Characteristics of soil water isotope and analysis of vegetation water consumption in sandy soil of the seasonally frozen region

Vegetation water consumption can be influenced by the phase changes of the soil water and low temperature, particularly in the seasonally frozen region. There are also significant differences between the freeze-thaw and non-freeze-thaw periods. While it is still unclear whether the effects of the seasonal freeze-thaw on the water sources, utilization strategies, and hydrological responses of the vegetation. Thus, the coupling of the ecohydrological process of the "soil-vegetation-atmosphere" system can be expected for the seasonally frozen soil regions, especially for the vegetation restoration and fragile ecological protection of the sandy area. In this study, an in-situ site was established to monitor the soil water and temperature in the different depths, vegetation water consumption, and meteorological elements. The research objects were selected as the Salix psammophila and Artemisia scoparia in the Mu Us Sandy Land. Meanwhile, the samples were collected from the rainfall, soil water, groundwater, and vegetation during regular monitoring. The isotope compositions of the different samples were tested for measurement. Bayesian mixed model and Random Forest were used to explore the distribution, variation patterns, influencing factors, and the significance of the soil water, temperature, and isotope under different vegetation systems during both freeze-thaw and non-freeze-thaw periods. Results showed that the distribution and transformation of the soil moisture and temperature displayed a significantly seasonal trend under freeze-thaw processes. The shallow 0-40 cm layer was the most active area for the soil water and heat dynamics. The isotopic composition of the soil water was closely related to the precipitation and evaporation. There were significant differences in the isotopic fractionation during freezing and melting. The variation of δ¹⁸O ranged from-7.03 to 1.77‰ during the freezing period, while during the melting period, δ¹⁸O was accumulated to -3.27~5.71‰. Meanwhile, the soil profile was divided into three layers, including the shallow layer (0-40 cm), the middle layer (40-90 cm), and the deep layer (90-150 cm), according to the distribution and variation of the soil water, temperature, and isotope. Soil water content and soil temperature were the two most important environmental influencing factors on the soil water isotope, thereby contributing 32.6% and 44% to the soil water isotope in the shallow layer during freeze-thaw periods, respectively, while the contribution decreased with increasing depth. Besides, the isotope of the rainfall was also one environmental influencing factor on the isotopic changes of the soil water in the middle and deep layers during the freeze-thaw period. The distribution of the vegetation roots and the groundwater level was dominated in the different sites. Among them, the Salix psammophila and Artemisia scoparia also exhibited different strategies of water consumption. In site 1 with the large groundwater depth, the Salix psammophila relied mainly on the soil water in the shallow and middle layer during the non-freeze-thaw periods, while the water absorbing layer shared the downward trend during the freezing period, with the ratio of deep soil water up to 43.5%. Meanwhile, the Artemisia desertorum was utilized in the soil water of the shallow and middle layers, with the highest water utilization rate in the shallow layer reaching 68.1%. There was a decrease in the water consumption of both Salix psammophila and Artemisia scoparia in the site 2 with the shallow groundwater depth. The seasonal freeze-thaw process can play a critical role in the soil water, temperature, isotope distribution, and vegetation water uptake, thus serving as a key influencing factor on the ecohydrological coupling in the arid areas. Isotope modules can be expected to integrate into the numerical models in order to explore the hydrological cycle of the typical vegetation in cold and arid regions. The soil water transport can be further predicted in seasonally frozen areas, from the perspectives of the hydrodynamics, isotope, and water cycle. The finding can also provide scientific support to the desertification prevention and vegetation restoration of the vulnerable areas.