Soil greenhouse gas fluxes during wetland forest retreat along the Lower Savannah River, Georgia (USA)

Tidal freshwater forested wetlands (tidal swamps) are periodically affected by salinity intrusion at seaward transitions with marsh, which, along with altered hydrology, may affect the balance of gaseous carbon (C) and nitrogen (N) losses from soils. We measured greenhouse gas emissions (CO₂, CH₄, N₂O) from healthy, moderately degraded, and degraded tidal swamp soils undergoing sea-level-rise-induced retreat along the lower Savannah River, Georgia, USA. Soil CO₂ flux ranged from 90.2 to 179.1 mg CO₂ m^-2 h^-1 among study sites, and was the dominant greenhouse gas emitted. CO₂ flux differed among sites in some months, while CH₄ and N₂O fluxes were 0.18 mg CH₄ m^-2 h^-1 and 1.23 μg N₂O m^-2 h^-1, respectively, with no differences among sites. Hydrology, soil temperature, and air temperature, but not salinity, controlled the annual balance of soil CO₂ emissions from tidal swamp soils. No clear drivers were found for CH₄ or N₂O emissions. On occasion, large ebbing or very low tides were even found to draw CO₂ fluxes into the soil (dark CO₂ uptake), along with CH₄ and N₂O. Overall, we hypothesized a much greater role for salinity and site condition in controlling the suite of greenhouse gases emitted from tidal swamps than we discovered, and found that CO₂ emissions–not CH₄ or N₂O–contributed most to the global warming potential from these tidal swamp soils.