Introduction Water use efficiency (WUE) is crucial in the fragile Horqin Sandy Land, yet how global change and ecosystem traits influence its patterns remains unclear.Methods This study focused on two ecosystems in the Horqin Sandy Land-semi-mobile dunes with Artemisia halodendron (SMAH) and a meadow wetland dominated by Phragmites australis (MPA). Using plant carbon isotope ratios and eddy covariance data from the 2022-2023 growing seasons, we revealed the distribution patterns of leaf-scale (WUEleaf) and ecosystem-scale (WUEeco) WUE and identified their environmental regulatory mechanisms.Results The results showed that the variation trend of WUEleaf in SMAH closely tracked soil moisture dynamics, whereas WUEeco lagged behind WUEleaf, demonstrating that a low T/ET (transpiration to evapotranspiration ratio) plays a key "attenuator" role. In MPA, WUEeco variation aligned with groundwater level and temperature. Overall, WUEleaf and WUEeco responded oppositely to key drivers, but were both abiotic-dominated. WUEleaf was influenced mainly by soil temperature (with an optimum of 22.8 degrees C) and soil organic carbon, while WUEeco was driven primarily by net radiation and vegetation cover.Discussion WUEeco variation arose from vegetation-environment interactions rather than vegetation type alone. Thus, WUEeco cannot be directly extrapolated from WUEleaf. These results underscore aligning WUE metric selection with research objectives in water-carbon coupling studies, highlighting the role of ecosystem structure and functional traits in regulating water-carbon dynamics.