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Global mangrove loss is occurring from deforestation and extreme climatic events, but changes to the coastal carbon cycle following mangrove mortality and/or loss are not well understood. In 2015/2016, a massive climate-driven mangrove dieback event occurred over~1000 km of Australian coastline. To assess carbon loss following mortality, carbon fluxes in adjacent living and dead forest areas were compared 8 and 20 months postforest dieback. Dead areas experienced an increase in soil CO 2 efflux by~189%, and a decrease in oceanic dissolved inorganic carbon (DIC) outwelling of~50% relative to living areas. DIC outwelling (predominantly carbonate alkalinity) and soil CO 2 efflux accounted for 81% and 16% of losses from the living forest, in comparison to 51% and 47%, respectively, from the dead forest. The dieback drove a shift from a dominance of oceanic carbon outwelling to increased atmospheric CO 2 emissions and decreased alkalinity exports. This shift was likely driven by increased oxygen sediment permeation and the loss of mangrove net primary productivity. Combining our new observations with literature data, we found a logarithmic relationship between soil carbon loss and time since mangrove loss. Using this relationship, we estimate ongoing global carbon losses from historical mangrove deforestation and dieback could be 13.7 AE 9.4 Tg C yr −1 , which is eightfold higher than previous estimates and offsets global mangrove carbon burial by~60%. Even if no future deforestation occurred, we estimate ongoing carbon losses to the atmosphere and ocean from current global mangrove losses of 27 Tg C over the next 30 yr.

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Abstract. Hypersaline tidal flats (HTFs) are coastal ecosystems with freshwater deficits often occurring in arid or semi-arid regions near mangrove supratidal zones with no major fluvial contributions. Here, we estimate that organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) were buried at rates averaging 21 (±6), 1.7 (±0.3) and 1.4 (±0.3) gm-2yr-1, respectively, during the previous century in three contrasting HTF systems, one in Brazil (eutrophic) and two in Australia (oligotrophic). Although these rates are lower than those from nearby mangrove, saltmarsh and seagrass systems, the importance of HTFs as sinks for OC, TN and TP may be significant given their extensive coverage. Despite the measured short-term variability between net air–saltpan CO2 influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal CO2 flux measurements, the OC sedimentary profiles over several decades suggest efficient OC burial at all sites. Indeed, the stable isotopes of OC and TN (δ13C and δ15N) along with C:N ratios show that microphytobenthos are the major source of the buried OC in these HTFs. Our findings highlight a previously unquantified carbon as well as a nutrient sink and suggest that coastal HTF ecosystems could be included in the emerging blue carbon framework.

Restoring degraded freshwater wetlands may help to maximize soil carbon sequestration. In this study, we use 18 210Pb‐dated sediment cores to determine the organic carbon (OC) accumulation rates from two hydrologically restored freshwater coastal acid sulfate soil (CASS) wetlands. Recent OC accumulation rates (from ~1980 to present) were estimated to be 251 ± 26 g·m−2·year−1 in the seasonally inundated CASS and 227 ± 50 g·m−2·year−1 in the permanently inundated CASS. The average OC accumulation during the previous century (190 ± 20 g·m−2·year−1) was within the range of blue carbon ecosystems (saltmarshes, mangroves, and seagrasses). Considering their large area and carbon accumulation rate, we estimate that Australian CASS wetlands sequester approximately 7.8 ± 0.8 Tg of carbon annually, which is equivalent to ~8% of the CO2 emission from fossil fuels in Australia. Hence, preserving or restoring CASS may be a good climate change mitigation strategy.