Sources, Sinks and Subsidies of Organic Carbon in Saltmarsh Habitats

Saltmarshes accumulate and store organic carbon through the drawdown of atmospheric CO₂ by photosynthesising vegetation (autochthonous carbon), and the deposition of externally derived carbon (allochthonous) during tidal inundation. These organic carbon sources can be different ages and remain stored in the soil for variable lengths of time, from minutes to millennia. International policy frameworks recognise that the management of saltmarshes can provide a climate mitigation service, yet uncertainties remain regarding the inclusion of allochthonous organic carbon in saltmarsh projects.

We employed a novel methodology to compare the radiocarbon (¹⁴C) contents of saltmarsh soils and CO₂ evolved from aerobic laboratory incubations to show that young (¹⁴C-enriched) organic carbon is preferentially respired over old (¹⁴C-depleted) organic carbon. The ¹⁴C contents of the respired CO₂ were compared to the ¹⁴C content of carbon pools defined by their thermal reactivity, measured by ramped oxidation. In most cases, the ¹⁴C content of the most thermally labile carbon pool was closest to the ¹⁴C content of the CO₂ evolved from aerobic incubations of the same soils, suggesting the thermal and biological lability of saltmarsh soil carbon in oxic conditions is closely related. These results highlight the role of saltmarshes as stores of both old, thermally recalcitrant organic carbon, as well as younger, thermally labile organic carbon. Management interventions, such as restoration, may help mitigate CO₂ emissions by limiting oxygen exposure and preserving these stores of thermally labile carbon.

We also highlight inconsistencies in the treatment of allochthonous carbon across blue carbon (saltmarsh, seagrass and mangrove) accounting methodologies. A review of these frameworks and their scientific basis reveals a lack of standardized, evidence-based approaches for determining the proportion of allochthonous carbon that should be discounted in additionality calculations. This research provides crucial evidence towards addressing these gaps and improving the robustness of blue carbon policy and accounting.