Carbon dioxide and methane exports from a southeastern floodplain swamp: Patterns, pathways, and sensitivity to climate

Patterns and pathways of terminal respiration are major gaps in knowledge of floodplain carbon dynamics. Exports of CO{dollar}sb2{dollar} and CH{dollar}sb4{dollar} from floodplain forests of the Ogeechee River in Georgia, U.S.A., were studied from July 1987 to September 1989. Despite frequent flooding, the fate of most organic carbon was aerobic respiration. 78% of litter inputs were ultimately exported as CO{dollar}sb2{dollar}. Only 1% of carbon was exported as CH{dollar}sb4{dollar}. Thus, though anaerobic and fluvial processes are significant features of floodplain biogeochemistry, they do not dominate total carbon flows. CH{dollar}sb4{dollar} emissions occurred irregularly during warmer seasons from flooded sites. All of net floodplain CH{dollar}sb4{dollar} emissions came from 30% of floodplain area (up to 271 mg C m{dollar}sp{lcub}-2{rcub}{dollar}d{dollar}sp{lcub}-1{rcub}{dollar}, averaging 16 g C m{dollar}sp{lcub}-2{rcub}{dollar}yr{dollar}sp{lcub}-1{rcub}{dollar} from these habitats). Methanogenesis comprised 7-20% of total respiration in these sites. CH{dollar}sb4{dollar} emissions showed a step-function response to temperature, with a sharp rise near 15C. Some CH{dollar}sb4{dollar} emissions took place from cypress knees, but contributed only 0.42% of total floodplain CH{dollar}sb4{dollar} emissions.; Long-term floodplain carbon dynamics were investigated with empirical and mechanistic simulation models based on field study results. Models were used to estimate both historical patterns and sensitivity to potential climate changes. Simulated soil organic carbon (SOC) and CO{dollar}sb2{dollar} emissions were relatively stable during 1937-1989. There were small increases in CO{dollar}sb2{dollar} emissions and declines in SOC since 1960 that seemed to arise from small seasonal shifts in temperatures and precipitation. In altered climate simulations, CO{dollar}sb2{dollar} emissions and total soil respiration increased with warmer temperatures but responded little to precipitation changes. Over time, SOC stocks at higher temperatures declined substantially (28% in 53 years at +4{dollar}spcirc{dollar}C) and total respiration approached unperturbed values. Simulated methane emissions varied up to 3-fold between years and responded strongly to changes in precipitation and streamflow. Simulated 20% decreases in rainfall resulted in 30-48% declines in methane emissions. Indirect consequences of temperature increases (altered hydrology leading to declines in CH{dollar}sb4{dollar} emissions) had larger effects on CH{dollar}sb4{dollar} emissions than did direct temperature effects, and were strongly dependent on responses of evapotranspiration to elevated temperatures.