# Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis
## 6 Whole-Ecosystem Carbon Mass Balance
); (2) other sources of allochthonous carbon, such as marine and terrestrial inputs are required to balance carbon flow; and/or (3) centuries-old \(\text{C}_{\text{ORG}}\) is also being mineralized, such as \(\text{C}_{\text{ORG}}\) buried prior to when the mangroves inhabited the unvegetated mudflat. As the export of DIC, DOC and \(\text{CH}_{4}\) from mangrove subsurface soils to adjacent tidal waters (175 Tg C a\({}^{-1}\)) is within the error estimate of the measured rates of total soil \(\text{C}_{\text{ORG}}\) mineralization (158 Tg C a\({}^{-1}\)), it is likely that allochthonous sources of carbon are important in mangrove carbon flow, such as imports from adjacent seagrass beds and coastal plankton. Additionally, the budget does not account for carbon fixed by benthic cyanobacterial mats and other nitrogen-fixing biota on the forest floor, roots and rhizomes, tree stems, litter and downed wood; these are very productive assemblages but too patchy in distribution to extrapolate beyond a square meter. Figure 3: Mass balance of mean carbon flow through the world’s mangrove forests. Units are Tg C a\({}^{-1}\). The budget assumes a global mangrove area of 86,495 km\({}^{2}\)[94]. Solid blue arrows depict mean values based on empirical data (see text for explanation and references). Dashed red arrows represent mean values estimated indirectly by difference. The \(\text{C}_{\text{ORG}}\) pool (both live and dead roots and other organic matter) in soils to a depth of 1 m is presented in a box in the forest floor with units of Tg C\({}_{\text{ORG}}\). Abbreviations: GPP = gross primary production; NPP = net primary production; \(\text{R}_{\text{A}}\) = algal production at soil surface and aboveground biomass; \(\text{R}_{\text{C}}\) = canopy respiration; \(\text{R}_{\text{S}}\) = soil respiration at soil surface; \(\text{R}_{\text{WATER}}\) = waterway respiration; POC = particulate organic carbon derived from litter and exported to tidal waters; DIC = dissolved inorganic carbon; DOC = dissolved organic carbon; \(\text{CH}_{4}\) = methane; EDOC = exchangeable dissolved organic carbon. Further, buried \(\text{C}_{\text{ORG}}\) is likely to be a source of respired carbon since measurements of isotopes in a subtropical Australian mangrove indicate that carbon deposited centuries ago is still susceptible to decomposition and subsequent tidal export [190]. Incubation experiments with coastal wetlands have also indicated the potential for sea-level rise to increase remineralization of previously buried carbon [191]. In both ecosystems, \(p\text{CO}_{2}\) and \(p\text{CH}_{4}\) supersaturation of adjacent tidal waters leads to significant \(\text{CO}_{2}\) and \(\text{CH}_{4}\) release to the atmosphere. The mean rates of total soil mineralization imply that the turnover time of the entire soil \(\text{C}_{\text{ORG}}\) pool (including dead and live roots) is in the order of 25 years for mangroves and 20 years for salt marshes, which is in agreement with the fact that mangrove roots decompose more slowly than marsh plant roots [189] and that most mangrove and salt marsh soil organic matter is composed of higher plant-derived material high in lignocellulosic and hemicellulose that decomposes slowly [189]. Mangrove discharge of dissolved carbon contributes nearly 60% of DIC and 27% of DOC export from the world's tropical rivers to the coastal ocean, based on comparison with the river export data in [PERSON] et al. [192]. Salt marshes and mangrove forests each inhabit only about 0.3% of global coastal ocean area, but, respectively, contribute 17% and 55% for a combined contribution of 72% of air-sea \(\text{CO}_{2}\) exchange from the world's wetlands and estuaries [193]. Salt marshes and mangrove ecosystems, respectively, export 6% and 28% of DIC export and 4% and 13% of DOC + POC export to the world's coastal ocean [194]. Thus, both wetland ecosystems contribute disproportionately to carbon flow in the global coastal ocean. Figure 4: Mass balance of mean carbon flow through the world’s salt marshes. Units are Tg C a\({}^{-1}\).