[M-IS14] Biogeochemistry

[MIS14-03] Estimation of time scales of carbon sequestration to outwelling in a mangrove ecosystem using stable carbon and radiocarbon isotopes

*Wataru Nakamura¹, Kenta Watanabe², Toshihiro Miyajima¹, Yosuke Miyairi¹, Yusuke Yokoyama¹, Yasuhiko Yamaguchi³, Tomohiro Kuwae², Jun Sasaki¹ (1.The University of Tokyo, 2.Port and Airport Research Institute, 3.Lake Biwa Environmental Research Institute)

Keywords:Blue carbon, Mangrove, Radiocarbon

Coastal vegetation such as mangroves, salt marshes, and seagrasses are known as blue carbon ecosystems, which absorb atmospheric CO₂ and store organic carbon in sediments. These ecosystems are estimated to sequester carbon at a rate of approximately 31.2–82.8 Tg C yr⁻¹, with carbon sequestration within the soil on a scale of 100 to 1000 years. While the conventional blue carbon context focuses on soil organic carbon, recent studies have shown that the largest carbon flow within mangrove and saltmarsh ecosystems is the outwelling of dissolved inorganic carbon (DIC) from the soil to the ocean, which exceeds the rate of carbon burial in the soil. This means that remineralized DIC is outwelled from the subsurface soil layer by tidal pumping due to the development of animal burrows. However, the time scale of the loss of sequestered organic carbon remains uncertain. To reveal this time scale, we measured DIC and DIC isotopes (δ¹³C_DIC and Δ¹⁴C_DIC) for 24h, as well as measuring the vertical profiles of radiocarbon isotope of soil organic carbon (Δ¹⁴C_SOC), in a subtropical mangrove forest. The result of this study indicated that the organic carbon sequestered in the average depth of 54–69 cm (at least 67–93 years old) would be outwelled as DIC. This means that organic carbon sequestered in the top 1 m of soil is potentially being lost again. Quantifying the climate change mitigation function of mangroves requires determining the time scale from sequestration to outwelling and the biogeochemical processes involved.