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Restoring degraded freshwater wetlands may help to maximize soil carbon sequestration. In this study, we use 18 Pb-210-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 similar to 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 similar to 8% of the CO2 emission from fossil fuels in Australia. Hence, preserving or restoring CASS may be a good climate change mitigation strategy.

Plain Language Summary Coastal freshwater wetlands are highly productive and can sequester large amounts of carbon. The carbon accumulation rates here suggest that coastal acid sulfate soils (CASS) are also a substantial sink of CO2. By extrapolating our observations, we show that approximately 7.79 Tg of carbon is sequestered in Australian CASS wetlands annually which is equivalent to similar to 8% of the CO2 emission from fossil fuels in Australia. Hence, managing CASS wetlands to protect their soil carbon store function may help offset anthropogenic sources of greenhouse gasses and their hydrological remediation could be an important component to the global carbon cycle.