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).