Carbon Dioxide and Methane Exports from a Southeastern Floodplain Swamp

Abstract

Patterns and rates of generation of CO2 and CH4 by aerobic and anaerobic soil respiration are a significant gap in knowledge of floodplain carbon dynamics. Gaseous and hydrologic exports of CO2 and CH4 from the forested floodplain of the Ogeechee River in Georgia, USA, were studied from July 1987 to September 1989. Net emissions to the atmosphere were measured with short (10—20 min) static chamber incubations. CO2 emissions were highly seasonal, with largest rates during summer, and were strongly correlated with soil temperatures. Annual total CO2 emissions were similar in both years of the study, and averaged 919 g/m2. The contribution of live root respiration to this total was estimated with in situ incubations of attached roots excavated from the soil. Over 55% of the total CO2 flux appeared to arise from live roots rather than mineralization of soil organic matter. Significant atmospheric CH4 fluxes were found only at flooded sites. CH4 emissions were highly variable, with high rates of release of methane carbon (up to 271 mg · m—2 · d—1) occurring irregularly during the warmer months. The temperature effect on CH4 emissions appeared to be a step function. No significant CH4 emissions occurred when soil temperatures were below 15°C; during warmer periods emission rates were generally positive, but showed no additional correlation with temperature. This effect is hypothesized to represent an interaction of the rates of oxygen consumption and replenishment in the soil. Spatial variability in CH4 emissions was also large. Over 90% of the total floodplain CH4 emissions came from the 30% of the floodplain that was most frequently inundated. Annual total fluxes of CH4 carbon from low habitats averaged 17 g · m—2 · yr—1. Methane oxidation rates were estimated in floodplain surface waters by in situ bottle incubations. Roughly half of the CH4 that entered the water column was consumed without reaching the atmosphere. Hydrologic exports of CO2 and CH4 via surface and groundwater were small, representing ≈1—2% of total export for both species, and were much less important than fluvial exports of organic carbon. Overall, floodplain detrital carbon processing was dominated by aerobic respiration and gaseous CO2 export, although methanogenesis did constitute up to 20% of total soil respiration at some sites.