Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

Riparian trees located in seasonally dry environments may be reliant on groundwater supplies, but the prevalence and magnitude of groundwater uptake is often unclear. Using soil water matric potential and water stable isotopes, we examined the relative contributions of soil water and groundwater to the dry season water uptake of five riparian tree species along an intermittent river of tropical northern Australia. Because xylem water was depleted in deuterium relative to source water (average offset -14.0 parts per thousand), we numerically removed this offset and assessed the effect of the correction on mixing model results. We also estimated the isotopic composition of unbound soil water (i.e., the portion of soil water not tightly bound to soil particles) from bulk soil water data by using an empirical formulation from the literature and tested whether considering unbound soil water as a source would affect our results. Despite the hot and dry surface environment, we found that soil moisture was available for trees at relatively shallow (similar to 0.7-1.5 m) depths. When unbound soil water and corrected xylem water data were considered, most tree species used a combination of this soil moisture source and groundwater from the capillary fringe. However, not correcting for isotopic effects resulted in large underestimations of the groundwater contributions to tree water uptake. Our findings suggest that ignoring soil isotopic effects and deuterium depletion in xylem water may reduce the validity of source water partitioning assessments. Further research is needed on the likely causes for deuterium depletion in xylem water.