Abstract
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.
Highlights
Recent research has highlighted the organic matter (OM) sequestration potential of vegetated coastal ecosystems such as seagrass meadows, salt marshes, and mangrove forests
Peat formation and accumulation occur in some mangrove forests
Saraswati et al showed that, using mangrove peat from the Everglades, peat supplemented with lignin-derived phenol solutions had significantly lower hydrolase activities, peat samples had significantly higher phenol oxidase activity under aerobic conditions, and peat supplemented with phenol oxidase had significantly lower (−8.3%) phenolic concentration [66]
Summary
Recent research has highlighted the organic matter (OM) sequestration potential of vegetated coastal ecosystems such as seagrass meadows, salt marshes, and mangrove forests. In marine sediments, a physical diffusion model has suggested that the encapsulation of OM by clay materials plays a key role in determining the degradation rate of OM [33] In soils, these mechanisms have been suggested to be several times more important than the inherent recalcitrance of roots [34]. Forests 2020, 11, x FOR PEER REVIEW of 14 persistence of soil C [28,31,35] Under this new paradigm, intrinsic recalcitrance may 3play only a secondary role and physico-chemical stabilization primarily determines the long-term persistence mechanisms have been suggested to be several times more important than the inherent recalcitrance of soil C.
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