Support for coastal wetland restoration projects that consider carbon (C) storage as a climate mitigation benefit is growing as coastal wetlands are sites of substantial C sequestration. However, the climate footprint of wetland restoration remains controversial as wetlands can also be large sources of methane (CH4). We quantify the vertical fluxes of C in restored fresh and oligohaline nontidal wetlands with managed hydrology and a tidal euhaline marsh in California's San Francisco Bay-Delta. We combine the use of eddy covariance atmospheric flux measurements with 210Pb-derived soil C accumulation rates to quantify the C sequestration efficiency of restored wetlands and their associated climate mitigation service. Nontidal managed wetlands were the most efficient in burying C on-site, with soil C accumulation rates as high as their net atmospheric C uptake (−280 ± 90 and −350 ± 150 g C m−2 yr−1). In contrast, the restored tidal wetland exhibited lower C burial rates over decadal timescales (70 ± 19 g C m−2 yr−1) that accounted for ∼13%–23% of its annual C uptake, suggesting that the remaining fraction is exported via lateral hydrologic flux. From an ecosystem radiative balance perspective, the restored tidal wetland showed a > 10 times higher CO2-sequestration to CH4-emission ratio than the nontidal managed wetlands. Thus overall, tidal wetland restoration resulted in a negative radiative forcing (cooling) through increased soil C accumulation, while nontidal wetland restoration led to an early positive forcing (warming) through increased CH4 emissions potentially lasting between 2.1 ± 2.0 to 8 ± 4 decades.
PLAIN LANGUAGE SUMMARY
Coastal wetlands have great potential to remove carbon dioxide from the atmosphere and mitigate climate change. This study aims to understand how effectively restored wetlands bury carbon in soils and sequester it, and the extent to which they produce methane, a potent greenhouse gas. We measured how much carbon dioxide and methane flow into and out of three restored wetlands differing in their restoration design, salinity, and tidal influence. We found that most of the carbon dioxide removed from the atmosphere by nontidal wetlands was stored in their soils, while restored tidal wetland soils stored a smaller fraction (13%–23%) of the removed carbon. Despite the lower carbon sequestration efficiency, the restored tidal wetland was a greater greenhouse gas sink and climate intervention because it emitted very little methane. Methane emissions in nontidal freshwater and brackish marshes fully offset the carbon dioxide removed via carbon burial for roughly the first 2–8 decades, while the tidal wetland contributed to greenhouse gas removal immediately after restoration. The merits of nontidal managed wetland restoration lie in increased soil and C accretion, but it should not be assumed that soil carbon storage results in an immediate climate mitigation benefit.