Climate‐Dependent Hydrogen Isotopic Offset of Stem Water and Its Effect on Quantification of Plant Water Sources

Abstract Increasing evidence reports hydrogen isotopic offset (HIO) between plant stem and source soil water, which introduces considerable uncertainties in estimating plant water sources. However, it remains unknown how HIO varies along precipitation gradients and its effect on quantification of plant water sources. We sampled soil water and apple tree samples at five sites along a precipitation gradient (420–610 mm) on China's Loess Plateau to characterize HIO's climatic drivers and hydrological implication. Clear and negative stem‐soil water HIO was observed at all sites, with magnitude increasing with mean annual precipitation (MAP). The overestimation of deep soil water use caused by HIO was clearly higher in sites with MAP > 500 mm (14.42%) than those with MAP < 500 mm (0.76%), indicating overestimation of deep soil water use by stable water isotopes is more susceptible in wetter regions. This study emphasizes the necessity of incorporating climate‐related HIO for accurate regional ecohydrological assessments.