Dept. of Environmental Systems Science, ETH Zürich, Zürich, Switzerland
Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
Abstract. Recent studies have demonstrated that plant and soilwater extraction techniques can introduce biases and uncertainties in stable isotope analyses. Here we show how recently documented δ2H biases resulting from cryogenic vacuum distillation of water from xylem tissues may have influenced the conclusions of five previous studies, including ours, that have used δ2H to infer plant water sources. Cryogenic extraction biases that reduce xylem water δ2H will also introduce an artifactual evaporation signal in dual-isotope (δ2H vs. δ18O) analyses. Calculations that estimate the composition of the source precipitation of xylem waters by compensating for their apparent evaporation will amplify the bias in δ2H, and also introduce new biases in the δ18O of the inferred pre-evaporation source precipitation. Cryogenic extraction biases may substantially alter plant water source attributions if the spread in δ2H among the potential end members is relatively narrow. By contrast, if the spread in δ2H among the potential end members is relatively wide, the impact of cryogenic extraction biases will be less pronounced, and thus suggestions that these biases universally invalidate inferences drawn from plant water δ2H are unwarranted. Nonetheless, until reliable correction factors for cryogenic extraction biases become available, their potential impact should be considered in studies using xylem water isotopes.
How to cite. Allen, S. T. and Kirchner, J. W.: Potential effects of cryogenic extraction biases on inferences drawn
from xylem water deuterium isotope ratios: case studies using stable
isotopes to infer plant water sources, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2020-683, 2021.
Received: 10 Jan 2021 – Discussion started: 19 Jan 2021
Extracting water from plant stems can introduce analytical errors in isotope analyses. We demonstrate that sensitivities to suspected errors can be evaluated and that conclusions drawn from extracted plant water isotope ratios are neither generally valid nor generally invalid. Ultimately, imperfect measurements of plant and soil water isotope ratios can continue to support useful inferences if study designs are appropriately matched to their likely biases and uncertainties.
Extracting water from plant stems can introduce analytical errors in isotope analyses. We...
The manuscript underlines existing issues with cryogenic vacuum extraction of plant water for isotope analysis. The authors demonstrate via five case studies how δ2H biases resulting from this extraction method may lead to misleading conclusions when interpreting plant water sources.
The authors heavily rely on results of a study by Chen et al. (2020) without going much into detail about this study. This is often tricky for the reader (since details are not mentioned) and it requires going back and reading the study by Chen et al.
The authors apply a δ2H bias correction to a handful of plant source water studies. But what are the selection criteria of these case studies? The manuscript does not go far beyond the discussion section of the Chen et al. (2020) study. The application of an average bias correction factor determined by Chen et al. to a limited selection of other studies is not enough. Are the tree species from the selected case studies even covered in the study by Chen et al (2020)? Otherwise, I highly question the overall applicability of an average bias correction factor to different tree species (from your case studies) that may react differently during cryogenic vacuum extraction. This would result in the necessity to calculate different bias correction factor for specific plant species. Chen et al. (2020) stress the fact that species-specific differences in the cryogenically obtained δ2H values exist and need to be accounted for (together with data on stem relative water content).
Overall, the manuscript is a bit light. It would benefit from 1) case studies that include the tested species of the Chen et al. study 2) selecting more than five case studies to underline the statements and conclusions of the present study and 3) from including recommendations on how to perform bias correction on your own data and which factors to consider for that (species-specific differences, extraction system bias correction, isotope measurement accuracy etc.). The authors further ignore the well-known fact that cryogenic vacuum extraction from certain soil types causes isotope effects that need to be corrected for (such effects may also differ for the selected case studies). I admit that this is difficult when the raw data of the selected case studies is not available but this could be overcome by choosing newer studies where authors might be willing to share their data or where data is already publically available.
Further, it would be interesting to know if a bias correction is necessary and how a potential bias correction would look like for artificial isotope labeling studies. How would results be shifted?
My specific comments can be found in the attached pdf.
Chen, Y., Helliker, B. R., Tang, X., Li, F., Zhou, Y. and Song, X.: Stem water cryogenic extraction biases estimation in deuterium isotope composition of plant source water, PNAS, 117(52), 33345–33350, https://doi.org/10.1073/pnas.2014422117, 2020.