Porewater-Derived Blue Carbon Outwelling and Greenhouse Gas Emissions in a Subtropical Multi-Species Saltmarsh

Saltmarshes can sequester atmospheric CO2 in sediments, but limited studies have quantified porewater-derived carbon exports and identified related carbon sources. Here, we estimated porewater exchange, carbon outwelling, and greenhouse gas emissions in a subtropical multi-species saltmarsh. The radon-based porewater exchange rate was estimated to be 5.60 +/- 2.78 cm d(-1). As the most dominant (~90%) carbon species, dissolved inorganic carbon (DIC) fluxes through porewater exchange and outwelling were 447 +/- 227 and 1200 +/- 61 mmol m(-2) d(-1), respectively, which were 1.2 and 3.2 times that of carbon burial. As most DIC can remain in the ocean for a long time, porewater-derived DIC outwelling represents another important carbon sink, in addition to carbon burial. CO2 and CH4 emissions from creek water were 54.6 +/- 0.5 and 0.19 +/- 0.01 mmol m(-2) d(-1), respectively, which could offset 16% of carbon burial. The delta C-13 and C/N ratios suggest that saltmarsh organic carbon mainly originates from the C3 plant Scirpus mariqueter rather than the C4 plant Spartina alterniflora. Overall, we suggest that porewater-derived DIC outwelling is an important long-term carbon sink in multi-species saltmarshes, providing a scientific basis for the protection and restoration of saltmarshes in the context of global climate change.