Cryogenic vacuum artifacts do not affect plant water-uptake studies using stable isotope analysis
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cryogenic distillation; ecohydrology; equilibrium isotope exchange; hydrogen isotopes; oxygen isotopes; plant water sourcing; soil water; xylem water
Water movement through the soil-plant-atmosphere continuum can be tracked through its hydrogen and oxygen isotopic composition. How the isotopic composition of water evolves as it moves through this continuum provides critical information on ecosystem hydrology and climate change. Cryogenic vacuum extraction is the most applied method to extract waters from soil and plant material for such studies. However, recent experiments where oven dried soils were spiked with a reference water suggest potential for these techniques to bias results. We present results from a greenhouse-based plant water uptake and cryogenic vacuum distillation experiment using Salix viminalis cuttings. We tested the capacity for cryogenic vacuum extraction to recover irrigation water and the plant-available water pool from 4 soils that were subject to continuous irrigation as would occur in nature. Results show that H-2 and O-18 values of extracted soil waters reflect those of irrigation water, but with some influence of evaporation that varied with differences in soil humic and clay content. This suggests that isotope effects observed in laboratory experiments with oven-dried soils may not be relevant under natural conditions. Cryogenic vacuum extraction also recovered xylem water H-2 values that matched the H-2 values of extracted soil water but revealed small, consistent offsets between xylem and soil water O-18 values of 0.84 +/- 1.13 parts per thousand. Although these effects may not significantly bias water isotope sourcing applications, they do result in large extrapolation errors when estimating original source water isotope composition from extracted xylem waters using the back extrapolation to the global meteoric water line method.