Shifting mineral and redox controls on carbon cycling in seasonally flooded mineral soils

LaCroix, Rachelle E.; Tfaily, Malak M.; McCreight, Menli; Jones, Morris E.; Spokas, Lesley; Keiluweit, Marco

doi:https://doi.org/10.5194/bg-16-2573-2019

Articles | Volume 16, issue 13

https://doi.org/10.5194/bg-16-2573-2019

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

https://doi.org/10.5194/bg-16-2573-2019

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

Articles | Volume 16, issue 13

Research article

|

04 Jul 2019

Research article |

| 04 Jul 2019

Shifting mineral and redox controls on carbon cycling in seasonally flooded mineral soils

Rachelle E. LaCroix, Malak M. Tfaily, Menli McCreight, Morris E. Jones, Lesley Spokas, and Marco Keiluweit

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Final revised paper (published on 04 Jul 2019)
Supplement to the final revised paper
Preprint (discussion started on 12 Oct 2018)
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 of LaCroix 2018', Aaron Thompson, 19 Nov 2018
- AC1: 'Resonse to reviewer 1 (Dr. Aaron Thompson)', Marco Keiluweit, 15 Jan 2019
RC2: '、', Anonymous Referee #2, 07 Dec 2018
- AC2: 'Response to reviewer #2', Marco Keiluweit, 15 Jan 2019

Peer-review completion

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

ED: Reconsider after major revisions (20 Jan 2019) by Luo Yu

AR by Marco Keiluweit on behalf of the Authors (23 Feb 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (01 Mar 2019) by Luo Yu

RR by Anonymous Referee #3 (29 Mar 2019)

Suggestions for revision or reasons for rejection

I have revised manuscript ‘Shifting Mineral and Redox Controls on Carbon Cycling in Seasonally Flooded Mineral Soils’. This work focuses on an estimation of various factors (Eh, extractable Fe and Al, and root biomass) on the CO2 emission from seasonally flooded forest soils in comparison to never flooded, and was done for the small scale transects. Moreover, the quality composition of SOM was determined in three soil horizons for these transect soils by the spectroscopic method (NEXAFS) and composition of DOC by the Fourier-transform ion cyclotron resonance mass spectrometry. During my revision, I found out that the manuscript has many drawbacks, especially in the methodological part and following results interpretation. My main concerns are: i) statement that upland soils are not affected by water fluctuations (actually, they are affected, which I can see well from the Eh values), ii) interpretation of the composition of SOM by the methods, which can only provide qualitative estimation, but not quantitative; iii) discussion of the data which are not statistically significant, iv) absence of any soil microbiological analyses, which can explain the differences for CO2 emission in various seasons.

General comments
The article focuses on the C cycling in seasonally flooded mineral soils, however, had looked only on the CO2 emission, what about the methane? How strong can be the contribution of this gas to C flux out from the soil? What is known from the literature?
Introduction in general: you have provided some information about the knowledge gaps, but did not explain any mechanisms for the processes going in the periodically flooded soils, and what is unclear. Please, provide more details. Besides, you have a lack of information about microbial driven processes. From the results, I can see that you did not touch this topic, but actually, C fate in the soil is driven by microorganisms. Moreover, you should point I the Introduction the reason why you focus only on the CO2, and not on methane estimations. No information is presented about the effect of temperature on the processes you are going to study.
Hypothesis1 – it is not clear: 1) capacity of which minerals do you mean? 2) greater C accumulation for which period? 3) what does macromolecular and chemically-reduced OM mean? 4) what are specific sites? Forest as well as grassland soils can be under periodic flood conditions in various climates and can be developed on different parent materials.
Moreover, you did not provide any information on how the quality of accumulated organic compounds in flooded condition is different from upland. There are many works done for the rice soils, for example, which are also under the pulse soil moisture conditions.
Materials and method: not clear how far where the plots from each other; is it possible to know the approximate age of these relief form (I mean they were permanently here or formed only when the experimental station was established?); how many soil cores were collected and how from each horizon and from each site? How many samples were collected for root analysis from each site and each landscape position?
Fourier-transform ion cyclotron resonance mass spectrometry – method is not so used in soil science and can confuse readers, please provide additional information whether a liquid sample is introduced as a liquid for the measurement, or prepared additionally (dried or by any other way).
For the results: there are many uncertainties in the method which you used to estimate the portion of various substances classes based on the FT-ICR-MS analysis, including ionization efficiencies, mobile phase composition, data acquisition parameters. Thus, I think it is better if you will re-do the figure 6, like it is originally advised in the article by Kim et al., 2003. Another suggestion can be to determine at least some components by the true analytical methods in the bulk soil samples and not in the DOC (for example lignin and carbohydrates contents).
Try to put conclusions at the end of each resulting part, summarizing the findings.
Everywhere – when you speak about C in the soil, please use ‘content’ (g kg-1 soil) and ‘concentration’ when you speak about DOC.
Table 1 – Actually shows no differences between CO2 effluxes, is it right?
Fig. 1 and Fig. 2 – from the Fig 1 I can see, that actually all horizons C for all soils are actually all the time underwater. I have than a question: how did you measure the Eh? And how did you collect soils samples? Moreover, Upland soil is also under the effect of water table rise (I can see it very well from the Eh values).
Fig 3 is not discussed in the result part.
Discussion part (4.3) Discussion about fig. 5 does not support by statistical analyses (near all differences are not statistically significant). I suggest that you try to test differences between the horizons. Besides, data on figure 6 are qualitative and not quantitative (see comments above).
Part 4.4. This is basically a repetition of the discussion above, and suggest to include these references into the discussion part to make it deeper in sense.

Specific comments
P3L11 ‘’ seasonally flooded soils are metabolically more active’’ – please be more precise here, what do you mean?
P4L5 Forgot oxidation state - SO4 2-
P3L25 – P4L12 Please be more precise here: the activity of which enzymes are inhibited; which exactly levels of Eh are usually observed in the seasonally flooded soils? This is important to write because the various electron-donor system is active for various Eh values; how does soil C content effect on the Eh conditions?
P8L2 What does spline function mean?
P8L23 Please, provide filter size for the DOC filtration.
P11L17-19 You can delete this sentence and start directly with results.
P11L20-P12L4 Please do not repeat data presented in the table, as well as the name of the stat. test you can remove, it is given in the table captions and in the mm section. Please, correct everywhere in the result section. Moreover, write only about significant results in the entire article, unless the goal of the paper is to show the absence of significance. Please, correct everywhere in the result section.
P14L1-P14L16 from Table S4, I can see that only the second horizon was different from for the landscape positions, thus, please, delete description about insignificant results. It is enough to write that no significant differences were found for other horizons or functional groups. Moreover, I do not see the reasons to present Fig 5 in the main result part, because it does not show any significant results. Please transfer to the supplementary materials.
P15L1-2 Please, delete this sentence. Start with the discussion directly.

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RR by Aaron Thompson (05 Apr 2019)

RR by Anonymous Referee #4 (25 Apr 2019)

ED: Reconsider after major revisions (08 May 2019) by Luo Yu

AR by Anna Wenzel on behalf of the Authors (11 Jun 2019) Author's response Manuscript

ED: Publish as is (12 Jun 2019) by Luo Yu

AR by Marco Keiluweit on behalf of the Authors (12 Jun 2019) Author's response Manuscript

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Short summary

Organic carbon (C) stocks within seasonal wetlands are vulnerable to increased severity and duration of droughts in response to climate change. Here we examined the mechanistic controls on C cycling in seasonally flooded mineral wetland soils. We found that different mechanisms preserve C in surface layers (oxygen limitations) compared to subsurface layers (mineral protection and lack of root C inputs).