Artefacts Associated With Cryogenic Vacuum Distillation Do Not Compromise Plant Source Water Extraction From Soil and Xylem for Isotope Analysis

Understanding the oxygen and hydrogen isotopic composition (δ 18 O and δ 2 H values, respectively) of plant source water is fundamental for tracing water uptake in ecohydrological studies. However, discrepancies between cryogenic vacuum distillation (CVD)‐extracted soil and xylem water and known plant source water δ 18 O and δ 2 H values continue to raise concerns about the accuracy of this approach, especially for δ 2 H. We conducted two irrigation experiments on juvenile temperate trees to evaluate the accuracy by which CVD soil and xylem water characterize the δ 18 O and δ 2 H values of plant source water. To do this, we compared the δ 18 O and δ 2 H of irrigation and mildly vacuum‐extracted (MVE) sap water, which has previously been shown to characterize plant source water δ 18 O and δ 2 H values with high accuracy, with those of CVD‐extracted water from soil, xylem and phloem δ 18 O and δ 2 H values. In both experiments, irrigation water and MVE sap δ 18 O and δ 2 H values were similar to each other. We yet observed a small and consistent offset between δ 18 O and δ 2 H values of irrigation water and CVD‐extracted soil water δ 18 O and δ 2 H values. These offsets indicate that CVD soil water captures a mixture of two isotopically distinct soil water pools: a mobile, plant‐available pool and an immobile, plant‐unavailable pool. Primarily δ 2 H values of CVD‐extracted xylem water differed from both CVD‐extracted water from MVE sap and phloem, while δ 18 O differences were minimal. This suggests that artefacts associated with CVD, rather than stem evaporation or phloem–xylem exchange, were the primary source of the observed δ 2 H offsets. In summary, our findings demonstrate that CVD is a valid extraction method for accurate plant source water δ 18 O values while δ 2 H values are associated with artefacts that require correction.