Vertical partitioning of CO<sub>2</sub> production in a forest soil

Wordell-Dietrich, Patrick; Wotte, Anja; Rethemeyer, Janet; Bachmann, Jörg; Helfrich, Mirjam; Kirfel, Kristina; Leuschner, Christoph; Don, Axel

doi:https://doi.org/10.5194/bg-17-6341-2020

Articles | Volume 17, issue 24

https://doi.org/10.5194/bg-17-6341-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-17-6341-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 24

Research article

|

15 Dec 2020

Research article |

| 15 Dec 2020

Vertical partitioning of CO₂ production in a forest soil

Patrick Wordell-Dietrich, Anja Wotte, Janet Rethemeyer, Jörg Bachmann, Mirjam Helfrich, Kristina Kirfel, Christoph Leuschner, and Axel Don

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Final revised paper (published on 15 Dec 2020)
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Preprint (discussion started on 26 Apr 2019)
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Status: closed

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

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- Supplement

RC1: 'Review of “Vertical partitioning of CO2 production in a Dystric Cambisol”', Anonymous Referee #1, 10 Jul 2019
- AC1: 'Authors response to Referees comments', Patrick Wordell-Dietrich, 07 Oct 2019
- AC3: 'Authors response to Referees comments Part II', Patrick Wordell-Dietrich, 07 Oct 2019
RC2: '2nd review', Anonymous Referee #2, 03 Aug 2019
- AC2: 'Authors response to Referees comments', Patrick Wordell-Dietrich, 07 Oct 2019

Peer-review completion

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

ED: Reconsider after major revisions (18 Nov 2019) by Luo Yu

AR by Patrick Wordell-Dietrich on behalf of the Authors (23 Jan 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Jan 2020) by Luo Yu

RR by Anonymous Referee #2 (10 Feb 2020)

RR by Anonymous Referee #1 (14 Apr 2020)

Suggestions for revision or reasons for rejection

Response to revised version – Referee 1

Comment 1:
The addition of the images and schematics in figures 1 and 2 really helps understand the methods much better.

Comment 2:
My preference is to normalize production rates to the volume of soil that is doing the producing, but if you prefer to keep all expressions on a surface area basis, that is fine as long as it is consistent and clear.

Comment 3:
The added explanations are helpful.

Comment 4:
Authors satisfactorily resolved the issue raised in comment.

Comment 5:
Authors satisfactorily resolved the issue raised in comment.

Comment 6:
I understand the challenges of determining isotope ratios of production with these data, and appreciate the efforts of the authors after my initial comment. Even for the unlabeled background case, the Keeling plot method is difficult if you do not have measurements very close to the surface where most of the isotope variation occurs. If all of your depths are around -24 to -25 per mil, then your overall source for the profile is probably very close to -29. The theoretical diffusive enrichment of 4.4 has been confirmed in many field studies. Given the large difference of isotope ratio of the litter (1241 and 1880 per mil), -29 per mil is probably a close enough approximation for your background endmember.

For determining amount of label in production, I still do not believe the paper gets it right, and I think the underlying problem is in trying to apply steady-state calculations to the non-steady state conditions induced by the label addition. I think the best way to interpret these isotopic data would be a non-steady-state model, which would be capable of accounting for the slow equilibration apparent in Fig 8b. After the litter addition, the label-derived CO2 slowly makes its way down the profile, while simultaneously diffusing out the top. Perhaps this effort could warrant a separate paper.

I agree with the authors’ response where they said, “Furthermore, we must first calculate the CO2 fluxes / production in the respective layers for each sampling time. Then we must apply Eq. 9 on the CO2 production to [obtain] the amount of litter mineralisation in the certain layer.” However, it appears in section 2.5.3 that the authors are still applying equation 9 to soil CO2, which is not valid, and then combining this “scaling factor” with total CO2 production calculated for each layer. Amount of label C in soil CO2, does not really tell you much about production by itself. If production rate or diffusivity have changed, the comparison goes out the window. If the authors were able to calculate 13CO2 fluxes by layer, then they can do it for 12CO2 as well, and then can apply equation 9 correctly to the CO2 produced within each layer (as stated in the reply, but not changed in the manuscript). If this does not produce reasonable results, it is probably due to a violation of steady state assumptions, requiring a non-steady state model.

Comments 7-9
Authors satisfactorily resolved the issues raised in comments.

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ED: Publish subject to minor revisions (review by editor) (16 Apr 2020) by Luo Yu

AR by Patrick Wordell-Dietrich on behalf of the Authors (25 Sep 2020) Author's response Manuscript

ED: Publish as is (16 Oct 2020) by Luo Yu

AR by Patrick Wordell-Dietrich on behalf of the Authors (29 Oct 2020) Author's response Manuscript

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Short summary

The release of CO₂ from soils, known as soil respiration, plays a major role in the global carbon cycle. However, the contributions of different soil depths or the sources of soil CO₂ have hardly been studied. We quantified the CO₂ production for different soil layers (up to 1.5 m) in three soil profiles for 2 years. We found that 90 % of CO₂ production occurs in the first 30 cm of the soil profile, and that the CO₂ originated from young carbon sources, as revealed by radiocarbon measurements.

Vertical partitioning of CO2 production in a forest soil

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Vertical partitioning of CO₂ production in a forest soil