Isotopic Apportionment of Water Sources for Representative Vegetation in the Soil-Covered Pisha Sandstone Area on the Northern Margin of the Ordos Plateau

In the context of artificial vegetation restoration in arid and semi-arid regions, competition for shallow soil water between shrubs and herbaceous species often hampers restoration efficacy. In this study, we investigated the water acquisition strategies and interspecific interactions of a typical shrub–herb community comprising Caragana korshinskii and Stipa capillata in a loess hilly region under relatively dry soil conditions, using stable hydrogen and oxygen isotopes combined with Bayesian mixing models. The results revealed that the 20–70 cm soil layer, characterized by a high sand content and sustained root water extraction, exhibited low soil moisture, which drove Stipa capillata into a passive defensive state during water shortage. In contrast, Caragana korshinskii exhibited considerable resource-acquisition plasticity, with deep soil water (>70 cm) accounting for 40.6–50.3% of its total water uptake. This deep-water exploitation sustained its survival and reduced direct hydrological competition with shallow-rooted species in the surface layers. Collectively, our findings elucidate the water-use dynamics of shrubs and herbaceous species, providing scientific support for vegetation restoration and water resource management under specific geological contexts.