Study on water use sources and ecohydrological niche characteristics of two dominant shrubs in the Tengger Desert

In arid regions with extremely limited water resources, understanding water-use strategies of dominant shrubs is critical for ecohydrology and vegetation restoration. This study investigated the water sources of Nitraria tangutorum and Zygophyllum xanthoxylum in the eastern Tengger Desert, using soil water monitoring and hydrogen-oxygen stable isotopes(δ²H, δ¹⁸O). Levins' index and the Proportional Similarity (PS) index were used to quantify niche breadth and similarity in water-source use. Soil water content in N. tangutorum sites was generally higher than that in Z. xanthoxylum sites, particularly in the deep layers. The slopes of the local meteoric water line and soil water line were lower than the global meteoric water line, indicating evaporative enrichment in precipitation and soil water. N. tangutorum xylem water aligned more closely with shallow to middle layer soil water, whereas Z. xanthoxylum corresponded more closely to middle to deep soil water, demonstrating vertical differentiation in water uptake. MixSIAR results revealed that N. tangutorum primarily utilized water from the 40-200 cm soil layer in the early growing season, shifted to shallow soil water (0-40 cm) during the peak precipitation period, and subsequently shifted back to middle and deep water sources. In contrast, Z. xanthoxylum consistently relied on water from the 120-200 cm soil layer throughout the growing season, with a minor groundwater contribution. Both shrubs exhibited relatively broad ecohydrological niche breadths across the growing season. However, during the peak precipitation period in July and August, their niche breadths contracted, and the similarity index of soil water and groundwater utilization decreased. Overall, the two shrubs exhibited contrasting water-source use patterns: N. tangutorum showed greater seasonal flexibility, shifting toward shallow soil water during July-August, whereas Z. xanthoxylum maintained a relatively stable reliance on deeper soil water. Because sampling was conducted in species-specific stands rather than mixed-species plots, niche metrics are interpreted as comparative indicators of water-source use patterns under shared regional conditions. These findings provide a basis for species selection and water management for desertification control and vegetation restoration.