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

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.