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 desertorum 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 desertorum 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 desertorum 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.

Vapor-water exchange among atmosphere, soil and groundwater plays an important role in the surface ecological restoration in the vadose zones of arid and semiarid regions. However, it is limited to in-situ monitoring of water vapor flux, due to the complex and uncertain process. The simulation of water vapor cannot be fully verified by the measurement. It is still lacking in the vapor-water migration at different spatial and temporal scales. This study aims to clarify the spatiotemporal variation and migration of vapor-water exchange in the vadose zone of Mu Us Sandy Land by in-situ vapor-water monitoring, isotope tracer and numerical analysis. The results showed that the δD_L and δ¹⁸O_L of precipitation were enriched in spring and summer, but depleted in autumn and winter, due to the atmospheric water vapor source and local circulation. Soil evaporation was enriched in the oxygen isotope (δ¹⁸O_a) of surface soil vapor-water in summer more than that of soil freezing in winter. Moreover, there was a significant positive correlation (P<0.01) in the δ¹⁸O profile of the soil vapor-water and liquid water at different depths. The reason was that the evaporation intensity source and migration mode of soil vapor-water varied greatly in seasons. The δ¹⁸O values of vapor water in spring and summer showed an extremely significant linear positive correlation with the liquid water (P<0.01), while there was no significant correlation in winter (P=0.12). Moreover, there was a positive linear relationship between water vapor flux and δ¹⁸O_a. Furthermore, the δ¹⁸O_a was enriched in the surface layer at the large water vapor flux, including both downward and upward migration. The surface δ¹⁸O_a was enriched in the period of soil freezing in winter, particularly with the decrease of water vapor density (November-March). But the surface layer δ¹⁸O_a increased in summer (May-October) as well with the increase of surface water vapor density. Driven by the gradient of soil temperature in the vadose zone, the replenishment relationship of water vapor throughout the profile varied greatly at different periods. The water vapor of shallow soil was the supply source of deep vapor-water in summer, while the recharge was received from the deep layer in winter. But there were the temperature convergence and divergence zero flux planes in spring and autumn, respectively. The recharge characteristics caused the complicated relationship of vapor recharge in the profile. The soil δ¹⁸O_a was controlled by the water vapor migration, atmospheric evaporation and soil freeze-thaw in the vadose zone. In winter, the reduced evaporation and upward transport of water vapor can concurrently cause to enrich the surface δ¹⁸O_a, thus resulting in a decrease in the correlation between soil water vapor δ¹⁸O_a and liquid water δ¹⁸O_L. While in summer, the dominant effect can be from the diurnal evaporation and condensation cycle of soil water. The findings can provide a scientific basis to clarify the migration of soil water vapor in the water circulation