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

摘要

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 (CO_2, CH_4, N_2O) from healthy, moderately degraded, and degraded tidal swamp soils undergoing sea-level-rise-induced retreat along the lower Savannah River, Georgia, USA. Soil CO_2 flux ranged from 90.2 to 179.1 mg CO_2 m~(-2) h~(-1) among study sites, and was the dominant greenhouse gas emitted. CO_2 flux differed among sites in some months, while CH_4 and N_2O fluxes were 0.18 mg CH_4 m~(-2) h~(-1) and 1.23 μg N_2O 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 CO_2 emissions from tidal swamp soils. No clear drivers were found for CH_4 or N_2O emissions. On occasion, large ebbing or very low tides were even found to draw CO_2 fluxes into the soil (dark CO_2 uptake), along with CH_4 and N_2O. 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 CO_2 emissions-not CH_4 or N_2O-contributed most to the global warming potential from these tidal swamp soils.