Reported offsets between hydrogen isotopes in source water and that of plant xylem water

Stable isotopes of water are frequently used as a means of inferring plant source water. However, a number of studies have described the potential for fractionation of water isotopes in soil (Oerter et al., 2014), fractionation of water isotopes at the time of plant root uptake (Ellsworth & Williams, 2007; Poca et al., 2019), and fractionation of plant xylem water (Barbeta et al., 2020; Chen et al., 2020). Recently, Chen et al., (2020) compared the hydrogen isotope ratio (δ²H) of plant xylem water obtained through cryogenic vacuum distillation and the δ²H of steady-state transpiration given a known source water provided to the plants. They identified a bias attributed to some combination of isotopic exchange of hydrogen in wood tissue with water in wood tissue and/or water stored in the xylem that is not participating in transpiration. They determined the offset between source water and xylem to be -8.1‰ based on observations from 9 species. Here, we present data on δ²H offsets compiled from 6 different studies and representing 30 plant species to facilitate the calculation of offsets for any number of different purposes. We also present δ¹⁸O offsets from the studies in which they were available.