The Choice of Isotope Analyzer Can Increase the Isotopic Discrepancy Between Xylem Water Extracted Via Cryogenic Vacuum Distillation and Via Cavitron

Cryogenic vacuum distillation (CVD) is the most widely used method to extract water from plant tissues aimed at the isotope-based identification of plant water sources. Yet, it can introduce isotopic biases that affect the identification of plant water sources. Alternative methods such as the cavitron (CAV) precisely retrieve the isotopic signal of the plant’s source water by the selective extraction of water from conducting tissues. However, CAV has only been tested for a reduced number of species. We compared CAV and CVD extractions across 17 woody species, analyzing CVD extracts by both cavity ring-down spectroscopy (CRDS) and isotope-ratio mass spectrometry (IRMS). CAV yielded enough water for isotopic analysis in 13 out of 17 species and produced samples without organic compounds that closely resembled local mobile waters. In contrast, CVD-extracted water deviated from local mobile waters, and from CAV values, with a larger discrepancy for δ²H than for δ¹⁸O. The choice of the analyzer was decisive: CVD samples analyzed by CRDS showed the largest δ²H offset (mean Δδ²H = -13.93‰), whereas CVD-IRMS values were closer to CAV although still slightly depleted in δ²H (mean Δδ²H = -4.62‰). The spectral interference was the main driver of isotopic differences in CRDS-based data, whereas extraction efficiency and gravimetric water content had no effect. Overall, CAV provides reliable isotopic signatures of plant water but it is not applicable to all plant species (not even to all large woody plants), while CVD can introduce substantial bias for δ2H, especially when combined with CRDS.