Hydrochemical Characteristics of Shallow Groundwater and Analysis of Vegetation Water Sources in the Ulan Buh Desert

The Ulan Buh Desert represents a quintessential desert ecosystem in the arid northwest of China. As the key factor to maintain the stability of ecosystem, the chemical characteristics of groundwater and its water relationship with vegetation need to be further studied. Through field sampling, hydrochemical analysis, hydrogen and oxygen isotope testing and the Bayesian mixing model (MixSIAR), this study systematically analyzed the chemical characteristics of groundwater, spatial distribution and vegetation water sources in the study area. The results show that the groundwater is predominantly of the Cl−–SO42− type, with total dissolved solids (TDS) ranging from 0.34 to 9.56 g/L (mean: 2.03 g/L), indicating medium to high salinity and significant spatial heterogeneity. These characteristics are jointly controlled by rock weathering, evaporative concentration, and ion exchange. Soil water isotopes exhibited vertical differentiation: the surface layer (0–20 cm) was significantly affected by evaporative fractionation (δD: −72‰ to −45‰; δ18O: −9.3‰ to −6.2‰), while deep soil water (60–80 cm) showed isotopic enrichment (δD: −29‰ to −58‰; δ18O: −6.8‰ to 0.9‰), closely matching groundwater isotopic signatures. Vegetation water use strategies demonstrated depth stratification: shallow-rooted plants such as Reaumuria soongorica and Kalidium foliatum relied primarily on shallow soil water (0–20 cm, >30% contribution), whereas deep-rooted plants such as Nitraria tangutorum and Ammopiptanthus mongolicus predominantly extracted water from the 40–80 cm soil layer (>30% contribution), with no direct dependence on groundwater.