Lateral Export and Sources of Subsurface Dissolved Carbon and Alkalinity in Mangroves: Revising the Blue Carbon Budget

Mangroves are carbon-rich ecosystems that store large quantities of carbon, mostly in soils. Early carbon (C) budgets indicated that >50% of mangrove C fixation was unaccounted for. This 'missing C' has now been discovered to be a large release (423 Tg C a(-1)) of porewater dissolved DIC (dissolved inorganic carbon), dissolved organic carbon (DOC), and total alkalinity (17 T-MOL a(-1)) via lateral export derived from bacterial decomposition of soil organic matter. This large export originates from DIC produced over at least a 1.0-1.5 m soil profile (280-420 Tg C a(-1)) via decomposition of autochthonous and allochthonous inputs and/or likely mineralization in deep (>= 1 m) ancient soils. DOC and DIC export from mangroves equate to 41% and approximate to 100% of export from the world's tropical rivers, respectively. A newly revised blue carbon budget for the world's mangroves indicates a mean ecosystem gross primary production (GPP(E)) to ecosystem respiration (R-E) ratio of 1.35 and a net ecosystem production (NEP) of 794 g C m(-2) a(-1) (= global NEP of 117 Tg C a(-1)), reflecting net autotrophy. C-ORG burial is 5% and 9% of GPP(E) and NEPE, respectively. Mean R(E/)GPP(E) is 0.74 and carbon use efficiency averages 0.57, higher than for tropical humid forests (0.35).

Automated summary

Mangroves are carbon-rich ecosystems that release a large amount of dissolved organic carbon, dissolved inorganic carbon, and total alkalinity via lateral export. This export accounts for a significant portion of carbon output from tropical rivers. The revised blue carbon budget indicates that mangroves have high net autotrophy and carbon use efficiency.