Enhanced Weathering and related element fluxes – a cropland mesocosm approach

Amann, Thorben; Hartmann, Jens; Struyf, Eric; de Oliveira Garcia, Wagner; Fischer, Elke K.; Janssens, Ivan; Meire, Patrick; Schoelynck, Jonas

doi:https://doi.org/10.5194/bg-17-103-2020

Articles | Volume 17, issue 1

https://doi.org/10.5194/bg-17-103-2020

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

https://doi.org/10.5194/bg-17-103-2020

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

Articles | Volume 17, issue 1

Research article

|

08 Jan 2020

Research article |

| 08 Jan 2020

Enhanced Weathering and related element fluxes – a cropland mesocosm approach

Thorben Amann, Jens Hartmann, Eric Struyf, Wagner de Oliveira Garcia, Elke K. Fischer, Ivan Janssens, Patrick Meire, and Jonas Schoelynck

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Final revised paper (published on 08 Jan 2020)
Supplement to the final revised paper
Preprint (discussion started on 02 Nov 2018)
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Interactive discussion

Status: closed

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

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- Supplement

SC1: 'Soil material, rate of olivine application, direct C acc, plant growth', Oliver Dilly, 06 Nov 2018
- AC1: 'tt', Thorben Amann, 29 Mar 2019
RC1: 'Comments to BGS Discussion paper by Amann et al.', Søren Jessen, 04 Dec 2018
- AC2: 'Our reply to your comments', Thorben Amann, 29 Mar 2019
RC2: 'comments on manuscript', David Manning, 17 Dec 2018
- AC3: 'Our reply to your comments', Thorben Amann, 29 Mar 2019

Peer-review completion

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

ED: Reconsider after major revisions (01 Apr 2019) by Andreas Ibrom

AR by Thorben Amann on behalf of the Authors (01 Apr 2019) Author's response Manuscript

ED: Reconsider after major revisions (03 Apr 2019) by Andreas Ibrom

ED: Referee Nomination & Report Request started (30 Apr 2019) by Andreas Ibrom

RR by Søren Jessen (13 Jun 2019)

Suggestions for revision or reasons for rejection

Dear authors

I would like to recognize first the effort you have done in order to improve figures and text of your manuscript. Overall, the paper reads much better now. Thank you.

***

However, still, please allow me to be a bit critical to the overall methodology that your analysis applies:

If I am not mistaken, your analysis uses the concentration differences ("in"-"out") of the non-conservative Mg to calculate a dilution/preferential flow effect, which is then used to calculate CO2 uptake. (If I am mistaken, please change the manuscript so that others don't get the same impression is I do.)

This approach warrants a much more careful treatment and discussion, than present in the current version of the manuscript.

p 16, l 26-28: Please explain exactly why "it is unlikely" and also include "adsorption" in your listing in the end of the sentence. Also, very low volumes of water actually comes through the mesocosms (or am I mistaken here too?), so the phrase 'large quantities of water bypassing ...' should be exchanged for something else.

***

This brings me to an alternative explanation for the Mg difference: The high Mg concentrations at 1.5 cm depth could be due to evaporation as the removal of water may leave Mg in solution at elevated concentrations (cf. Thaysen et al., 2014a (doi.org/10.5194/bg-11-7179-2014) and 2014b (doi.org/10.2136/vzj2014.07.0083)). Very low water volumes actually passes through the column despite continuous irrigation, and no water balance which can account for the residence time was or could be made. Who knows, maybe water moves upwards in large parts of the column? Therefore, please elaborately discuss this alternative explanation for the Mg concentration differences, and soften your statements accordingly throughout the manuscript. For example, the first time the dilution is mentioned is in bi-sentence (..., recognizing processes that lead to dilution, ...; p 13 l 6-7) by which time the reader apparently has to just accept this as a fact?

I probably just have overlooked a point in your work - if so, please argue fully and carefully in the text of the diminutive effects of the non-conservative Mg behavior and evaporation.

***

Several of the authors are highly experienced researchers, who I am sure are able to help avoid 'circular conclusions' and 'overly speculative conclusions' in the final version. (Excuse my direct address here, please.)

An example of what I see as a circular conclusion appears in the abstract (p 1 l 21-23): "Calculations that explicitly factor out the dilution effect of bypassed water, lead to relative fluxes about a magnitude higher, confirming that preferential flow paths and surface runoff in the field must be included in assessments for the CO2 consumption potential of Enhanced Weathering in general." The statement appears to assume that the previous CO2 sequestration rate is correct and that a rate ten times higher cannot be correct?

An example of what I see as an 'overly speculative conclusion' appears also in the abstract: Here I agree and find it correct to conclude that "Porewater Mg/Si molar ratios suggest that dissolved Si from the added minerals stays in the system over the observation period,..." but the reason that follows is not warranted: "...because a cation depleted Si layer forms
on the reactive mineral surface of freshly ground rocks.". And the anticipation of such a layer does not prove its actual existence, which is strongly inferred in the sentence that then follows: "This layer has not reached equilibrium thickness within the first two years."

For a reader of the abstract it must be made clear that no direct observation of the Si-enriched layer was observed, but only inferred from water chemistry. Again, perhaps I missed a point. If so, when you correct the text, please state the method by which layer was observed.

***

Finally, it goes without saying that I still find this research very valuable and important for climate mitigation. I certainly hope the authors will address the issues I experienced when reading the paper to enable its final publication in the near future.

I know, for typical authors, at least myself, criticism like the above may seem to change a lot and challenge the publication (again - that is, if I haven't overlooked some important points). The hardest part is if one has to abandon a conclusion, perhaps even replace one with the opposite. Yet, I hope one realizes that even in that case the paper is actually really close to being finalized.

Best regards
Soren

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ED: Publish subject to minor revisions (review by editor) (14 Jun 2019) by Andreas Ibrom

AR by Thorben Amann on behalf of the Authors (28 Aug 2019) Author's response Manuscript

ED: Reconsider after major revisions (30 Aug 2019) by Andreas Ibrom

AR by Thorben Amann on behalf of the Authors (23 Sep 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (28 Sep 2019) by Andreas Ibrom

ED: Publish subject to minor revisions (review by editor) (24 Oct 2019) by Andreas Ibrom

Dear Authors

after a long review process, I now made my decisions on how to proceed.

You will agree that the manuscript changed substantially and the discussion brought some very relevant limitations of the study, or better of its original interpretation. I briefly thought that this was an argument to reject it, but I admit that your experiment, given it's limitations, is a very valuable contribution to the Biogeochemistry, especially in terms of methodology but not so much or only very indirectly for the evaluation of enhanced weathering as vial CO2 sequestration mechanism.

This change should be reflected in the title that promises constraints on enhanced weathering, which were, given the uncertainties, not as tight as one might have wished.

The interesting discussion of uncertainties of deriving constraints on enhanced weathering from very intensive and careful field mesocosm studies, such as the presented one, should rather be the focus. Apart from the title, I find this very much reflected in the manuscript, so there are only some technical changes needed.

Regarding the uncertainty of estimates of the global CO2 consumption potential of enhanced weathering, your study might become a valuable contribution, if your results have unveiled a new source of uncertainty, hence not appropriately considered in these estimations.

Required technical changes
1. You write "Enhanced Weathering" throughout the text. Please don't use upper case letters, maybe rather use and abbreviation (not in the abstract) or a symbol.
2. (1|23) "one order of magnitude"
3. (16|8) insert "were" before "around"
4. (16|25 and 17|28) is "downward percolation" not a tautology in a gravitational field? Consider omitting "downward" or explain, why that is a necessary specification.
5. (19|26) replace "a magnitude" by "one order of magnitude"

With kind regards,
Andreas Ibrom

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AR by Thorben Amann on behalf of the Authors (11 Nov 2019) Author's response Manuscript

ED: Publish as is (13 Nov 2019) by Andreas Ibrom

AR by Thorben Amann on behalf of the Authors (20 Nov 2019)

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Short summary

Weathering is a major control on atmospheric CO₂ at geologic timescales. Enhancement of this process can be used to actively remove CO₂ from the atmosphere. Field results are still scarce and with this experiment we try to add some near-natural insights into dissolution processes. Results show CO₂ sequestration potentials but also highlight the strong variability of outcomes that can be expected in natural environments. Such experiments are of the utmost importance to identify key processes.