## 5 Carbon Flow through the World's Mangrove Ecosystems and Contributions to the Coastal Ocean
This time frame is supported by empirical findings that mangrove roots decompose slowly [106] and that mangrove soil organic matter is composed mostly of allochthonous, highly-refractory, plant-derived material that is high in lignocellulosic, and hemicellulose derived mostly from leaves [116] that decompose slowly [117]. About 58% of soil carbon is mangrove-derived, a value that comes from stable isotope signatures of mangrove soils [118], and about one-third of the total soil carbon pool is composed of dead roots [2; 10] that also decompose slowly [106]. Assuming that 4 Tg C a\({}^{-1}\) of litter is buried and that all POC export is derived from litter, then the remaining 16 Tg a\({}^{-1}\) of litter produced must fall to the forest floor where it is presumably incorporated into detritus food webs and eventually mineralized in situ. Wood that falls to the forest floor may be eventually incorporated into the soil pool, but decomposition is very  
Figure 5: A mass balance model of carbon flow through the world’s mangrove ecosystems, updated from [2]. Units are Tg C a\({}^{-1}\). The budget assumes a global mangrove area of 86,495 km\({}^{2}\)[116]. Solid blue arrows represent mean values based on empirical data (see text for explanation and references.) Dashed red arrows represent mean values estimated indirectly (by difference). The COrg pool (both roots and soil) in soils to a depth of 1 m is presented as a box in the forest floor with units of Tg C. Unquantified inputs of dissolved carbon from land-derived groundwater and organic matter inputs from adjacent marine and catchments are not depicted. Abbreviations: GPP = gross primary production; NPP = net primary production; RA\({}_{\text{A}}\) = algal respiration; RC\({}_{\text{C}}\) = canopy respiration; RS\({}_{\text{S}}\) = soil respiration at soil surface; RW\({}_{\text{WATER}}\) = waterway respiration; POC = particulate organic matter; DIC = dissolved inorganic carbon; DOC = dissolved organic carbon; CH\({}_{4}\) = methane; EDOC = exchangeable dissolved organic carbon. slow [119] and likely a minor flux, so it is not included in the mass balance; also not included are bentnos and zooplankton production and chemical defenses. The total soil mineralization equates to approximately 140% of mangrove NPP. This anomaly suggests that: (1) inputs from allochthonous marine and terrestrial sources are necessary to balance the mineralization outputs (158 Tg C a\({}^{-1}\)); (2) a large proportion of the soil pool and its subsequent decomposition is derived from the intertidal mudflat prior to mangrove colonization; (3) wood, algae, and fauna contribute to the soil pool; and/or (4) the subsurface soil mineralization rates and subsequent export data are overestimated. It is also conceivable that root production is underestimated given that the empirical dataset (mostly for estimates of fine roots) is small and that there are some methodological shortcomings in deriving production estimates [10; 107]. Analysis of the origin of mangrove soil organic matter indicates that about 42% of the organic matter may be derived from external sources [14; 118]. Measurements of radiogenic and stable isotopes in a subtropical Australian mangrove indicate that century-old sequestered carbon is still susceptible to remineralization and tidal export [120], supporting the idea that organic carbon deposited prior to mangrove colonization continues to be decomposed, as all mangroves colonize intertidal mudflats that have considerable amounts of soil C\({}_{\text{ORG}}\)[6]. Mangrove DIC export contributes nearly 60% of DIC, and 27% of DOC discharged from the world's tropical rivers to the coastal ocean, based on comparison with tropical riverine export values in [PERSON] et al. [121]. Mangroves inhabit only 0.3% of global coastal ocean area but contribute 55% of air-sea exchange compared with the global average [122], 28% of DIC export, 14% of C burial, and 13% of DOC + POC export, compared to global averages in [123] for the world's coastal ocean.