Technical note: Uncovering the influence of methodological variations on the extractability of iron-bound organic carbon

Fisher, Ben J.; Faust, Johan C.; Moore, Oliver W.; Peacock, Caroline L.; März, Christian

doi:https://doi.org/10.5194/bg-18-3409-2021

Articles | Volume 18, issue 11

https://doi.org/10.5194/bg-18-3409-2021

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

https://doi.org/10.5194/bg-18-3409-2021

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

Articles | Volume 18, issue 11

Technical note

|

07 Jun 2021

Technical note |

| 07 Jun 2021

Technical note: Uncovering the influence of methodological variations on the extractability of iron-bound organic carbon

Ben J. Fisher, Johan C. Faust, Oliver W. Moore, Caroline L. Peacock, and Christian März

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Final revised paper (published on 07 Jun 2021)
Preprint (discussion started on 06 Nov 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review to the article by Fisher et al.', Susann Henkel, 20 Nov 2020
- AC1: 'Response to review by Susann Henkel', Ben Fisher, 26 Nov 2020
AC2: 'Response to comments from Monique Patzner and colleagues.', Ben Fisher, 26 Nov 2020
RC2: 'Interactive comment on “Technical Note: Uncovering the influence of methodologicalvariations on the extractability of iron boundorganic carbon” by Ben J. Fisher et al', Tom Jilbert, 30 Nov 2020
- AC3: 'Response to review by Tom Jilbert', Ben Fisher, 18 Dec 2020
RC3: 'Review of Technical Note: Uncovering the influence of methodological variations on the extractablilty of iron bound organic matter', Anonymous Referee #3, 30 Nov 2020
- AC4: 'Response to review by Anonymous Reviewer 3', Ben Fisher, 18 Dec 2020
RC4: 'Review of bg-2020-399', Peter Kraal, 02 Dec 2020
- AC5: 'Response to review by Peter Kraal', Ben Fisher, 18 Dec 2020

Peer-review completion

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

ED: Reconsider after major revisions (04 Jan 2021) by Jack Middelburg

AR by Ben Fisher on behalf of the Authors (05 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (08 Feb 2021) by Jack Middelburg

RR by Jeffrey C. Cornwell (10 Mar 2021)

RR by Bo Thamdrup (24 Mar 2021)

Suggestions for revision or reasons for rejection

The manuscript presents a methodological study of a widely used wet chemical extraction method for determining ”reactive iron” and associated organics (and phosphate) in soils and sediments – the dithionite-citrate-bicarbonate (CBD) extraction. It demonstrates how the selectivity of the method might deviate strongly from its intended target by leaving relatively large proportions of the reactive iron pool untouched and how this is affected by associated organics, and it provides guidelines for how to use the extraction to minimize these issues.

Wet chemical extractions for Fe, organics and P are a jungle with the exact protocols varying between labs, and calibrations with standard materials frequently being neglected. In this perspective, the present study has merit, although it also fails to some extent with respect to standards by only testing a homemade iron mineral. While this suffices to demonstrate the issues with the method, and OC-Fe materials might not be available, tests with some iron phases would have been useful.

The experiments have been carried out with care, the results are of good quality, and the manuscript is generally well written. My greatest concern with the paper relates to a mixing up of Fe and organic carbon (OC) extractability. It starts with the title, which suggests that the subject is the extractability or Fe-bound OC, but really 90% of the results are about the extractability of Fe. It might be that the authors equate Fe and OC extractability, but then this needs to be discussed and justified. Indeed, Fig. 1 demonstrates that these two parameters do not go hand in hand. In the introduction, l. 39-68, I recommend a clearer separation of studies focusing at Fe, OC, and P extraction with DCB, and I would like to see some discussion of the expected congruency between the extraction of these substances. For instance, I am relatively sure (though I couldn’t find my copy and check this) that Mehra and Jackson did NOT use CBD for OC-bound to reactive iron (as claimed in l. 39-40) but for the reactive iron itself. Fe extraction would seem the natural starting point in this introduction, with OC-Fe and P-Fe extraction being later adaptations. Now it seems the other way around.

Specific comments

24: Please leave it to the reader to decide on the value of your work.

61: Did they really say that CBD will REDUCE the Fe-associated organic C?

64: Most of the tests concern Fe extraction, not OC-Fe

86: Same as above, most tests concern Fe extraction, not OC-Fe

Methods, sect. 2.1 and 2.2: Are the procedures here not same as in Fisher et al. 2020? Or how do the procedures in l. 117-26 and 128-37 differ from the previous paper? If procedures for synthesis differ, how was the structure verified? If they don’t, please make this clear and shorten the methods here.

Results: Please use past tense for your results as customary.

Section 3.1: The first halves of each of the two paragraphs repeat results and should be strongly abbreviated.

244: “incomplete Fe extractions” is difficult to understand, please rephrase.

248: 40% and 50% of total OC are confusing coming and not comparable to 7 wt% Fe in the previous sentence. These are not useful values here without information about the total OC content. If the systems were OC poor, the OC-Fe content could be low.

257-279: I suggest merging and condensing these two paragraphs. The discussion of 30-40% OC-Fe first and then 50% OC-Fe seems too repetitive and does not convey the big picture very well.

Fig. 1: The curves in the figure are not explained and look like parabolas with decreasing values at the highest dithionite additions. I recommend use of hyperbolas or other asymptotic functions instead to reflect the idea that the extraction saturates.

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ED: Publish subject to minor revisions (review by editor) (25 Mar 2021) by Jack Middelburg

AR by Ben Fisher on behalf of the Authors (07 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (15 Apr 2021) by Jack Middelburg

AR by Ben Fisher on behalf of the Authors (16 Apr 2021) Manuscript

Short summary

Organic carbon can be protected from microbial degradation in marine sediments through association with iron minerals on 1000-year timescales. Despite the importance of this carbon sink, our spatial and temporal understanding of iron-bound organic carbon interactions globally is poor. Here we show that caution must be applied when comparing quantification of iron-bound organic carbon extracted by different methods as the extraction strength and method specificity can be highly variable.