hand, soil water is the primary source of plant water, and 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 self-growth until October. On the other hand, soil water isotopic compositions resulting from soil water evaporative differences influenced the plant water uptake pattern of plants (Rothfuss&Javaux, 2017).
The isotopic values and SWC exhibited larger variability in shallow soil layers, compared with deeper soil layers from May to October for both the two sites. Groundwater isotopes were consistent in the deeper soil and remained relatively stable. This result is consistent with results of previous studies in 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 (Figure 5). The enriched soil water in the shallow soil layers corresponded well to the lower SWC in July, the isotopically depleted soil water matched well with the high SWC, the results probably attributable to less rainfall and intensive evaporation (Gazis&Feng, 2004) in July, and rainfall recharge with negative isotopic values (Wang, et al., 2019) in August. All the results indicate that precipitation recharge, evaporation, and antecedent moisture all influence soil water isotopic compositions (Hsieh et al., 1998; Brooks et al., 2015).