Quantitative Contribution of Cryogenic Vacuum Extraction and Radial Water Transport to Xylem-Source Water Deuterium Offsets

The positions and magnitudes of deuterium offsets between bulk xylem and corresponding source waters are under debate and quantifying them is essential for isotope-based ecohydrological investigations. In this study, stable isotopes (delta H-2, delta O-18, and delta C-13), iteration method, and rehydration experiments were combined to quantitatively determine the magnitude of cryogenic vacuum extraction (CVE)- and radial water transport (RWT)-induced deuterium offsets using one riparian tree species Salix babylonica L. A modified potential water source line (MPWL) was proposed to identify the total delta H-2 offsets between bulk xylem and source waters. The relationships between delta H-2 offsets induced by CVE or RWT and plant water content, leaf delta C-13 values, soil water content (SWC), and the depth to the water table (WTD) were investigated. Results showed that the bulk xylem waters in different tissue positions of S. babylonica showed -7.0 parts per thousand to -4.0 parts per thousand deuterium depletion relative to MPWL at four different sites (p < 0.01). The isotopic compositions of sap water coincided well with MPWL on the dual-isotope plot at the four sites. The CVE- and RWT-induced delta H-2 offsets accounted for 75.1 % and 24.9 % of the total delta H-2 offsets, respectively. The CVE-induced delta H-2 offsets were significantly negatively correlated with plant water content. In comparison, the RWT-induced delta H-2 offsets were negatively related to plant leaf delta C-13 values, trunk water content, and SWC, but positively correlated with WTD. This study provides a quantitative contribution of two major sources of deuterium offsets. The results provide critical insights into isotope-based plant water source identification and evapotranspiration partitioning.