###### Abstract  
Carbon cycling within the deep mangrove forest floor is unique compared to other marine ecosystems with organic carbon input, mineralization, burial, and advective and groundwater export pathways being in non-steady-state, often oscillating in synchrony with tides, plant uptake, and release/uptake via roots and other edaphic factors in a highly dynamic and harsh environment. Rates of soil organic carbon (C\({}_{\text{ORG}}\)) mineralization and belowground C\({}_{\text{ORG}}\) stocks are high, with rapid diagenesis throughout the deep (>1 m) soil horizon. Pocketed with cracks, fissures, extensive roots, burrows, tubes, and drainage channels through which tidal waters percolate and drain, the forest floor sustains non-steady-state diagenesis of the soil C\({}_{\text{ORG}}\), in which decomposition processes at the soil surface are distinct from those in deeper soils. Aerobic respiration occurs within the upper 2 mm of the soil surface and within biogenic structures. On average, carbon respiration across the surface soil-air/water interface (104 mmol C m\({}^{-2}\) d\({}^{-1}\)) equates to only 25% of the total carbon mineralized within the entire soil horizon, as nearly all respired carbon (569 mmol C m\({}^{-2}\) d\({}^{-1}\)) is released in a dissolved form via advective porewater exchange and/or lateral transport and subsurface tidal pumping to adjacent tidal waters. A carbon budget for the world's mangrove ecosystems indicates that subsurface respiration is the second-largest respiratory flux after canopy respiration. Dissolved carbon release is sufficient to oversaturate water-column \(p\)CO\({}_{2}\), causing tropical coastal waters to be a source of CO\({}_{2}\) to the atmosphere. Mangrove dissolved inorganic carbon (DIC) discharge contributes nearly 60% of DIC and 27% of dissolved organic carbon (DOC) discharge from the world's low latitude rivers to the tropical coastal ocean. Mangrove inhabit only 0.3% of the global coastal ocean area but contribute 55% of air-sea exchange, 14% of C\({}_{\text{ORG}}\) burial, 28% of DIC export, and 13% of DOC + particulate organic matter (POC) export from the world's coastal wetlands and estuaries to the atmosphere and global coastal ocean. biogeochemistry; carbon; carbon cycling; coastal ocean; export; non-steady-state; mangrove; mangrove carbon budget; moil; subsurface transport [color=black]0.0000-0.0000-0.00000-0.00000-0. Mangroves function like other forested ecosystems in exchanging gases with the atmosphere but are like other coastal tidal wetlands in exchanging gases and dissolved and particulate materials with adjacent coastal waters. Mangrove ecosystems are thus tightly linked to land, ocean, and atmosphere, yet still, store vast quantities of CORG and other elements in their soils and forest biomass. While mangroves have some features of both terrestrial and coastal ecosystems, they do have a number of unique ecological and physiological characteristics [4] that enable them to efficiently utilize and sequester carbon and nutrient elements, especially nitrogen, phosphorus, iron, and copper [5; 6]. The biogeochemical connections between mangroves and adjacent tidal waters are complex, with variable concentrations of dissolved organic and inorganic solutes and particulates imported and exported by both tides, porewater pumping, and subsurface groundwater advection [7]. Tidal circulatory processes, such as a pronounced asymmetry between ebb and flood tides (with the ebb tide being shorter in duration but with stronger current velocity than the flood tide); friction, flow resistance, drag of tidal waters due to the presence of aboveground and belowground structures, lateral trapping (water flowing in and out of a waterway is temporarily retained in the forest to be returned to the main channel later), and lateral gradients due to high evapotranspiration [8].