Preprints
https://doi.org/10.5194/egusphere-2024-3281
https://doi.org/10.5194/egusphere-2024-3281
13 Nov 2024
 | 13 Nov 2024

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.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
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Journal article(s) based on this preprint

23 Sep 2025
Old carbon, new insights: thermal reactivity and bioavailability of saltmarsh soils
Alex Houston, Mark H. Garnett, and William E. N. Austin
Biogeosciences, 22, 4851–4864, https://doi.org/10.5194/bg-22-4851-2025,https://doi.org/10.5194/bg-22-4851-2025, 2025
Short summary
Alex Houston, Mark H. Garnett, Jo Smith, and William E. N. Austin

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-3281', Jordon Hemingway, 18 Dec 2024
    • AC1: 'Reply on RC1', Alex Houston, 17 Jan 2025
  • RC2: 'Comment on egusphere-2024-3281', Anonymous Referee #2, 17 Mar 2025
    • AC2: 'Reply on RC2', Alex Houston, 25 Mar 2025

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-3281', Jordon Hemingway, 18 Dec 2024
    • AC1: 'Reply on RC1', Alex Houston, 17 Jan 2025
  • RC2: 'Comment on egusphere-2024-3281', Anonymous Referee #2, 17 Mar 2025
    • AC2: 'Reply on RC2', Alex Houston, 25 Mar 2025

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
ED: Reconsider after major revisions (07 Apr 2025) by Tyler Cyronak
AR by Alex Houston on behalf of the Authors (07 May 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (07 May 2025) by Tyler Cyronak
RR by Jordon Hemingway (02 Jun 2025)
ED: Publish subject to minor revisions (review by editor) (20 Jun 2025) by Tyler Cyronak
AR by Alex Houston on behalf of the Authors (14 Jul 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (04 Aug 2025) by Tyler Cyronak
AR by Alex Houston on behalf of the Authors (05 Aug 2025)  Author's response 

Journal article(s) based on this preprint

23 Sep 2025
Old carbon, new insights: thermal reactivity and bioavailability of saltmarsh soils
Alex Houston, Mark H. Garnett, and William E. N. Austin
Biogeosciences, 22, 4851–4864, https://doi.org/10.5194/bg-22-4851-2025,https://doi.org/10.5194/bg-22-4851-2025, 2025
Short summary
Alex Houston, Mark H. Garnett, Jo Smith, and William E. N. Austin
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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