Mangroves are one of the most carbon‐dense forests on the Earth and have been highlighted as key ecosystems for climate change mitigation and adaptation. Hundreds of studies have investigated how mangroves fix, transform, store, and export carbon. Here, we review and synthesize the previously known and emerging carbon pathways in mangroves, including gains (woody biomass accumulation, deadwood accumulation, soil carbon sequestration, root and litterfall production), transformations (food web transfer through herbivory, decomposition), and losses (respiration as CO₂ and CH₄, litterfall export, particulate and dissolved carbon export). We then review the technologies available to measure carbon fluxes in mangroves, their potential, and their limitations. We also synthesize and compare mangrove net ecosystem productivity (NEP) with terrestrial forests. Finally, we update global estimates of carbon fluxes with the most current values of fluxes and global mangrove area. We found that the contributions of recently investigated fluxes, such as soil respiration as CH₄, are minor (35 Tg C year⁻¹). Dissolved inorganic carbon exports are an order of magnitude higher than the other processes investigated and were highly variable, highlighting the need for further studies. Gross primary productivity (GPP) and ecosystem respiration (ER) per area of mangroves were within the same order of magnitude as terrestrial forests. However, ER/GPP was lower in mangroves, explaining their higher carbon sequestration. We estimate the global mean mangrove NEP of 109.1 Tg C year⁻¹ (7.4 Mg C ha⁻¹ year⁻¹) or through a budget balance, accounting for lateral losses, a global mean of 66.6 Tg C year⁻¹ (4.5 Mg C ha⁻¹ year⁻¹). Overall, mangroves are highly productive, and despite losses due to respiration and tidal exchange, they are significant carbon sinks.

Coastal wetland restoration is an important activity to achieve greenhouse gas (GHG) reduction targets, improve water quality, and reach the Sustainable Development Goals. However, many uncertainties remain in connection with achieving, measuring, and reporting success from coastal wetland restoration. We measured levels of carbon (C) abatement and nitrogen (N) removal potential of restored coastal wetlands in subtropical Queensland, Australia. The site was originally a supratidal forest composed of Melaleuca spp. that was cleared and drained in the 1990s for sugarcane production. In 2010, tidal inundation was reinstated, and a mosaic of coastal vegetation (saltmarshes, mangroves, and supratidal forests) emerged. We measured soil GHG fluxes (CH₄, N₂O, CO₂) and sequestration of organic C in the trees and soil to estimate the net C abatement associated with the reference, converted, and restored sites. To assess the influence of restoration on water quality improvement, we measured denitrification and soil N accumulation. We calculated C abatement of 18.5 Mg CO_2−eq ha⁻¹ year⁻¹ when sugarcane land transitioned to supratidal forests, 11.0 Mg CO_2−eq ha⁻¹ year⁻¹ when the land transitioned to mangroves, and 6.2 Mg CO_2−eq ha⁻¹ year⁻¹ when the land transitioned to saltmarshes. The C abatement was due to tree growth, soil accumulation, and reduced N₂O emissions due to the cessation of fertilization. Carbon abatement was still positive, even accounting for CH₄ emissions, which increased in the wetlands due to flooding and N₂O production due to enhanced levels of denitrification. Coastal wetland restoration in this subtropical setting effectively reduces CO₂ emissions while providing additional cobenefits, notably water quality improvement.

Coastal wetlands are essential for regulating the global carbon budget through soil carbon sequestration and greenhouse gas (GHG – CO₂, CH₄, and N₂O) fluxes. The conversion of coastal wetlands to agricultural land alters these fluxes' magnitude and direction (uptake/release). However, the extent and drivers of change of GHG fluxes are still unknown for many tropical regions. We measured soil GHG fluxes from three natural coastal wetlands – mangroves, salt marsh, and freshwater tidal forests – and two alternative agricultural land uses – sugarcane farming and pastures for cattle grazing (ponded and dry conditions). We assessed variations throughout different climatic conditions (dry–cool, dry–hot, and wet–hot) within 2 years of measurements (2018–2020) in tropical Australia. The wet pasture had by far the highest CH₄ emissions with 1231±386 $\mathrm{mg}{\mathrm{m}}^{-\mathrm{2}}{\mathrm{d}}^{-\mathrm{1}}$ , which were 200-fold higher than any other site. Dry pastures and sugarcane were the highest emitters of N₂O with 55±9 $\mathrm{mg}{\mathrm{m}}^{-\mathrm{2}}{\mathrm{d}}^{-\mathrm{1}}$ (wet–hot period) and 11±3 m$\mathrm{g}{\mathrm{m}}^{-\mathrm{2}}{\mathrm{d}}^{-\mathrm{1}}$ (hot-dry period, coinciding with fertilisation), respectively. Dry pastures were also the highest emitters of CO₂ with 20±1 $\mathrm{g}{\mathrm{m}}^{-\mathrm{2}}{\mathrm{d}}^{-\mathrm{1}}$ (wet–hot period). The three coastal wetlands measured had lower emissions, with salt marsh uptake of $-\mathrm{0.55}\pm \mathrm{0.23}$ and $-\mathrm{1.19}\pm \mathrm{0.08}$ $\mathrm{g}{\mathrm{m}}^{-\mathrm{2}}{\mathrm{d}}^{-\mathrm{1}}$ of N₂O and CO₂, respectively, during the dry–hot period. During the sampled period, sugarcane and pastures had higher total cumulative soil GHG emissions (CH₄+N₂O) of 7142 and 56 124 ${\mathrm{CO}}_{\text{2-eq}}\mathrm{kg}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ compared to coastal wetlands with 144 to 884 ${\mathrm{CO}}_{\text{2-eq}}\mathrm{kg}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ (where CO_2-eq is CO₂ equivalent). Restoring unproductive sugarcane land or pastures (especially ponded ones) to coastal wetlands could provide significant GHG mitigation.

Coastal wetlands have long been identified as ecosystems that can ameliorate N inputs into the ocean. The processes associated with N uptake, transformation, and losses are relatively well understood for temperate wetlands; however, information on tropical wetlands is scarce. In this study, we conducted a whole ecosystem approach to measure N processes within tropical mangroves in the Moresby estuary in northeast Australia. We measured N stocks (trees and soils), inputs from sedimentation, fixation, and accumulation as woody biomass, and outputs through denitrification, anammox, and soil respiration (N₂O emissions). Potential denitrification was detected along anammox (average, min‐max) at 883 (485–1,450) gN ha⁻¹ day⁻¹, followed by sediment accumulation with 108 (0–375) gN ha⁻¹ day⁻¹, and tree uptake with 93 (13–153) gN ha⁻¹ day⁻¹. Lower rates were found for N fixation with 45 (0–260) gN ha⁻¹ day⁻¹ and soil respiration as N₂O with uptakes of −0.36 (−2.7 – 0.40) gN ha⁻¹ day⁻¹. Overall, mangroves in the Moresby estuary are fixing some N in their standing litter while removing NO₃ and NH₄⁺ from the water column through denitrification and anammox, temporarily storing N as woody biomass, and accumulating particulate N in their sediments. These mangroves are also functioning as sinks of N₂O. Thus, the protection and restoration of these mangroves provide water quality and climate benefits.

Tidal coastal wetlands are significant to the global carbon budgets through carbon sequestration and greenhouse gas (GHG; CO₂, CH₄ and N₂O) emissions. The conversion of tidal coastal wetlands to agriculture land alters soil processes changing GHG emissions. The GHG emissions associated with land-use change are important for restoration strategies that rely upon financial incentives such as carbon credits. We measured GHG fluxes from mangroves, saltmarsh and freshwater tidal forest and their alternative agricultural lands including sugarcane and ponded pastures. We investigated seasonal variations between June 2018 and February 2020 in tropical. Australia. The wet ponded pasture had by far the highest CH₄ emissions with 1,231 ± 386 mg m⁻² d⁻¹, which were 200-fold higher than any other land use. Agricultural lands were the most significant sources of N₂O emissions with 55 ± 9 mg m⁻² d⁻¹ from dry ponded pasture (wet-hot period) and 11 ± 3 mg m⁻² d⁻¹ from sugar cane (hot-dry period), coinciding with fertilisation. The N₂O fluxes from the tidal coastal wetlands ranged between −0.55 ± 0.23 and 2.76 ± 0.45 mg m⁻² d⁻¹ throughout the study period. The highest CO₂ fluxes of 20 ± 1 g m⁻² d⁻¹ were from the dry ponded pasture during the wet-hot period, while the saltmarsh had the lowest CO₂ fluxes having an uptake of −1.19 ± 0.08 g m⁻² d⁻¹ in the dry-hot period. Overall, agricultural lands had significantly higher total cumulative GHG emissions (CH₄ + N₂O) of 7142 to 56,124 CO_2-eq kg ha⁻¹ y⁻¹ compared to those of any type of tidal coastal wetlands, which ranged between 144 and 884 CO_2-eq kg ha⁻¹ y⁻¹. Converting agricultural land, particularly wet ponded pasture, to tidal coastal wetlands could provide large GHG mitigation gains and potential financial incentives.