Habitat evolution of a peat swamp forest and belowground carbon storage was examined in a coastal lowland along the eastern coast of Central Sumatra, Indonesia. Boring surveys using a hand-operated peat sampler for 32 sites and radiocarbon dating for 29 samples revealed that the peat swamp forest had begun formation by 6500 cal BP and expanded rapidly between 6500 and 5000 cal BP and between 4000 and 2000 cal BP to nearly the present distribution area. The thickness of the peat layer formed under the peat swamp forest reached approximately 4 m, even in the margin of the peat dome, and more than 9 m in the center. Stored carbon per unit volume of the peat layer was estimated to be between 30 and 44 kg C m−3 in the margin of the peat dome and approximately 26 kg C m−3 in the center, which was less than the general carbon values of the mangrove peat. The relatively higher amount of stored carbon per unit volume in the margin was possibly caused by an inflow of allochthonous carbon from flood deposits. Belowground carbon burial rate of the peat layer was calculated as between 31 and 105 g C m−2 years−1 in the peat dome margin and between 27 and 76 g C m−2 years−1 in the center, which were identical to mangrove peat. These data suggest that peat swamp forests have played a significant role as places for carbon sequestration belowground as well as mangrove forest with mangrove peat.

Carbon storage processes in mangrove ecosystems are summarized and future research directions are discussed based on findings from our long-term monitoring studies on Pohnpei Island in the Federated States of Micronesia. On Pohnpei, where coral reef-type mangrove forests dominate, Rhizophora communities maintain their habitat by accumulating mangrove peat at over 5 mm year-1 in response to rapid sea-level rise, but surface erosion is progressing in communities where the tree density of Rhizophora spp. has declined through succession. However, high-resolution aerial photographs taken by drones have identified trees with reduced vigor even in Rhizophora forests, and if sea-level rise occurs at a rate close to the IPCC's maximum prediction, then Rhizophora forests, which are valuable carbon storage sites due to mangrove peat accumulation, are likely to disappear. The impact of relative sea-level rise is determined by the sum of the rate of ground-level change by the external sediment budget and the rate of ground-level rise with mangrove peat accumulation. In the future, each region will need to conduct its own quantitative evaluation.

Carbon export from mangrove forests to the oceans partly acts as a sink for atmospheric CO₂, exceeding the rate of carbon burial in mangrove soils. Primary production in ecosystems adjacent to mangroves may prevent degassing and enhance further carbon export from mangroves to the oceans. In this study, we continuously monitored carbonate chemistry parameters (pCO₂, dissolved inorganic carbon (DIC), total alkalinity (TA)) and dissolved organic carbon (DOC) in a tidal flat adjacent to a fringe mangrove forest over a spring-neap tidal cycle. Mean pCO₂ during the entire period was 923 ± 318 μatm, and the export of TA, DIC, and DOC from the mangroves to the ocean was 36 ± 26 mmol m⁻² d⁻¹, 42 ± 39 mmol m⁻² d⁻¹, and 10 ± 9 mmol m⁻² d⁻¹, respectively. Semi-monthly pCO₂ variations in the mangrove front were controlled by the tidal level during spring tide and by photosynthesis and respiration on the tidal flat during neap tide. This means that during neap tide, photosynthesis on the tidal flat offset the increase in pCO₂ caused by the porewater export from the mangrove soil. The DIC/TA export ratio in this study was 1.17 ± 0.08, which was lower than the global average of 1.41 ± 1.39, indicating that the tidal flat adjacent to the mangrove forest may act as a buffer zone to mitigate the increase in pCO₂, resulting in much of the exported DIC being stored in the ocean.