Preprints
https://doi.org/10.5194/hess-2022-178
https://doi.org/10.5194/hess-2022-178
 
09 May 2022
09 May 2022
Status: this preprint is currently under review for the journal HESS.

Technical note: On uncertainties in plant water isotopic composition following extraction by cryogenic vacuum distillation

Haoyu Diao1,2,3, Philipp Schuler1, Gregory R. Goldsmith4, Rolf T. W. Siegwolf1, Matthias Saurer1, and Marco M. Lehmann1 Haoyu Diao et al.
  • 1Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf 8903, Switzerland
  • 2CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
  • 3University of Chinese Academy of Sciences, Beijing 100049, China
  • 4Schmid College of Science and Technology, Chapman University, Orange, CA 92866 USA

Abstract. Recent studies have challenged the interpretation of plant water isotopes obtained through cryogenic vacuum distillation (CVD) based on observations of large 2H-fractionations. These studies have hypothesized the existence of a H-atom exchange between water and organic tissue during CVD extraction with the magnitude of H-exchange related to relative water content of the sample; however, clear evidence is lacking. Here, we systematically tested the uncertainties in the isotopic composition of CVD-extracted water by conducting a series of incubation and rehydration experiments using isotopically depleted water, water at natural isotopic abundance, and organic materials with and without exchangeable H. We show that the offsets between hydrogen and oxygen isotope ratios and expected reference values (Δ2H and Δ18O) have inversely proportional relationships with the absolute amount of water being extracted, i.e., the lower the water amount, the higher the Δ2H and Δ18O. However, neither Δ2H, nor Δ18O, values were related to sample relative water content. The Δ2H pattern was more pronounced for materials with exchangeable H-atoms than with non-exchangeable H-atoms. This is caused by the combined effect of H-exchange during the incubation and evaporation and sublimation enrichments that depend on absolute water amount. The H-exchange during CVD extraction itself was negligible. Despite these technical issues, we observed that the water amount-dependent patterns were much less pronounced for samples at natural isotope abundance and particularly low when sufficiently high amounts of water were extracted (> 600 μl). Our study provides new insights into the mechanisms causing isotope fractionation during CVD extraction of water. The methodological uncertainties can be controlled if large samples of natural isotope abundance are used in ecohydrological research studies.

Haoyu Diao et al.

Status: open (until 04 Jul 2022)

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Short summary
We systematically investigate the uncertainties in previously observed isotopic offsets between plant source water and water extracted by cryogenic vacuum distillation. Our results show that hydrogen isotope exchange between organic material and water is a real phenomenon. However, the isotopic offsets are rather influenced by the actual amount of extracted water, sublimation, and evaporation. Our findings will help improving interpretations of ecohydrological processes in isotope-based studies.