Preprints
https://doi.org/10.5194/egusphere-2024-3281
https://doi.org/10.5194/egusphere-2024-3281
13 Nov 2024
 | 13 Nov 2024
Status: this preprint is open for discussion.

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.

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

Status: open (until 09 Jan 2025)

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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.