Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO<sub>2</sub> in calcareous and acidic soils

Joseph, Jobin; Külls, Christoph; Arend, Matthias; Schaub, Marcus; Hagedorn, Frank; Gessler, Arthur; Weiler, Markus

doi:https://doi.org/10.5194/soil-5-49-2019

Articles | Volume 5, issue 1

https://doi.org/10.5194/soil-5-49-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/soil-5-49-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 5, issue 1

Original research article

|

31 Jan 2019

Original research article |

| 31 Jan 2019

Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO₂ in calcareous and acidic soils

Jobin Joseph, Christoph Külls, Matthias Arend, Marcus Schaub, Frank Hagedorn, Arthur Gessler, and Markus Weiler

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Final revised paper (published on 31 Jan 2019)
Supplement to the final revised paper
Preprint (discussion started on 30 May 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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SC1: 'Suggest adding a few soil CO2 measurement papers to the intro', Erik Oerter, 11 Jun 2018
- AC3: 'Response', Jobin Joseph, 30 Sep 2018
RC1: 'review of Joseph et al.', Anonymous Referee #1, 24 Jun 2018
- AC1: 'Final response Reviewer 1', Jobin Joseph, 30 Sep 2018
RC2: 'SOIL', Rolf Siegwolf, 01 Aug 2018
- AC2: 'Final Response Reviewer 2', Jobin Joseph, 30 Sep 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (08 Oct 2018) by Raúl Zornoza

AR by Jobin Joseph on behalf of the Authors (16 Nov 2018)

ED: Referee Nomination & Report Request started (20 Nov 2018) by Raúl Zornoza

RR by Anonymous Referee #1 (05 Jan 2019)

Suggestions for revision or reasons for rejection

I thank the authors for seriously considering the reviewer comments and for conducting the additional calibrations—in my view these have greatly strengthened the paper, and it will be a useful contribution to the literature. However, several important points remain that are unclear or could use clarification.

The text on 63-70 would likely lead a casual reader to think that your work was the first to use high-frequency measurements of isotopes in soil gas profiles, but this is not the case. I made a previous comment (not addressed in the response) about the importance of mentioning recent studies that have similarly worked with high-frequency measurements of C and O isotopes in soil profiles, using different analytical techniques. For example, the papers by Jochheim et al. 2018 (10.1002/jpln.201700259), and Bowling et al. 2015 (doi:10.5194/bg-12-5143-2015) are directly related to your topic and should be acknowledged. Also perhaps see Stumpp et al. 2018 (doi:10.2136/vzj2018.05.0096) and papers in that issue.

I also previously made a comment with respect the “1 hz sampling frequency” that was not understood: “Finally, it should be noted that the useful temporal resolution of the measurements will never actually be 1hz as reported given the Allan variance results.”
The point here, shown in Figure 3, is that the practical resolution of the measurement cannot be 1 hz because of the high variance in the measured delta values (especially for 18O, ~1.3 per mil) when estimated using 1 hz data. If we assume that precision of say ~0.1 - 0.2 per mil is adequate (obviously, this would depend on the specific study), the useful sampling frequency would be ~10 – 20 seconds (0.05 – 0.1 hz). In fact, your in-situ soil measurements were conducted over 6 minute intervals to allow establishment of steady-state conditions. Please clarify accordingly in the Abstract. This detail is important for readers considering other applications of the method that might demand a higher sampling frequency.

Specific comments

30-31: This statement is a truism, as atmospheric CO2 will always diffuse into soil. What is relevant is the degree to which atmospheric CO2 is diluted by soil-respired CO2. There is abundant previous work in this area. Please rephrase.

31: What is the corollary—were 18O values decoupled from soil water at the calcareous site?

38: “accurate monitoring and modeling of these fluxes are inevitable” I think you mean essential here, rather than inevitable?
38: “Approximately 30 - 35% …” note that the anthropogenic CO2 flux has doubled since this paper was written so this statement is no longer correct.
233: replace “inevitable” with “necessary” (that seems to be what you mean?)
270-277: There is a subtle misinterpretation of your carbonate end member 13C values. It is stated “According to Cerling (1984), the distinct oxygen and carbon isotopic composition of soil carbonate depends on the isotopic signature of meteoric water and to the proportion of C4 biomass present at the time of carbonate formation (Cerling, 1984).” Note that pedogenic carbonate 13C reflects the 13C of the CO2 source of that carbonate (after accounting for fractionation), which is not simply a function of C3 vs. C4 biomass, but rather all of the other myriad factors that determine the 13C value of soil CO2. This should be clarified. Then it is stated “CO2 released as a result from carbonates have a distinct δ13C value close to 0‰ vs. VPDB.” Really, the CO2 released from carbonate will have whatever 13C value the carbonate had to begin with (assuming complete conversion through bicarbonate to CO2, without fractionation). Note that these in fact have 13C values much lower than zero per mil at your site! You have now measured carbonate 13C so you can be more precise here.
283: You measured total inorganic C, not bicarbonate, correct? Is this a typo or something else?
291: Also organic acids (or any other source of H+), which are perhaps most important. Note that acidity generated from CO2 (carbonic acid) will dissolve carbonate to form bicarbonate, but this is a net zero CO2 flux, even though you will observe the 13C from the carbonate due to exchange. See for example Zamanian et al. 2016, http://dx.doi.org/10.1016/j.earscirev.2016.03.003

328-329: This is another place where relevant recent studies of soil gas isotope dynamics should be cited.
Figure 6: What do the colored dashed lines represent? There is no indication in the legend. Are they some kind of error around the solid colored lines?
Figure 7: What CO2 mole fractions were used to generate this figure? This seems important in light of Figure 6

 

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RR by Anonymous Referee #3 (09 Jan 2019)

ED: Publish subject to minor revisions (review by editor) (09 Jan 2019) by Raúl Zornoza

AR by Jobin Joseph on behalf of the Authors (14 Jan 2019) Author's response Manuscript

ED: Publish subject to technical corrections (16 Jan 2019) by Raúl Zornoza

ED: Publish subject to technical corrections (18 Jan 2019) by Jorge Mataix-Solera (Executive editor)

AR by Jobin Joseph on behalf of the Authors (21 Jan 2019) Manuscript

Short summary

By coupling an OA-ICOS with hydrophobic but gas-permeable membranes placed at different depths in acidic and calcareous soils, we investigated the contribution of abiotic and biotic components to total soil CO₂ release. In calcareous Gleysol, CO₂ originating from carbonate dissolution contributed to total soil CO₂ concentration at detectable degrees, probably due to CO₂ evasion from groundwater. Inward diffusion of atmospheric CO₂ was found to be pronounced in the topsoil layers at both sites.

Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO2 in calcareous and acidic soils

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Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO₂ in calcareous and acidic soils