Soil is the most critical source of water for plant growth. Quantitatively assessing the contribution of water from different soil layers to plants has been a long-standing challenge in the field of ecohydrology. Stable isotope analysis, which tracks plant water sources by measuring the ratios of hydrogen (^2H) and oxygen (^18O) isotopes in soil water, has been a primary method for addressing this issue. However, the hydrogen and oxygen stable isotope compositions of soil water obtained via the commonly used cryogenic vacuum distillation (CVD) method may not accurately reflect the actual water sources utilized by plants.
Dr. Xu Xiang from the School of Earth and Environmental Sciences, Lanzhou University, in collaboration with Associate Professor Grzegorz Skrzypek from the University of Western Australia, selected Picea crassifolia (Qinghai spruce) and Sabina przewalskii (Qilian juniper) as representative tree species in the Tianlaochi Basin on the northern slopes of the Qilian Mountains to investigate the spatial distribution of soil water isotopes. The study analyzed the relative contributions of water from different soil layers to vegetation water use. Unlike the single use of the cryogenic vacuum distillation method, this research compared the isotopic compositions of soil water extracted by three different methods: cryogenic vacuum distillation (CVD), centrifugation (CEN), and suction (SUC).
The results showed that the traditional CVD method overestimates or underestimates the actual water sources absorbed by plants. The stable isotope values of soil water extracted by the CVD method were significantly more negative, showing substantial isotopic differences compared to the other two methods. A significant isotopic disequilibrium phenomenon was observed among the soil water reservoirs corresponding to different water potentials when extracted using the CVD, CEN, and SUC methods. Bayesian mixing model MixSIAR calculations revealed that the results based on the CVD method failed to pass statistical diagnostics and were excluded. Results based on the centrifugation method indicated that neither Qinghai spruce nor Qilian juniper exhibited preferences for water from specific soil depths during the growing season.
This study is the first to reveal, under field conditions, the significant bias in plant water source estimation caused by isotopic disequilibrium in soil water. It presents new challenges for research on plant water sources, highlighting that relying solely on the CVD method could lead to serious misestimations of actual plant water sources. By combining the centrifugation and CVD methods, the study offers a more accurate approach to assessing the depth and source of plant water use. This advancement helps to better understand plant-water interactions, providing critical insights for watershed water resource management and early warning systems.
The research, titled“Stable isotope disequilibrium between soil bound water and soil bulk water – implications for estimations of plant water sources”, was published online on December 20, 2024, in the internationally renowned geoscience journalJournal of Hydrology(Category 1 in geosciences, IF=5.90). This work was supported by the National Natural Science Foundation of China (Grant Nos. 41807148 and 42071048), the Open Fund of the Key Laboratory of Western China's Environmental Systems, Ministry of Education, Lanzhou University, and the Fundamental Research Funds for the Central Universities, Lanzhou University (Grant No. lzujbky-2019-kb01).
Figure 1.Study site: Picea crassifolia slope and Sabina przewalskii slope in the Tianlaochi Basin, located in the upper reaches of the Heihe River in northwest China. Dual-isotope distribution diagrams and boxplots of soil water and branch water show the quartile distributions of δ^18O in soil water extracted using cryogenic vacuum distillation (CVD), centrifugation (CEN), and suction (SUC), along with branch water.
Link to the paper: https://authors.elsevier.com/a/1kIM652cufB-9
