Isotopic deviations of water extracted from carbonate soil by cryogenic vacuum extraction: implication for root water uptake analysis.

Purpose: Isotope-based root water uptake analysis usually involves cryogenic vacuum extraction (CVE) of water from soil and other water-holding materials, according to the assumption that no isotopic fractionation occurs during the related processes. Recently, a growing body of literature identified isotopic deviations that were caused by CVE. However, the isotopic deviations of water extracted from carbonate soils by CVE are undefined, which prohibits dependable utilization of stable water isotopes in CVE involved studies, especially in karst regions. Methods: Three types of water-holding material, weathered carbonate bedrock, typical karst soils that were kept intact or washed with diluted HCL thus with varying levels of carbonate content were oven-dried and spiked with various amounts of reference water. The recovery of known isotope values was tested in all rewetted samples by using one CVE system. Results: No significant differences were detected between the isotope values of the water extracted from the weathered carbonate bedrock and the input water. However, isotope values of water extracted from the soils were significantly more negative than the input water. Moreover, the magnitude of δD deviation was highly related to soil water content, while the magnitude of δ18O values was strongly dependent on the amount of carbonate present. Conclusions: Considering the isotopic deviations caused by CVE and the obstacles associated with the calibration process, the application of an isotope-based method that involves CVE needs to be adapted to the specific properties of karst regions. [ABSTRACT FROM AUTHOR]