| 26 Jun 2026
Technical note: Evaluation of a new cryogenic airtight vapor extraction (CRAVE) method for soil and plant water
Abstract. Accurate extraction of soil and plant water for stable isotope analysis remains a methodological challenge in ecohydrology, particularly due to isotopic biases introduced by heating or selective pore-water extraction in conventional techniques. This study developed and evaluated a cryogenic airtight vapor extraction (CRAVE) method from soil and vegetation samples at ambient temperature within a recirculating vapor-liquid pathway. This approach avoids heating-induced non-equilibrium effects and reduces matrix-dependent artifacts and organic contamination, thereby facilitating direct comparison of isotopic compositions between soil and plant water. The results demonstrate that CRAVE-derived isotopic signatures align with both cryogenic vacuum distillation (CVD) and suction lysimeter (SL) benchmarks. However, systematic deviations were observed based on specific matrix properties. For xylem water, the d2H offset between CRAVE and CVD was strongly modulated by gravimetric water content (dry-weight basis), with CVD exhibiting greater hydrogen isotope depletion under low-moisture conditions (< 0.8 g·g-1). For soils, the isotopic divergence between CRAVE and CVD was driven primarily by soil texture, with offsets increasing as clay content and depth increased (r = 0.82–0.94), where CVD-extracted bulk water became depleted progressively in both δ2H and δ18O relative to the mobile-capillary pool captured by SL and CRAVE. The Rayleigh-based framework provides a physically grounded means to reconstruct source-water isotope values from condensate measurements; its potential use for mobile–immobile partitioning should, however, be treated as a future application pending targeted validation. Overall, CRAVE represents a promising ambient-temperature extraction method for tracing water partitioning and source-uptake dynamics within the soil–plant–atmosphere continuum.
Water stable isotope biogeochemistry and, by extension, its domain of applications (e.g. isotope ecohydrology) is limited by how water is collected from mineral media and living tissues. Aside from technical issues potentially leading to incomplete recovery, questions recurrently arise about the representativeness of the extracted water as a pool or source mobilized by some process (e.g. plant root water uptake). In recent years, new methods and novel declinations of well-established ones have been published, aiming at simplifying procedures or at targeting water sources to address the aforementioned issues.
Xiuqiang Liu & colleagues’ method, namely the cryogenic airtight vapor extraction (CRAVE), is another relevant and potentially useful addition; it should, for instance, remove potential effects associated with the heating of the samples, their texture, and water availability.
I found the manuscript to be well-written, although would require streamlining in some sections (e.g in the introduction) and editing (isotope and physics terminology) – please see my specific comments below.
I have one general issue with the use of the Rayleigh distillation framework in the specific context of this study and urge the authors to address it. I simply do not think Eq. (1) applies here. Rayleigh distillation only applies during condensation of a water vapor reservoir into liquid water – with or without the removal of the condensate from the vapor-liquid system during “rain-out”. The authors’ case is rather different, they have instead:
The authors should also provide a comparison between the isotopic composition values of the water collected in the “condensation” U-tube and the values after correction with Eq. (3). This is missing for now and would help the readers to assess the magnitude of these corrections.
My specific comments follow below:
Title
I encourage the authors note to use the adjective “new”. The presented method will not indefinitely be “new” but the title will remain as is…
Abstract
Please harmonize and simplify the isotopic terminology (“composition” vs “signature”, “δ” instead of “d”).
L25. “greater hydrogen isotope depletion” -> “lower isotope composition”.
L26. “g water (g of dry soil)-1”. You may delete mention of “dry-weight basis”
L27. “depth” is not explained. Why would depth be a factor here? Information about the experimental design is missing.
L27. “where CVD-extracted bulk water δ2H and δ18O progressively decreased…”
L28. “The Rayleigh-based framework” is not evident and should be shortly explained before.
L31. This method does not qualify to “ambient-temperature extraction” as it operates with liquid Nitrogen, but the reader might think otherwise from reading the abstract alone.
Introduction
L40. Consider citing Ceperley et al.
Ceperley, N., Gimeno, T. E., Jacobs, S. R., Beyer, M., Dubbert, M., Fischer, B., Geris, J., Holko, L., Kuebert, A., Le Gall, S., Lehmann, M. M., Llorens, P., Millar, C., Penna, D., Prieto, I., Radolinski, J., Scandellari, F., Stockinger, M., Stumpp, C., Tetzlaff, D., van Meerveld, I., Werner, C., Yildiz, O., Zuecco, G., Barbeta, A., Orlowski, N., and Rothfuss, Y.: Toward a common methodological framework for the sampling, extraction, and isotopic analysis of water in the Critical Zone to study vegetation water use. Wiley Interdisciplinary Reviews-Water, 11(4), https://www.doi.org/10.1002/wat2.1727, 2024.
L47. I personally question any study extracting water at temperatures that high! This is contrary to standard recommendations (see study of Ceperley et al.). You want to achieve inverse-sublimation of water, not pyrolysis!
L55. Suction cups are not an alternative. On the one hand you have a destructive method and on the other hand an invasive one. The true alternatives are presented in the next paragraph and table 1 (e.g. centrifugation, direct soil vapor equilibration…).
L59. Please cite properly, either by year or first author name – please check journal guidelines.
L60. “These approaches offer high throughput and, in some cases, in situ monitoring capability when coupled with LS.” That cannot be the case, in situ observation are made where the experiments are conducted.
L63. You cannot compare in situ high frequency approaches with in labo low frequency ones: the latter measure repeatedly from the same sample, the latter includes spatial heterogeneities in isotopic composition.
Table 1 should be streamlined, e.g. do not write sentences in full, rather use bullet points. Waasenaar et al. DVE method is not to be pooled together with the rest…The rest of the “equilibration methods”, the in situ one, are not 1:1 comparable to the rest – please see my comment above – and should be treaded differently. I would actually not even bother to cite them here and in the intro at all.
L74-104. Text is not well structured despite the attempt to list the features of the presented method. Please streamline this and hierarchize the information in order of importance.
L75, L82. “sublimating”, not “condensing”
L74-79. Not clear at all what “quantified” and “measured” mean here.
L78. “However” is not correct. Also the three items (i)-(iii) do not add structuration and do not relate as is to the text preceding them. Please streamline this.
L83. A desiccant cannot trap VOCs. A solid phase extractor may do the job well, depending on the nature of the VOC (e.g. methanol, ethanol).
L85. Again, you are comparing apples with oranges and the statement is rather unfair to the in situ methods: condensation is not inherent to the in situ family of methods after the collection point and can be largely avoided. Condensation is, on the other hand, a major problem in water vapor continuous analysis.
L93. Which method uses “empirical post hoc corrections”? Also CRAVE uses a rayleigh distillation framework, for which α is determined empirically and a posteriori. So I don’t think the statement is valid.
L95. “QA/QC” is not defined.
L101. “leakage pathways”? “leakage” should be enough.
L102. Because of the somewhat chaotic nature of the text L74-104, I don’t know what “finally” exactly refers to. What came first, then second etc. before “finally”?
L110. Water does not condensate in liquid N, it inversely sublimates (sometimes referred as to “deposition”). This is an important terminology point.
L114. This is a “deposition U-tube”, not a “condensation U-tube”.
L120ff. “[deposition] vials (Gcv) [are] pre-weighed”. This is a general statement – did not happen once in the past. Same for “soil and plant materials [are] placed immediately” etc.
L129. What is meant by “pressure equilibrium”? Do you want to say “to avoid under/overpressure in the closed loop system due to changes in temperature overtime”? Either way, please add details in the text.
L131. “Cryogenic condensation” is “inverse sublimation”.
L132. How was the protocol “optimized”? Please add details.
L150. Water pools of what? Please explain / detail.
L160. I have a problem with using this equation in this particular context. Rayleigh distillation only applies during condensation of a water vapor reservoir into liquid water – with or without the removal of the condensate from the vapor-liquid system. Your case is rather different, you have:
Therefore, “reconstruction” of the “isotopic compositions of soil and plant water derived from the condensed vapor” is not granted with the Rayleigh distillation framework.
On a side note, all parameters and variables in Eq. (1) (α, δ, f) are unitless, therefore the factor “1000” is unnecessary. It should just read “1”.
L180. Please provide the definition of “immobile” versus “mobile” water pools in terms of matric potential.
L193. Did you sample pairs in unicates (no replicates)?
L194. Again, 200 degC is much too high for soil already, but not acceptable for plant organs or tissues. Are you sure we are talking about the actual sample temperature or the set point temperature of the heating device (, which may significantly differ)??
L232ff. Parts of this section should be moved to the material and methods.
L238. The reader cannot verify the statement “These metrics indicate that the Rayleigh-based equilibrium correction (Eq. 3) accurately reconstructs the source water composition by neutralizing effectively fractionation during vapor circulation and condensation.”) himself without the authors providing a comparison with and without “reconstruction” with Eq. 3. Please provide the corresponding data in the form of a table or figure.
L256. Providing two digits after the decimal point for δ2H and δ18O when in fact the precision is 10 or 100 times more (±1.0-2.0‰ for δ2H and ±0.2-0.8‰ for δ18O) is not correct.
Fig. 3. Nice figure! One can see from the spread in CVD versus CRAVE reference water extractions that CRAVE is generally producing more depleted water that CVD, which points out to possible incomplete water deposition (inverse sublimation) and/or non-equilibrium between water vapor and liquid water in the sample vial.
Fig. 5. Apparently, only CVD shows a depletion in both isotopes in deeper (>1 m) water. CRAVE and SL see less of a vertical isotopic composition gradient.
Fig. 6. Only the relationships between Eref and two of the three soil texture classes is needed since the 3rd relationship is a function of the first two. In other words, the displayed data is partly redundant.
Fig. 7. Providing the R2 for linear regressions that have a y-intercept different than zero makes little sense if you are aiming to evaluate how two procedures plot with respect to the 1:1 line.
Fig. 8. See previous comment.
Fig. 9. 1- Do you assume that the water content is the same between samples of each xylem pair? 2- How do you determine the xylem water content of the CRAVE sample? 3- I see that there are, for example, less samples between 1.0 and 1.5 g/g for δ18O than δ2H, how come?
L366. A clear definition of mobile/immobile water in the context of the present study is still missing at this point.
L371. CRAVE does not run isothermally, there is roughly a 200 degC difference between the sample and the trap.
406ff. The advantages of the CRAVE method, especially in terms of ease of use, number of samples to be extracted simultaneously etc. should be discussed here as well.