Blue Carbon Additionality: New Insights from the Radiocarbon Content of Saltmarsh Soils and their Respired CO2

International policy frameworks recognize the net drawdown and storage of atmospheric greenhouse gases through management interventions on blue carbon ecosystems (saltmarshes, mangroves, seagrasses) as potential emissions offset strategies. However, key questions remain around the ‘additionality’ of the carbon sequestered by these ecosystems, and whether some fraction of the organic carbon (OC) that does not derive from in-situ production (allochthonous) should be included in carbon budgets. This study compares the radiocarbon (¹⁴C) contents of saltmarsh soils and CO₂ evolved from aerobic laboratory incubations to show that young OC is preferentially respired over aged OC, and that the latter is also vulnerable to remineralisation under oxic conditions. The results from this study are the first to empirically show the remineralisation of aged OC from a blue carbon ecosystem. This highlights that management interventions which reduce the exposure of saltmarsh soils to oxic conditions support the inclusion of some portion of allochthonous OC in carbon budgets. Elevated temperature incubations provide preliminary evidence that the predominant source of respired OC will not change under predicted future warmer conditions. Saltmarsh typology also influences the ¹⁴C content of both the bulk soil and respired CO₂, highlighting the importance of site selection for optimized blue carbon additionality.