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Despite the recognized organic carbon (OC) sequestration potential of mangrove forests, the ongoing climate change and anthropogenic disturbances pose a great threat to these ecosystems. However, we currently lack the ability to mechanically understand and predict the consequences of such impacts, primarily because mechanisms underlying OC stabilization in these ecosystems remain elusive. Research into OC stabilization has focused on terrestrial soils and marine sediments for decades, overlooking the vegetated coastal ecosystems including mangroves. In terrestrial soils and marine sediments, it is widely accepted that OC stabilization is the integrated consequence of OM’s inherent recalcitrance, physical protection, and interactions with minerals and metals. However, related discussion is rarely done in mangrove soils, and recalcitrance of roots and high net ecosystem production (high primary production and low heterotrophic respiration) have been considered as a primary OC sequestration mechanism in mangrove peat and mineral soils, respectively. This review presents the available information on the mechanisms underlying OC stabilization in mangrove soils and highlights research questions that warrant further investigation. Primary OC stabilization mechanisms differ between mangrove peat and mineral soils. In mangrove mineral soils, physico-chemical stabilization processes are important, yet grossly understudied OC stabilization mechanisms. In mangrove peat, recalcitrance of mangrove roots and the inhibition of phenoloxidase under the anoxic condition may be the primary OC stabilization mechanisms. Salinity-induced OC immobilization likely plays a role in both type of soils. Finally, this review argues that belowground production and allochthonous inputs in mangrove forests are likely underestimated. More studies are needed to constrain C budgets to explain the enigma that mangrove OC keeps accumulating despite much higher decomposition (especially by large lateral exports) than previously considered.

The significance of aquatic lateral carbon (C) export in mangrove ecosystems highlights the extensive contribution of aquatic pathways to the net ecosystem carbon budget. However, few studies have investigated lateral fluxes of dissolved organic carbon (DOC) and inorganic carbon (DIC), partly due to methodological difficulty. Therefore, we evaluated area-based lateral C fluxes in a small mangrove estuary that only had one exit for water exchange to the coast. We sampled water from the mouth of the creek and integrated discharge and consecutive concentration of mangrove-derived C (&Delta, C). Then, we estimated the area-normalized C fluxes based on the inundated mangrove area. DIC and DOC concentrations at the river mouth increased during ebb tide during both summer and winter. We quantified the &Delta, C in the estuary using a two-component conservative mixing model of freshwater and seawater. DIC and DOC proportions of &Delta, C concentrations at the river mouth during ebb tide was between 34% and 56% in the winter and 26% and 42% in the summer, respectively. DIC and DOC fluxes from the estuary were estimated to be 1.36 g C m&minus, 2 d&minus, 1 and 0.20 g C m&minus, 1 in the winter and 3.35 g C m&minus, 1 and 0.86 g C m&minus, 1 in the summer, respectively. Based on our method, daily fluxes are mangrove area-based DIC and DOC lateral exports that can be directly incorporated into the mangrove carbon budget.