Samples of Nitraria tangutorum, groundwater and soil water were collected in the arid area of Northwest China. Generally, there is no hydrogen and oxygen isotope fractionation when roots absorb water. However, this study found that the isotope fractionation occurs when roots absorb water, and the hydrogen isotope fractionation is more obvious. The vegetation in arid area will redistribute water from the moist soil layer to the roots in the arid soil layer to alleviate the root drought stress caused by too low soil water. The Nitraria tangutorum in this study area absorbed more water molecules containing 1H from groundwater and released them into soil through hydraulic redistribution, resulting in much more negative hydrogen isotope of soil water. Why did Nitraria occur such obvious isotope fractionation? Is the negative isotope of soil water caused by hydraulic redistribution? What does this have to do with the ‘two water worlds’ hypotheses?
There are some papers on isotope fractionation of root water uptake :
[1] Ellsworth P Z, Williams D G. Hydrogen isotope fractionation during water uptake by woody xerophytes[J]. Plant & Soil,2007,291(1-2):93-107.
[2]Lin G，Sternberg LSL. Hydrogen isotope fractionation by plant roots during water uptake in coastal wetland plants// Ehleringer J R，Hall AE，Farquhar GID，eds. Stable Isotopes and Plant Carbon-water Relations. London: Academic Press,1993.
[3] Vargas A I, Schaffer B, Yuhong L, et al. Testing plant use of mobile vs immobile soil water sources using stable isotope experiments[J]. New Phytologist,2017,215.
[4] Schachtschneider K, February EC. The relationship between fog, floods, groundwater and tree growth along the lower Kuiseb River in the hyperarid Namib［J］. Journal of Arid Environments, 2010,74(12):1632-1637.
[5] 姜生秀,安富博,马剑平,等.石羊河下游青土湖白刺灌丛水分来源及其对生态输水的响应[J].干旱区资源与环境,2019,33(09):176-182.
[6] 赵鹏,徐先英,纪永福,等.民勤绿洲边缘不同演替阶段白刺灌丛水分利用动态[J].干旱区资源与环境, 2019, 033(009):168-175.
The samples should be plotted on a dD/d18O plot to see if there are linear relationships that would indicate that evaporation is the dominant source of differentiation between samples. Look for a linear deviation from the LMWL which would suggest that the system is sampling water that is at different stages of an evaporative regime. Only samples that fall off of the LMWL and an evaporative line should be candidates for root level fractionation.
Look at the work of Kvr Priyadarshini who showed that water uplift is often made available in the soil to other plants. A shift to a groundwater source in a plant may indicate accessing of deeper water sources, but it could also indicate that other plants in the community are doing the uplift.
From what depth are the groundwater samples taken? The GW seems very variable indicating a dynamic system. Could the plant dD values that are more negative than any of your soil or GW samples be a temporal sampling issue?
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