Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

Kalks, Fabian; Noren, Gabriel; Mueller, Carsten W.; Helfrich, Mirjam; Rethemeyer, Janet; Don, Axel

doi:https://doi.org/10.5194/soil-7-347-2021

Articles | Volume 7, issue 2

https://doi.org/10.5194/soil-7-347-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/soil-7-347-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 7, issue 2

Original research article

|

06 Jul 2021

Original research article |

| 06 Jul 2021

Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

Fabian Kalks, Gabriel Noren, Carsten W. Mueller, Mirjam Helfrich, Janet Rethemeyer, and Axel Don

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Final revised paper (published on 06 Jul 2021)
Supplement to the final revised paper
Preprint (discussion started on 05 Jun 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review SOIL 2020-34 Kalks et al. 2020', Anonymous Referee #1, 16 Aug 2020
- AC1: 'Reply to reviewer 1', Axel Don, 25 Sep 2020
RC2: 'Interactive comments on “Geogenic organic carbon in terrestrial sediments and its contributions to total soil carbon” by Fabian Kalks et al', Anonymous Referee #2, 27 Aug 2020
- AC2: 'Reply to reviwer #2', Axel Don, 25 Sep 2020

Peer-review completion

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

ED: Reconsider after major revisions (12 Oct 2020) by Cornelia Rumpel

AR by Axel Don on behalf of the Authors (23 Nov 2020) Manuscript

ED: Referee Nomination & Report Request started (05 Dec 2020) by Cornelia Rumpel

RR by Anonymous Referee #3 (23 Dec 2020)

Suggestions for revision or reasons for rejection

I read the manuscript of Kalks et al. with great interest. It is a good study that brings the importance of geogenic derived organic C in (sub)soil carbon stocks to attention and increases our understanding of its (potential) role in soil C cycling. As such, it fits well the topic of the Soil journal.

However, despite the points addressed already by the previous reviewers, there are still some things to be cleared up. Especially the part on δ13C is only very shortly mentioned in the introduction, without explanation or reference, and not at all used in the discussion section. These results have to be better integrated in the paper.

Furthermore, it is better to stick with geogenic organic carbon (GOC) as term and avoid using other terminologies like “sedimentary OC” or “Sedimentary contribution” as it causes some confusion. For example, it is not very clear if OC of section 3.1 was already corrected for GOC contribution. A better distinguish between OC and “corrected OC” has to be made, if this is the case.

Here below I gave more details on the issues encountered.

Major points to be addressed:
- Is there something known about the origin of GOC in the three different sites (i.e. buried organic matter / soot / coal / …)? As it is not stated/hypothesized, it makes also difficult to evaluate the used temperatures (450 ℃) for preheating samples to remove OC.

- Line 54: “GOC in most cases is devoid of 14C and thus may lead to an overestimation of ancient OC sources although a number of studies showed the importance of root derived, young OC inputs to subsoils.” This is quite a fundamental point of your study and could be better stated here already. Suggested: “As 14C has a half lifetime of 5730 years, carbon deposited from the Weichselian and older are depleted in 14C, thereby diluting the overall 14C concentration. Especially in C poor subsoil, where GOC forms a relative larger part of the overall C content, this leads to an age overestimation of relative fresh OM, like root derived components. “

- Line 80: “Thus, using both carbon isotopes can reveal if the OC is a mixture of GOC and OC”. It is not clear how δ13C can be used (from the introduction) and more detail how these different isotopes can be used to disentangle the different C components should be added. Above this line it is only made clear why GOC and 14C are important to study.

- Line 101-102: Restructure and rephrase questions, especially as question II is fundamental for the disentanglement of geogenic and more recent OC. It is suggested to start with “Is (G)OC free of 14C”, than “how much does GOC contribute to (sub)soil OC?” and “will sedimentary GOC be degraded and/or incorporated in recent OC”

- Line 261-266: This part of the results does not create confidence in your data. First it is stated all samples were within detection limit (to my opinion an understatement, as otherwise samples should not be included or represented by the value 0) and next there is speaking of “random noise”. Better to simply state what the mean relative standard deviation was (or overall measurement/methodological error) and the lowest measured value (0.04 g C kg-1 soil). Note that mg C g-1 soil and g C kg-1 soil are both used in the text.

- You could consider to discuss first the “How much GOC contributes to soil organic carbon?” before going into the bioavailability of it. This would make the “flow” of the discussion more logical.

Minor suggestions for improvement:
Line 22: “this gap” -> this knowledge gap
Line 24: “sedimentary OC” -> GOC
Line 51: “an contribution” -> “a contribution”
Line 60: “have been investigated” Missing the results of these studies, probably rephrase.
Line 68-69: “more information about the amounts of OC in sediments is needed.” -> “GOC in sediments” or “contribution of GOC in sediments”
Line 74: “hydraulic conductivity” -> “Pore distribution” or “porosity” fits the context better.
Line 127-128: “This means e.g. for a sample increment from 1-2 m, the sample represents the 1.85-1.95 m depth” -> “This means that for example the increment 1-2m is represented by a sample from 1.85-1.95m depth.”
Line 148: “removing carbonates” Same as 14C?
Line 250: “lm” -> “the function lm”
Title 3.1: “…sedimentary and subsoil organic carbon” -> be consistent with terminology. Better to use GOC / geogenic organic C instead of “sedimentary”, especially as “subsoil organic carbon” can be all OC found in the subsoil
Line 296/277: Fig 1a. -> Fig 2a.
Line 284 “they all were in the range of C3 plant material. A value above -25 ‰ for the Red Sandstone in 4 m depth can be explained by corresponding high values of inorganic carbon (IC) in this depth” -> better for discussion
Line 308: “Fig. 2 a” -> “Fig. 3a”
Line 322: “Fig. 2 c” -> “Fig. 3c”
Line 424: “the same site assigned the different” -> “the same site and assigned the different”
Line 425: “sedimentary OC” -> “OC”
Line 426/427: “…extremely low concentrations of 426 OC is more prone for infiltration of biogenic OC” Not completely clear what is meant, but probably best to say:”… very low OC contents increases the relative importance of biogenic C input for the over OC”
Line 463: “sedimentary bedrock” Loess is no bedrock, but an (aeolian) deposit or sediment
Line 523: Not clear what is meant with “a resistant part”
Line 525: “distinguished” -> “Distinguish”
Line 526: “bedrock OC” -> “GOC”
Line 557: “despite differences between sediments” -> “, despite differing between sediments,”
Line 570-571: Combine sentences
Line 572: “high age” -> “high 14C age”

Referee Report: PDF

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RR by Anonymous Referee #1 (18 Jan 2021)

Suggestions for revision or reasons for rejection

Feedback revision KALKS et al. SOIL.

The submission is a revision of a former manuscript. I checked the responses to the reviewer comments and also the revised manuscript. In general, I think the authors did a generally good job in revising the MS. Most comments were addressed well I consider the remaining comments all solveable as part of a minro/medium revision. The authors should consider shortening the discussion, it contains some speculative elements and many parts seem to be not relevant for the final conclusions. Also please check the entire manuscript thoroughly for language. I found many shortcomings still (also in the newly added parts). Please find my below list of comments. They follow the flow of the document. Line numbers refer to the track changes version of the MS which was added to the authors’ response and do not refer to the cleaned version.

l.18 this sentence is not necessary in the abstract and implicit from l 16.

l. 30-31. These statements of the GOC contribution to SOC should be phrased depth explicit.

l.32 this interpretation is speculative and should be removed. Subsoils without GOC show also depth trends and you did not evaluate the overall strength of GOC to these 14C depth across a wide enough range of soil types. In consequence, l. 33-34 need revision too. (Dot missing in l. 34)

l.49. grammar/wording
l. 51 grammar/wording
l. 59. grammar/wording
l. 64 grammar/wording

l.77-78. This sentence is only partly correct since human activities cause much more soil and sediment redistribution than any natural process. So I would limit your statement to natural deposition processes, and for those specifically again on which soils have developed.

l.81-82. This sentence is out of place and breaks the flow of the paragraph.

l. 84-85. This statement is in parts speculative. GOC is resistant under the conditions of deposition, but you cannot assume much from that under any other (in particular surface) conditions.

l.88 grammar/wording

l.91-93. Give this sentence a direction. Are you referring here to priming effects?

l.95 grammar/wording

l. 101-104. Question 1) and 4) seem related/repetitive. Combine

l. 145 grammar/wording

l. 160 name again what acid at what concentration

l. 177 this “transition to sediments” does not match to what you show in your profile pictures as beginning C horizons.

l.178 grammar/wording

l. 214-215. Revise the added sentence for clarification. Maybe add “…degradation rates at lower temperatures”.

l. 213. What was the incentive to incubate the crushed sandstone for 63 days, but the non-crushed for 50 days?

l. 225 unclear what you mean with “poured bulk density”.

l. 293-294. This strange finding is interesting, but seems to be an outlier when looking at figure 2? Has this measurement been confirmed? Or checked with measurements of a similar depth for this sample? It seems to drive your OC stock calculation in table 1 and needs to be confirmed and checked very carefully.

l. 287-311. Figure numbering refers to the former version.

l. 302 grammar/wording

l.333-334. This statement is discussion and interpretation, not results.

l.339 grammar/wording

l. 326-353. Also in this section the figure numbering seems to refer to the old MS version. Please check throughout the MS that his is addressed. The same is the case in the next section, I stopped checking after that one.

l. 375 grammar/wording

l. 378-380. This sentence is important but seems to be out of place here. Better placed in the methods? Besides, you state in your responses that O2 was >20% throughout the incubation experiment. How have you checked that ? Related to this Figure A4 panels are not well described. To me it does look like you have CH4 production increasing over time. In this regard, I would then make sure to not use the 553 days datapoint in the manuscript (Figure A2) for any interpretation or calculations (it is also not constrained like the other points shown in Figure A3).

l. 469. A reason why an annotated photo figure of the cores would be important (see comment at “figures”).

l.470-471. grammar/wording

l.474-478. This statement needs a proper reference. DOC infiltration rates have not been measured in any of the substrates.

l. 515-549. What are typical subsoil temperatures here? MAT? And its not only the temperature. Its also aeration, water content, structure etc. So I don’t think your experiment can reveal too much on the decomposability of deep C in situ but you can compare to other similar experiments or what the mineralization rates would be under conditions similar to your experiments. Revise section accordingly and reduce to avoid speculation.

l. 586. Remove “long-term” since you work without the uncertain 533 days datapoint, no?

l. 612-613. grammar/wording

l. 631-632. This 30% statement is speculative and unnecessary. Remove sentence.

l. 653 grammar/wording

l. 655 grammar/wording

l. 677. Delete “seem to”

l. 678. Add a sentence that your experiment shows that the GOC is degradable under the conditions you created during your incubation, no?

General comment discussion: The discussion is quite long. That’s OK as it is mostly data focused. However, I am missing a bit of a critical view on the assumptions behind the 14C signatures used to calculated the biogenic and geogenic. Uncertainties are discussed here and there a bit but there are some really big assumptions here with DOC and root exudates being involved over assumptions on what age range to expect from biogenic OC and so on. A critical view on this before going into all the data interpretation details would be helpful for readers to get a better feeling on the uncertainties surrounding the data.

General comment conclusions: I like the conclusion a lot. It really focuses on what the data can clearly show. It makes me think however, why all the data from the very deep cores was discussed in such great detail in the first place. Frankly, the manuscript would profit on more focus on the soil part, and not so much of the rather (speculative) mineralization or origin discussion for the deeper sediment cores (I see this as rather supplemental).

Comments on figures and tables:

Figure 1. Reformat Figure. Replace scale bars and signs with a digital version. Write horizon labels either on the left or to the right but not central into the image. Cut image to focus area you want to display. Use USDA or WRB system of horizon classification, not KA5. Besides, I cannot follow the classification easily and also some of the interpretation later in the text when relating to it. Some examples: Al horizon features at 66cm, or even 82cm depth (image A). How is that possible? If there was no disturbance in the forests, where is the rather big M horizon coming from (image B) which is also not discussed anywhere. Why would the sandstone profile (image C) have the most shallow development depth? Furthermore, since your MS is about long sediment cores, please add annotated images of the cores too (in the supplement), especially as you stress that the sediments were homogenous for 10m.

Figure 2. Some of the panels have weir line features at the x axis and y axis zero lines. Remove them. Further, red sandstones cores seem to be quite different from each other. This contradicts to your statements of the homogeneity of the core samples. How is this addressed?

Figure 5 essentially shows the same as Figure A3. I think you can remove Fig. A3

Table 2: Make sure the mineralization rates using the linear model are done based on the 63 day timeframe of the experiment and not include the uncertain extrapolation to the 533 days. Unclear from Fig A2.

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ED: Revision (26 Jan 2021) by Cornelia Rumpel

AR by Axel Don on behalf of the Authors (09 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (16 May 2021) by Cornelia Rumpel

AR by Axel Don on behalf of the Authors (18 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (27 May 2021) by Cornelia Rumpel

ED: Publish as is (27 May 2021) by Jorge Mataix-Solera (Executive editor)

AR by Axel Don on behalf of the Authors (31 May 2021) Manuscript

Short summary

Sedimentary rocks contain organic carbon that may end up as soil carbon. However, this source of soil carbon is overlooked and has not been quantified sufficiently. We analysed 10 m long sediment cores with three different sedimentary rocks. All sediments contain considerable amounts of geogenic carbon contributing 3 %–12 % to the total soil carbon below 30 cm depth. The low ¹⁴C content of geogenic carbon can result in underestimations of soil carbon turnover derived from ¹⁴C data.