Carbon degradation in agricultural soils flooded with seawater after managed coastal realignment

Sjøgaard, Kamilla S.; Treusch, Alexander H.; Valdemarsen, Thomas B.

doi:https://doi.org/10.5194/bg-14-4375-2017

Articles | Volume 14, issue 18

https://doi.org/10.5194/bg-14-4375-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-14-4375-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 18

Research article

|

29 Sep 2017

Research article |

| 29 Sep 2017

Carbon degradation in agricultural soils flooded with seawater after managed coastal realignment

Kamilla S. Sjøgaard, Alexander H. Treusch, and Thomas B. Valdemarsen

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Final revised paper (published on 29 Sep 2017)
Preprint (discussion started on 04 Nov 2016)

Interactive discussion

Status: closed

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

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RC1: 'RC MS bg-2016-417', Anonymous Referee #1, 20 Jan 2017
RC2: 'bg-2016-417 Reviewer 2', Anonymous Referee #2, 07 Feb 2017
RC3: 'review of the manuscript "Carbon degradation in agricultural 1 soils flooded with sea2 water after managed coastal realignment"', Edouard Metzger, 10 Feb 2017
AC1: 'Final author comments', Kamilla Sjøgaard, 13 Mar 2017

Peer-review completion

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

ED: Reconsider after major revisions (19 Mar 2017) by Caroline P. Slomp

AR by Kamilla Sjøgaard on behalf of the Authors (29 Apr 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 May 2017) by Caroline P. Slomp

RR by Anonymous Referee #1 (08 May 2017)

Suggestions for revision or reasons for rejection

Dear dr. Sjøgaard,
Thank you for this substantial revision of the manuscript, it does look much better now, and I only have a few (minor) comments and suggestions left.

Reply to comment 1.6:
I was not very clear in this comment, my apologies. My question was:
If you analyzed OC of only 2 time points, did that proved a big enough range to warrant a linear regression (i.e., if the OC vs. OM contents cluster at the two end-points of the regression, you will have a significant regression, but the equation will not necessarily be correct as the slope will be very dependent on the end-points).
Could you perhaps add a plot of the OM vs OC as Supplementary Information?

P4L14: were the cores sliced in an anoxic atmosphere?

P5L17: did you use a statistical test to check for outliers? Or did you remove them on sight (which is acceptable, if they are obvious, but that has to be mentioned)

P6L5: ’accumulated porewater TCO2 at different time points …’ -> I assume that this is the accumulation, correct for the accumulation at the time before (i.e. integrated PW TCO2 at month 2 - integrated PW TCO2 at week 1 = produced TCO2 between week 1 and month 2)?

P6L16-18: Do you have any idea of the accumulation rate of the sediment? Can you estimate which depth part of the soil is still marine? Considering that you can still find shell material, some part will be the old marine sediment. And thus, how much of the organic carbon is actually soil organic carbon?

Section 3.2 and lower: give standard deviations if you show average values

Section 4.2: the two paragraphs are kind of repetitive (first one talks about SR and mentions O2 and other electron acceptors, the second one talks about O2 and others and mentions SR); you could shorten this by combining them and streamlining the text.

Section 4.4: it might be nice to try and make a rough estimation of the time scale over which this buffering will stay active (assuming the rate of sulfate reduction at the end + the percentage that precipitates + the content of reactive non –sulfurized FeII and FeIII left.

P12L25 could these high effluxes of CO2 also be due to the absence of CaCO3 precipitation, or is this a negligible effect?

Table 3: the relative contribution numbers do not seem to add up, so I am not sure if I understand the table correctly:
First row: TCO2 measured based on the whole core incubations
second row: TCO2 production in anaerobic jar incubations
third row: Sulfate reduction in carbon units (so converted 2:1)
Fourth row: relative contribution of SR to anaerobic respiration (so third row/second row * 100)
fifth row: relative contribution of other anaerobic pathways (so (1- third row/second row)*100)
If this is correct, then your percentages do not make sense (if I estimate the relative contribution of SR to total anaerobic respiration, I get these values):
0.3/0.8 = 0.375
4.7/8.7 = 0.54
12.9/19.9=0.64
etc.
Also, I would suggest to maybe change the table: Keep the first two row, and then show the relative contributions of aerobic respiration – sulfate reduction – other anaerobic pathways.

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RR by Anonymous Referee #2 (16 May 2017)

Suggestions for revision or reasons for rejection

Sjøgaard et al. present an impressive data set exploring the effects of seawater flooding on carbon biogeochemistry in (1) agricultural and (2) a freshwater reedswamp with high temporal resolution over an extended 1 year time period (again, impressive). While the experiment itself was clearly well executed and the authors do a very nice job constructing a metabolic budget given the experimental constraints, the study design does not address the original hypotheses and cannot support the type of strong conclusions drawn by the authors.

The authors have responded constructively to the original reviewer comments regarding methodology and literature, but have not addressed the key concerns from the original reviewers regarding the bulk of their interpretation of the data. Because there are no measurements from unflooded soils (C) or freshwater flooded soils (UC), there is no ambient condition with which to compare the effects of seawater flooding. Furthermore, there is no methodological control (i.e. cores from a seawater lagoon) to assess whether the laboratory handling (not seawater flooding) were responsible for the observed patterns. This does not doom the study, but severely restricts the type of solid conclusions that can be drawn to those regarding the effect of seawater on agricultural versus reedswamp soils only. In fact, the authors barely discuss the implications of antecedent conditions in the two sites (C & UC) (besides organic content) but it seems that the fact UC was a saturated (anaerobic) reedswamp is very important for framing and interpreting the results.

While some speculation regarding impacts on broader C cycling is appropriate, the current manuscript extends speculation to strongly-worded conclusions regarding coastal carbon sinks that convey far more certainty than is appropriate. All three original referees objected to the author's conclusions as they are based on a fundamental misunderstanding of carbon-climate feedbacks and it is disappointing that the authors continue to emphasize this point over more evidence-based conclusions from their rich data set. Therefore the paper must undergo major revisions before it can be considered for publication.

The following hypotheses and discussion pertinent to them must be significantly re-written:

H1 & H2: The authors cannot asses whether the origin or lability of organic matter plays any role in SOC degradation because lability is not measured and there is no control provided to show that recently flooded soils have a higher C loss that marine sediments under the same experimental conditions. While the chemical structure of material certainly impacts degradation, this is simply not measured in the current study and should not be the main focus.

H3: the study cannot support the key conclusions of the paper, namely that seawater flooding will preserve C, because the authors do not measure SOC degradation prior to flooding and their comparisons to agricultural rates rely on rates from disparate systems (some tropical, some global averages) measured largely in-situ (they therefor include autotrophic (root) respiration and are not comparable to the present study. Furthermore, respiration is meaningless if we don't know what gross primary productivity is. What is important is the net exchange of C, which is not addressed in this comparison with other studies. We can assume GPP it is 0 for the cores, so respiration is the only number in the equation (GPP-R=NEE) and the net exchange is negative (i.e. always out of the system). This is not the case for the other ag systems from the review. While it is clear that flooded soils preferentially preserve carbon in general, the authors should support this with outside literature as there results do not directly address this.

The conclusion that flooded site will constitute and immediate C-sink/negative carbon-climate feedback is highly objectionable and has great potential for misuse and misunderstanding. This is not a matter of data interpretation or an over-extension of data, it is a incorrect. I implore the authors to consult the IPCC or Verified Carbon Standards (VCS) for finite definitions of the terms sink, stock, and source as they apply to greenhouse gas feedbacks. These terms are well established and clearly defined by the global change community.

Why the conclusion that flooded site will constitute and immediate C-sink/negative carbon-climate is inaccurate:

Agricultural soils can and do sequester C through the accumulation of crop residue (C sink), albeit at a lower rate than natural systems. As is the case with some drained agricultural land, it is also possible that SOC from reclaimed marine/intertidal/marsh sediments is still being lost to aerobic oxidation at a higher rate than crop residue is accumulating (C source). The authors present no evidence for either case as the antecedent (pre flood) condition was not measured. Thus there is no baseline to conclude how the direction of C flux has changed.

While it is not clear what the end-point of this particular coastal managed realignment is (subtidal or intertidal mudflat? subtidal seagrass bed? intertidal wetland?), the current study supposes it is a subtidal flat (always flooded, no vegetation) which can lead to 1 of 3 outcomes:

(1) assume ag land (C) was a C source (carbon emissions as respiration>crop C uptake) and flooding preserved C (100% preservation in 1 year= C stock) this is not the case in this study and even if it was a system simply cannot be a C sink/negative feedback unless primary production is removing C from the atmosphere. Prevented emissions do not equal negative carbon-climate feedbacks because C is NOT being removed from the atmosphere. Zero emission scenario.

(2) assume ag land (C) was a C sources, even at the measured 93% preservation in 1 year, the site is small C source and is a candidate for reduced emissions only, again not a sink, no negative climate feedback.

(3) ag land was a small sink (carbon uptake from crops > carbon emissions from ecosystem respiration) and flooding preserves 93% of the SOM. Flooding (without vegetation establishment) now makes the site a net source of C and thus there is a positive climate feedback. Furthermore, in the reedswamp (UC) soil, all indications are that saltwater increases respiration in freshwater anaerobic environments(Weston, Neubauer, and many other citations), this this represents the potential for positive feedback, not to mention the death of vegetation.

If the soils were vegetated (subtidal seagrass or intertidal wetland) then we have a candidate for a negative feedback.

I will reiterate that the data the authors have produced is interesting and impressive and should be published. It will be of great interest to the coastal community. As is, I have no qualms regarding the methods or data, only the interpretation. I encourage the authors to consider re-writing the hypotheses and discussion/conclusions in a way that emphasizes a direct connection to their results.

Minor comments are included in attached PDF.

Referee Report: PDF

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ED: Reconsider after major revisions (15 Jun 2017) by Caroline P. Slomp

AR by Kamilla Sjøgaard on behalf of the Authors (06 Jul 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (03 Aug 2017) by Caroline P. Slomp

ED: Publish as is (28 Aug 2017) by Caroline P. Slomp

AR by Kamilla Sjøgaard on behalf of the Authors (29 Aug 2017)

Short summary

Permanent flooding of low-lying coastal areas is a growing threat due to climate-change-related sea-level rise. To reduce coastal damage, buffer zones can be created by managed coastal realignment where existing dykes are breached and new dykes are built further inland. We studied the impacts on organic matter degradation in soils flooded with seawater by managed coastal realignment and suggest that most of the organic carbon present in coastal soils will be permanently preserved after flooding.