Insights into the Comprehensive Carbon Cycle in a Mangrove Ecosystem: a Case Study for Understanding Carbon Burial, Outgassing, and Outwelling in a Subtropical Island
A significant quantity of soil organic carbon (SOC) is buried in mangrove ecosystems. However, recent research has revealed that substantial amounts of carbon are exported to the atmosphere as CO2 or to the ocean as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). This carbon outflow highlights the need for a comprehensive understanding of carbon burial, outgassing, and outwelling to clarify the role of mangroves in net atmospheric CO2 removal. To elucidate the carbon cycle within a mangrove ecosystem, we quantified CO2 outgassing through high-resolution measurements CO2 and porewater-derived fluxes of total alkalinity, DIC, DOC, and POC using radium isotopes (224Ra and 223Ra) in an island mangrove ecosystem in Japan. Our findings showed that the dominant carbon flux was DIC outwelling, at 189 ± 18 mmol m-2 day -1, approximately 2.3 times higher than the global median. This pronounced DIC outwelling likely reflects the presence of a vast reservoir of poorly stabilized SOC. A comparison with ecosystem-scale CO2 emissions revealed that approximately 89% of the DIC was transported into the estuary without being emitted as CO2. This high DIC transport appears to result from the efficient water exchange characteristic of island mangroves, with a creek residence time of ~1 day. Surprisingly, the adjacent estuary acted as a net CO2 sink, surpassing CO2 outgassing from the mangrove creeks. These results suggest that efficient water exchange in island mangroves, coupled with high biological productivity at the adjacent estuary, promotes long-term sequestration of mangrove-derived DIC in the ocean.