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

Influenced by the phase changes of soil water and low temperature, there are significant differences in the water consumption characteristics of vegetation between the freeze-thaw and non freeze-thaw periods, while it is still unclear how seasonal freeze-thaw effects affect the water sources, utilization strategies, and hydrological effects of vegetation systems. Thus, conducting researches on the coupling ecohydrological process of the "soil-vegetation-atmosphere" system are the key issues in seasonally frozen soil regions, especially for vegetation restoration and fragile ecological protection of the sandy area. This study selected the Salix psammophila and Artemisia scoparia in the Mu Us Sandy Land as the research objects and an in-situ site for monitoring soil water and temperature in different depths, vegetation water consumption, and meteorological elements has been established. Meanwhile, the samples of rainfall, soil water, groundwater, and vegetation were collected regular during the analyzed and the isotope compositions of different samples were tested. On this basis, Bayesian mixed model and Random Forest method were used to deeply explore the distribution characteristics, variation patterns, influencing factors, and indicative significance of 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 process of soil moisture and temperature displayed a significantly seasonal trend affected by freeze-thaw processes, and the shallow 0-40 cm layer was the most active area for soil water and heat dynamic changes. The isotopic composition of soil water was closely related to precipitation and evaporation processes, and there were significant differences in isotopic fractionation characteristics during freezing and melting processes. The variation of δ¹⁸O ranged from-7.03 to 1.77‰ during the freezing period, while during the melting period, δ¹⁸O accumulated to -3.27～5.71‰. According to the distribution and variation of soil water, temperature, and isotope, the soil profile can be divided into three distinct layers, including the shallow layer (0-40 cm), the middle layer (40-90 cm), and the deep layer (90-150 cm). Soil water content and soil temperature were important environmental factors that affected soil water isotope changes, contributing 32.6% and 44% to soil water isotope changes in the shallow layer during freeze-thaw periods, respectively, while the contribution decreased with increasing depth. Besides, the isotope of rainfall is also a main environmental factor affecting the isotopic changes of soil water in the middle and deep layers during the freeze-thaw period. Controlled by the distribution of vegetation roots and the groundwater level in different sites, Salix psammophila and Artemisia scoparia exhibited different water consumption strategies. In site 1 with large groundwater depth, Salix psammophila relied on shallow and middle layer’s soil water during the non freeze-thaw periods, while the water absorbing layer showed downward trend during the freezing period, with the ratio of deep soil water up to 43.5%. Artemisia desertorum mainly utilized shallow and middle layer’s soil water, with the highest water utilization rate in shallow layer reaching 68.1%. In site 2 with shallow groundwater depth, the water consumption of both Salix psammophila and Artemisia scoparia decreased. The results mentioned above prove that the seasonal freeze-thaw process plays critical role in affecting soil water, temperature, and isotope distribution and vegetation water uptake and is a key influencing factor of the ecohydrological coupling system in arid areas. By integrating isotope modules into numerical models and exploring the hydrological cycle process of typical vegetation systems in cold and arid regions, it is expected to further deepen the theory of soil water transport from the perspectives of hydrodynamics and isotope, which could deepen the theory of water cycle in seasonally frozen areas and provide scientific support for desertification prevention and vegetation restoration of the vulnerable areas.