Radiocarbon analysis reveals decomposition of old soil organic carbon into dissolved inorganic carbon in a subtropical mangrove ecosystem

The outwelling of remineralized dissolved inorganic carbon (DIC) from the subsurface soil layer through tidal pumping has recently been noted for its importance in the blue carbon budget of mangroves. However, the age of soil organic carbon (SOC) that has been decomposed into DIC remains uncertain. In this study, two distinct models (a two endmembers model and three endmembers model) were utilized to verify the age of the decomposed SOC, employing radiocarbon (Δ14C) as a tracer. We conducted a 24‐h measurement of DIC and DIC isotopes (δ13CDIC and Δ14CDIC), in addition to vertical profiles of Δ14C of SOC, in a subtropical mangrove ecosystem in Japan. The Δ14C expected for the source SOC that decomposed into DIC exhibited a comparable trend for the two distinct models, with mean values ranging from −86.6‰ to −16.8‰. The Δ14C values were converted to calendar age using the OxCal program, with the calibration curve from IntCal20 being employed. This conversion revealed that DIC in the mangrove creek originated from SOC buried over a century ago. Given that mangroves are confined to the intertidal zone, it is likely that there are spatial constraints on the burial of SOC in mangrove ecosystems. Consequently, the transition from SOC to the DIC carbon pool in the ocean could be a key process in supporting long‐term carbon sequestration over millennia.