Mangroves’ ability to store carbon (C) has long been recognized, but little is known about whether planted mangroves can store C as efficiently as naturally established (i.e., intact) stands and in which time frame. Through Bayesian logistic models compiled from 40 years of data and built from 684 planted mangrove stands worldwide, we found that biomass C stock culminated at 71 to 73% to that of intact stands ~20 years after planting. Furthermore, prioritizing mixed-species planting including Rhizophora spp. would maximize C accumulation within the biomass compared to monospecific planting. Despite a 25% increase in the first 5 years following planting, no notable change was observed in the soil C stocks thereafter, which remains at a constant value of 75% to that of intact soil C stock, suggesting that planting effectively prevents further C losses due to land use change. These results have strong implications for mangrove restoration planning and serve as a baseline for future C buildup assessments.

Tidal transport from coastal wetlands (“outwelling”), together with riverine fluxes, provide the most important sources of terrigenous organic matter (OM) to the ocean. The flux of dissolved organic carbon (DOC) from the mangrove swamps accounts for 10% of the terrestrial DOC flux to the coastal water. This study examines the sources, distribution, and export of dissolved OM at interannual, seasonal, and diurnal bases along the estuaries located at the Sundarbans, the world’s largest deltaic mangrove and heritage site. Sampling was carried out from the riverine (Hooghly) and mangrove-dominated tidal estuaries (Saptamukhi, Thakuran, Matla), covering all three seasons (pre-monsoon, monsoon, and post-monsoon) during 2012–2017. DOC varied at a broad range, from 109 to 462 μM (n = 146), with higher concentration observed in the Hooghly (383 ± 120 μM, n = 35) than the mangrove estuaries (246 ± 82 to 266 ± 120 μM, n = 111). Non-conservative mixing of DOC along the salinity gradient attested to mangrove input, particularly in the polyhaline waters. Upper and mid estuarine zones of the mangrove estuaries showed slightly higher DOC concentration (270 ± 92 μM, n = 84, salinity range 18–25) than in the mouth (250 ± 85 μM, n = 27, salinity range 26–27), because of the dilution with marine waters having low DOC concentration and shorter water residence time downstream. Seasonally, higher DOC concentration during the post-monsoon might be linked to higher litterfall, promoting leaching of organic compounds to the water. In that connection, colored dissolved organic matter (CDOM) could be a by-product of mangrove litter leaching, and its absorption coefficient (at 350 nm) exhibited non-conservative mixing pattern at wide ranges from 2.5 to 7.6 m⁻¹ (n = 40). CDOM enrichment was observed in the surface water during the low tide when outwelling maximized. Overall, the central and eastern parts of the Indian Sundarbans showed enrichment of more terrigenous type CDOM (evident from optical proxies, e.g., S_275–295 and SUVA₂₅₄) than the western part, probably due to greater mangrove productivity in the eastern side. Flux estimates of DOM resulted in higher yield and export of mangrove-derived DOC but lower export of CDOM to the Bay of Bengal as compared to their riverine transport.

Mangrove forest plays a key role in regulating climate change, earth carbon cycling and other biogeochemical processes within blue carbon ecosystems. Therefore, mangrove forests should be incorporated into Earth system climate models with the aim of understanding future climate change. Despite multiple carbon stock and flux assessments taking place over the past couple of decades, concrete knowledge of carbon source/sink patterns is largely lacking, particularly in the biodiversity-rich Asia-Pacific (AP) region with its 68 493 km² of mangrove area. Thus, to understand the gaps in mangrove blue carbon research in the AP region, we summarize a recent decade-long inventory of carbon stock pools (aboveground, belowground and soil) and biogeochemical flux components (burial, export/import, soil-air and water-air CO₂ flux) across 25 AP countries to understand the current knowledge and gaps. While carbon stock assessments of individual components are available for all 25 countries, whole ecosystem carbon stocks—including live and standing dead aboveground and belowground, downed woody debris and soil carbon stocks—are often lacking, even in highly researched countries like Indonesia. There is restricted knowledge around biogeochemical carbon fluxes in 55% of the countries, suggesting poor carbon flux research across the region. Focusing on flux components, reports on sediment-to-sea carbon exports are extremely limited (coming from just nine countries in the AP region). There is notable scarcity of data on carbon export fluxes in Indonesian mangroves. Given the key role AP mangroves play in climate change mitigation worldwide, more detailed and methodologically comparable investigation of biogeochemical source/sink processes is required to better understand the role of this large carbon source in global carbon stocks and fluxes, and hence, global climate.