CARBON-DIOXIDE AND METHANE EXPORTS FROM A SOUTHEASTERN FLOODPLAIN SWAMP

Patterns and rates of generation of CO2 and CH4 by aerobic and anaerob ic soil respiration are a significant gap in knowledge of floodplain c arbon dynamics. Gaseous and hydrologic exports of CO2 and CH4 from the forested floodplain of the Ogeechee River in Georgia, USA, were studi ed from July 1987 to September 1989. Net emissions to the atmosphere w ere measured with short(10-20 min) static chamber incubations. CO2 emi ssions were highly seasonal, with largest rates during summer, and wer e strongly correlated with soil temperatures. Annual total CO2 emissio ns were similar in both years of the study, and averaged 919 g/m(2). T he contribution of live root respiration to this total was estimated w ith in situ incubations of attached roots excavated from the soil. Ove r 55% of the total CO2 flux appeared to arise from live roots rather t han mineralization of soil organic matter. Significant atmospheric CH4 fluxes were found only at flooded sites. CH4 emissions were highly va riable, with high rates of release of methane carbon (up to 271 mg.m(- 2).d(-1)) occurring irregularly during the warmer months. The temperat ure effect on CH4 emissions appeared to be a step function. No signifi cant CH4 emissions occurred when soil temperatures were below 15 degre es C; during warmer periods emission rates were generally positive, bu t showed no additional correlation with temperature. This effect is hy pothesized to represent an interaction of the rates of oxygen consumpt ion and replenishment in the soil. Spatial variability in CH4 emission s was also large. Over 90% of the total floodplain CH4 emissions came from the 30% of the floodplain that was most frequently inundated. Ann ual total fluxes of CH4 carbon from low habitats averaged 17 g.m(-2).y r(-1). Methane oxidation rates were estimated in floodplain surface wa ters by in situ bottle incubations. Roughly half of the CH4 that enter ed the water column was consumed without reaching the atmosphere.