The natural abundance of stable water isotopes method may overestimate deep-layer soil water use by trees

[摘要] Stable water isotopes have been used extensively to studythe water use strategy of plants in various ecosystems. In deep vadose zone(DVZ) regions, the rooting depth of trees can reach several meters to tensof meters. However, the existence of roots in deep soils does notnecessarily mean the occurrence of root water uptake, which usually occursat a particular time during the growing season. Therefore, quantifying thecontribution of deep-layer soil water (DLSW) in DVZ regions using thenatural abundance of stable water isotopes may not be accurate because thismethod assumes that trees always extract shallow- and deep-layer soil water.We propose a multi-step method for addressing this issue. First, isotopiclabeling in deep layers identifies whether trees absorb DLSW and determinesthe soil layer depths from which trees derive their water source. Next, wecalculate water sources based on the natural abundance of stable isotopes in the soil layer determined above to quantify the water use strategy of trees. We also compared the resultswith the natural abundance of stable water isotopes method. The 11- and17-year-old apple trees were taken as examples for analyses on China's Loess Plateau. Isotopic labeling showed that the water uptake depth of 11-year-old apple trees reached 300 cm in the blossom and young fruit (BYF) stage and only 100 cm in the fruit swelling (FSW) stage, whereas 17-year-old trees always consumed water from the 0–320 cm soil layer. Overall, apple trees absorbed the most water from deep soils ( > 140  cm) during the BYF stage, and 17-year-old trees consumed more water in these layers than 11-year-old trees throughout the growing season. In addition, the natural abundance of stable water isotopes method overestimated the contribution of DLSW, especially in the 320–500 cm soil layer. Our findings highlight that determining the occurrence of root water uptake in deep soils helps to quantify the water use strategy of trees in DVZ regions.

[学科分类] 妇产科学