Regulation of Water-Use Efficiency by Ecosystem-Type-Driven Soil–Plant–Water–Salt Coupling in Cold Arid Regions

Understanding how ecohydrological processes shape soil–plant–water interactions across various ecosystem types is vital for elucidating vegetation water-use strategies in cold, arid regions. This study focused on mountain ecosystems along the northeastern margin of the Qinghai–Tibet Plateau. By comprehensively collecting data on soil profiles, soil moisture and salinity characteristics, and plant leaf functional traits, we systematically evaluated the mechanisms by which these factors influence plant water-use efficiency (WUE). Our findings revealed that soil carbon, nitrogen, phosphorus, and moisture were markedly enriched in the surface layer (0–20 cm), with spatial heterogeneity largely controlled by ecosystem type. Patterns of water–salt interactions followed two distinct regimes: either moisture deficit accompanied by salt accumulation or simultaneous supplementation of water and salt, depending strongly on the hydrological and evapotranspiration context. Moreover, under water-limited conditions, WUE was mainly driven by photosynthetic and water-related traits, whereas in environments with ample water and nutrients, nutrient availability and metabolic traits played a dominant role. These results suggest that ecosystem-specific resource environments shape distinct adaptive strategies for plant water use. This study reveals the mechanism by which ecosystem types regulate water-use efficiency (WUE) through the coordinated effects of soil moisture patterns and plant functional traits. It provides a theoretical basis for understanding vegetation water adaptation strategies and guiding resource management in cold alpine mountain ecosystems.