Soil Greenhouse Gas Fluxes during Wetland Forest Retreat along the Lower Savannah River, Georgia (USA)

Abstract

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.