δ2H isotopic offsets in xylem water measurements under cryogenic vacuum distillation: Quantifying and correcting wood-water hydrogen exchange influences

Abstract

Stable isotopes δ¹⁸O and δ²H are used to infer vegetation water sources. In some studies, significant xylem water δ²H offsets from potential source waters have been observed. The offsets appear to be more prevalent with cryogenic vacuum distillation (CVD) of plant water. Hypothesized mechanisms for these offsets include changes during plant water uptake and transport, and methodological problems. We propose that a large portion of the offsets are due to hydrogen isotope exchange between xylem water and non-crystalline hydroxyl groups of wood cellulose and hemicellulose during CVD. We present a method for estimating the hypothesized isotopic exchange between wood tissues and water, which is the result of Rayleigh and equilibrium fractionation. To estimate the exchange, we use published wood properties for North American tree species and isotope chemical relationships as a function of moisture content, CVD temperature and water extraction efficiency. A simple model of exchange between xylem water and hydroxyl groups captures the range of observations in studies in which CVD and non-CVD methods were compared. To evaluate the model, we compared observed δ²H offsets (sw-excess) values from two field datasets (90°C, n = 364, and 170°C, n = 43) to δ²H offsets estimated with our chemical model. We found good agreement between observed and estimated δ²H offsets for samples extracted at 90°C (r² = 0.69) but not for samples extracted at 170°C (r² = 0.20). The offset may be eliminated by increasing the extraction temperature to 229°C or by adding a standard sufficient to raise the moisture content to >150%. A correction can also be approximated by applying a theoretical calculation based on the extraction temperature, moisture content and water extraction efficiency.