Old Carbon, New Insights: Thermal Reactivity and Bioavailability of Saltmarsh Soils

Houston, Alex; Garnett, Mark H.; Smith, Jo; Austin, William E. N.

doi:https://doi.org/10.5194/egusphere-2024-3281

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https://doi.org/10.5194/egusphere-2024-3281

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13 Nov 2024

| 13 Nov 2024

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Old Carbon, New Insights: Thermal Reactivity and Bioavailability of Saltmarsh Soils

Alex Houston, Mark H. Garnett, Jo Smith, and William E. N. Austin

Abstract. Saltmarshes are globally important coastal wetlands which can store carbon for millennia, helping to mitigate the impacts of climate change. They accumulate organic carbon from both autochthonous sources (above- and belowground plant production) and allochthonous sources (terrestrial and marine sediments deposited during tidal inundation). Previous studies have found that long-term organic carbon storage in saltmarsh soils is driven by the pre-aged allochthonous fraction, implying that autochthonous organic carbon is recycled at a faster rate. However, it is also acknowledged that the bioavailability of soil organic carbon depends as much upon environmental conditions as the reactivity of the organic carbon itself. Until now, there has been no empirical evidence linking the reactivity of saltmarsh soil organic carbon with its bioavailability for remineralization.

We found that the ¹⁴C age of CO₂ produced during ramped oxidation of soils from the same saltmarsh ranged from 201 to 14,875 years BP, and that ¹⁴C-depleted (older) carbon evolved from higher temperature ramped oxidation fractions, indicating that older carbon dominates the thermally recalcitrant fractions. In most cases, the ¹⁴C content of the lowest temperature ramped oxidation fraction (the most thermally labile organic C source) was closest to the previously reported ¹⁴C content of the CO₂ evolved from aerobic incubations of the same soils, implying that the latter was from a thermally labile organic carbon source. This implies that the bioavailability of saltmarsh soil organic carbon to remineralisation in oxic conditions is closely related to its thermal reactivity. Management interventions (e.g. rewetting by tidal inundation) to limit the exposure of saltmarsh soils to elevated oxygen availability may help to protect and conserve these stores of old, labile organic carbon and hence limit CO₂ emissions.

Received: 21 Oct 2024 – Discussion started: 13 Nov 2024

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Alex Houston, Mark H. Garnett, Jo Smith, and William E. N. Austin

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RC1:
'Comment on egusphere-2024-3281', Jordon Hemingway, 18 Dec 2024 reply

Please find my review in the attached pdf file.

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Citation: https://doi.org/10.5194/egusphere-2024-3281-RC1
- AC1:
  'Reply on RC1', Alex Houston, 17 Jan 2025 reply
  We thank the reviewer for their focused and detailed review of our manuscript. We appreciate the constructive feedback and the opportunity to improve this manuscript. We welcome the comment that the topic of this manuscript is an interesting and underexplored area of research. In this response letter we will address the key points of the review, while stopping short of providing a revised manuscript until a formal editorial decision is made.
  Primary R(P)O literature and context
  
  We agree with the reviewer’s comment that there is insufficient citing of the primary ramped (pyrolysis) oxidation (R(P)O) literature and thank them for sharing a list of sources to potentially include. This issue stems from an attempt to keep the manuscript concise, however, we acknowledge that this has resulted in some of the primary RO literature being excluded or not covered in sufficient detail. We will cite these more extensively in the revised manuscript.
  RO data analysis and interpretation
  
  We acknowledge that calculating activation energies could improve the interpretation of the data as the reviewer suggests and welcome the suggestion to utilise their ‘rampedpyrox’ package. However, we believe that the novel research questions that this manuscript sets out to answer do not require the calculation of activation energies, given the existence of other published RO literature which do not calculate these and make the same inferences about temperature and thermal reactivity as we do (e.g. Grant et al., 2019; Peltre et al., 2013; Sanderman and Grandy, 2020; Stoner et al., 2023; Williams et al., 2018).
  We initially understood that this model would not be suitable for our samples as they have not been decarbonated which is stated as a requirement (Hemingway et al., 2017). Therefore, we proceeded with inferring thermal reactivity from the chosen temperature intervals in the submitted manuscript.
  Hemingway et al., 2017; P3: “the presence of carbonate will result in thermograms that cannot be accurately described by the model presented here, and we therefore argue in favor of acid treatment when using the RPO instrument to determine reaction energetics of carbonate-containing samples.”
  Following receipt of this review, we contacted the reviewer to determine if the ‘rampexpyrox’ model was potentially suitable for our samples, despite the above published advice that they were not. The reviewer has now confirmed (pers. comm. 16/01/2025) that due to the low amount of carbonates in our samples that it would be worth attempting to calculate activation energies. We will therefore implement the ‘rampedpyrox’ model for analysis and presentation of the thermograms and isotope data in the revised manuscript; we thank the reviewer for this clarification.
  Regarding the normalisation of thermograms to maximum peak size, this is simply to aid visual comparison of the curves between the samples. The regressions were not based on the normalised curves, but on the volume of CO₂ evolved over the defined temperature intervals.
  As we will be implementing the ‘rampedpyrox’ model for our data analysis and presentation of the thermograms and isotope data, the figures used in the results section will be updated in the revised manuscript. Therefore, the normalised thermograms and regressions presented in the initial manuscript will likely be removed from the revised version and replaced by a discussion of the distribution of activation energies (and related ¹⁴C contents).
  Analysis and measurement detail
  
  A published paper verifying the RO methodology used in this manuscript already exists (Garnett et al., 2023) and is cited in this paper. We will add the required details such as carrier gas composition, sample masses, and flow rate to the revised manuscript. We thank the reviewer for their suggested verification checks and we will add the mass balance verifications to the revised manuscript where possible.
  The decision to not remove carbonates from our samples was taken to avoid the potential loss of carbon from labile fractions during acid treatment (Bao et al., 2019). Importantly, we note that in Hemingway et al. (2017), acid treatment of samples prior to RO resulted in a shift of 0.04 Fm ¹⁴C. This effect could shift one of our samples from having a pre-bomb ¹⁴C content to a post-bomb ¹⁴C content, or vice-versa. A similar shift in ¹⁴C content for our samples could seriously impact the interpretations in our study, and our ability to compare the ¹⁴C content of the CO₂ respired from bulk (untreated) soils in the incubation experiments (Houston et al., 2024) to the ¹⁴C content of the CO₂ evolved during the RO procedure. This is a crucial component of this work and hence why samples were not decarbonated.
  Furthermore, the detail on the treatment of these samples is stated in the manuscript, L109: “The RO sub-samples were individually dried to constant mass before milling to a fine powder to homogenise and limit potential shielding effects from aggregates. The samples were then sent to the NEIF Radiocarbon Laboratory for the RO procedure, which is described in Garnett et al. (2023)”. We acknowledge the fact that we did not acid-treat our samples is implicit rather than explicitly stated, so we will add in this detail to the revised manuscript to avoid confusing future readers. We will also add a discussion section detailing why we did not decarbonate our samples and will also report the inorganic carbon content of the bulk soil samples in the supplementary information (these are <0.5 % inorganic carbon). In this new section we will also discuss how not decarbonating our samples may have impacted our results and interpretations.
  Regarding the normalisation of ¹⁴C values, it is incorrect to state that using δ¹³C values (determined using isotope ratio mass spectrometry (IRMS) from an aliquot of the sample CO₂) to normalise ¹⁴C results represents a major break from typical procedure. Use of IRMS δ¹³C values for normalising AMS data is clearly supported in several key AMS methods papers (Donahue et al., 1990; McNichol et al., 2001) and while online AMS ¹³C measurements can be advantageous particularly for small samples (Santos et al., 2007), many labs employ IRMS values for normalisation (McIntyre et al., 2017) and quality assurance results show that the method is clearly reliable (Ascough et al., 2024).
  Data validation and verification
  
  The RO samples are sub-sets of those analysed in Houston et al. (2024), where the bulk sample %OC, %IC, pMC, and δ¹³C values are all reported (Houston et al., 2024). To aid comparison for readers, we will add these into the supplementary material in the revised manuscript. We will also add sense-checks such as isotope mass-balance calculations to the supplementary material.
  We welcome the reviewer’s point about the potential (or lack thereof) for ¹³C fractionation during the RO procedure. We will alter this section of the manuscript and discuss in greater detail the mechanisms such as different compounds combusting at different temperature ranges and the importance of organo-mineral interactions for some compound classes. However, ¹⁴C is the main focus of this manuscript (comparison of ¹⁴C content of respired CO₂ from incubation experiments and ¹⁴C content of thermal reactivity fractions).
  We will also discuss the potential for carbonate contribution as low as ~550 °C here. We note that the ¹³C values for each of the 500-650 °C RO fractions were within expected ranges for OC in these samples, but that there could be contributions from carbonate sources. As previously stated, we will be implementing the ‘rampedpyrox’ model for our data analysis and presentation of the thermograms and isotope data, so the interpretations of ¹⁴C content and temperature will be replaced in the revised manuscript by comparing ¹⁴C contents of the evolved CO₂ to activation energies.
  Concepts and terminology
  
  We will improve on our terminology usage in the revised manuscript and thank the reviewer for their suggested terms of “thermal recalcitrance” and “thermal lability” which we will use when referring to the thermal reactivity of the OC pools. We will retain the use of “bioavailable OC” when referring to the CO₂ respired from the incubation experiments. We agree with the reviewer that it is important to highlight to the reader that thermal reactivity does not have to correlate with biological turnover times. Interestingly, this was one of the motivations for undertaking this work as much previous work has been undertaken on the ‘reactivity’ of soil/sediment OC but there has been limited work comparing this to ‘bioavailable’ carbon under set environmental conditions.
  We take on board the reviewer’s comments of not using ¹⁴C ‘ages’. We had included this to provide context to the most ¹⁴C-depleted samples to highlight the potential for ¹⁴C-dead contributions but that they aren’t necessary to achieve these low ¹⁴C contents for the region. However, we will remove this interpretation in the revised manuscript to avoid inappropriate reporting of complex OC mixtures.
  Regressions, plotting, and statistics
  
  We agree with the reviewer that the regressions are not ideal as they are calculated from the mean temperature, but we now acknowledge that the ¹⁴C (or ¹³C) content could be predominantly evolved from a narrower temperature interval within the defined temperature range of CO2 collection. However, we agree with the reviewer that these plots could be clearer and welcome their data visualisation suggestions which we will implement in the revised manuscript.
  Additionally, we agree that the box and whisker plots result in a loss of crucial information about individual samples.
  The omission of the 650-800 °C RO fraction from Figure 3 was to compare the OC pools. However, we strongly disagree with the reviewer that this was a dishonest omission as the δ¹³C for the 650-800 °C RO fraction are reported in Table 2, directly below this figure.
  As we will be implementing the ‘rampedpyrox’ model for our data analysis and presentation of the thermograms and isotope data a lot of the figures used in the results section will be updated in the revised manuscript.
  We agree with the reviewer that we should remove any interpretation of statistically insignificant trends from the revised manuscript.
  
  Yours sincerely,
  Alex Houston (on behalf of all authors).
  
  Reference List
  Ascough, P., Bompard, N., Garnett, M. H., Gulliver, P., Murray, C., Newton, J.-A., and Taylor, C.: 14C MEASUREMENT OF SAMPLES FOR ENVIRONMENTAL SCIENCE APPLICATIONS AT THE NATIONAL ENVIRONMENTAL ISOTOPE FACILITY (NEIF) RADIOCARBON LABORATORY, SUERC, UK, Radiocarbon, 66, 1020–1031, https://doi.org/10.1017/RDC.2024.9, 2024.
  Bao, R., McNichol, A. P., Hemingway, J. D., Gaylord, M. C. L., and Eglinton, T. I.: Influence of Different Acid Treatments on the Radiocarbon Content Spectrum of Sedimentary Organic Matter Determined by RPO/Accelerator Mass Spectrometry, Radiocarbon, 61, 395–413, https://doi.org/10.1017/RDC.2018.125, 2019.
  Dasari, S., Garnett, M. H., and Hilton, R. G.: Leakage of old carbon dioxide from a major river system in the Canadian Arctic, PNAS Nexus, 3, pgae134, https://doi.org/10.1093/pnasnexus/pgae134, 2024.
  Dean, J. F., Billett, M. F., Turner, T. E., Garnett, M. H., Andersen, R., McKenzie, R. M., Dinsmore, K. J., Baird, A. J., Chapman, P. J., and Holden, J.: Peatland pools are tightly coupled to the contemporary carbon cycle, Global Change Biology, n/a, e16999, https://doi.org/10.1111/gcb.16999, 2023.
  Donahue, D. J., Linick, T. W., and Jull, A. J. T.: Isotope-Ratio and Background Corrections for Accelerator Mass Spectrometry Radiocarbon Measurements, Radiocarbon, 32, 135–142, https://doi.org/10.1017/S0033822200040121, 1990.
  Garnett, M. H. and Hardie, S. M. L.: Isotope (14C and 13C) analysis of deep peat CO2 using a passive sampling technique, Soil Biology and Biochemistry, 41, 2477–2483, https://doi.org/10.1016/j.soilbio.2009.09.004, 2009.
  Garnett, M. H., Hardie, S. M. L., and Murray, C.: Radiocarbon analysis reveals that vegetation facilitates the release of old methane in a temperate raised bog, Biogeochemistry, 148, 1–17, https://doi.org/10.1007/s10533-020-00638-x, 2020.
  Garnett, M. H., Pereira, R., Taylor, C., Murray, C., and Ascough, P. L.: A NEW RAMPED OXIDATION-14C ANALYSIS FACILITY AT THE NEIF RADIOCARBON LABORATORY, EAST KILBRIDE, UK, Radiocarbon, 65, 1213–1229, https://doi.org/10.1017/RDC.2023.96, 2023.
  Grant, K. E., Galy, V. V., Chadwick, O. A., and Derry, L. A.: Thermal oxidation of carbon in organic matter rich volcanic soils: insights into SOC age differentiation and mineral stabilization, Biogeochemistry, 144, 291–304, https://doi.org/10.1007/s10533-019-00586-1, 2019.
  Hemingway, J. D., Rothman, D. H., Rosengard, S. Z., and Galy, V. V.: Technical note: An inverse method to relate organic carbon reactivity to isotope composition from serial oxidation, Biogeosciences, 14, 5099–5114, https://doi.org/10.5194/bg-14-5099-2017, 2017.
  Houston, A., Garnett, M. H., and Austin, W. E. N.: Blue carbon additionality: New insights from the radiocarbon content of saltmarsh soils and their respired CO2, Limnology and Oceanography, n/a, https://doi.org/10.1002/lno.12508, 2024.
  McIntyre, C. P., Wacker, L., Haghipour, N., Blattmann, T. M., Fahrni, S., Usman, M., Eglinton, T. I., and Synal, H.-A.: Online 13C and 14C Gas Measurements by EA-IRMS–AMS at ETH Zürich, Radiocarbon, 59, 893–903, https://doi.org/10.1017/RDC.2016.68, 2017.
  McNichol, A. P., Jull, A. J. T., and Burr, G. S.: Converting AMS Data to Radiocarbon Values: Considerations and Conventions, Radiocarbon, 43, 313–320, https://doi.org/10.1017/S0033822200038169, 2001.
  Peltre, C., Fernández, J. M., Craine, J. M., and Plante, A. F.: Relationships between Biological and Thermal Indices of Soil Organic Matter Stability Differ with Soil Organic Carbon Level, Soil Science Society of America Journal, 77, 2020–2028, https://doi.org/10.2136/sssaj2013.02.0081, 2013.
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  Stoner, S. W., Schrumpf, M., Hoyt, A., Sierra, C. A., Doetterl, S., Galy, V., and Trumbore, S.: How well does ramped thermal oxidation quantify the age distribution of soil carbon? Assessing thermal stability of physically and chemically fractionated soil organic matter, Biogeosciences, 20, 3151–3163, https://doi.org/10.5194/bg-20-3151-2023, 2023.
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  Citation: https://doi.org/10.5194/egusphere-2024-3281-AC1

Alex Houston, Mark H. Garnett, Jo Smith, and William E. N. Austin

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Alex Houston, Mark H. Garnett, Jo Smith, and William E. N. Austin

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Short summary

The organic carbon stored in saltmarsh soils can be up to 15,000 years old. We found that less energy is required to decompose young carbon than old carbon, i.e., young carbon tends to be more labile. We show that the labile carbon can still be up to 2,000 years old, implying that even old carbon in saltmarsh soils may contribute to greenhouse gas emissions. Protecting saltmarshes from degradation may help conserve these stores of old, labile organic carbon and hence limit CO₂ emissions.


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