Old carbon, new insights: thermal reactivity and bioavailability of saltmarsh soils

Saltmarshes are globally important coastal wetlands which can help to mitigate the impacts of climate change. They accumulate organic carbon from both modern and aged sources through in-situ biological production and the capture of ex-situ sources which are deposited during tidal inundation. Previous studies have found that long-term organic carbon storage in saltmarsh soils is driven by the net contribution from the older fraction, implying that the inputs of young organic carbon derived from in situ production are recycled at a faster rate. Using ramped oxidation, we assessed the composition (14C and 13C) of saltmarsh soil carbon pools defined by their thermal reactivity. By relating 14C measurements of the soil carbon pools to CO2 respired in aerobic incubations of the same soils, we provide the first empirical evidence linking the thermal reactivity of saltmarsh soil organic carbon with its bioavailability for remineralization. We found that old (14C-depleted) carbon dominates the thermally recalcitrant organic carbon pools, whereas the thermally labile carbon is composed of younger organic carbon sources. In most cases, the 14C content of the most thermally labile carbon pool was closest to the previously reported 14C content of the CO2 evolved from aerobic incubations of the same soils, implying that the bioavailability of saltmarsh soil organic carbon to remineralisation in oxic conditions is closely related to its thermal lability. Our results highlight the importance of saltmarshes as stores of both old, thermally recalcitrant organic carbon, as well as younger, thermally labile organic carbon that is vulnerable to decomposition under oxic conditions. 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 thermally labile organic carbon and hence limit CO2 emissions. We also present evidence to support the inclusion of thermally labile allochthonous OC stored in saltmarsh soils in additionality assessments for projects which aim to prevent the drainage of saltmarshes, with relevance to international carbon crediting projects and National GHG Inventories.