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Extensive interfluvial wetlands occur in theupper Negro River basin (Brazil) and contain a mosaicof vegetation dominated by emergent grasses andsedges with patches of shrubs and palms. To characterizethe release of carbon dioxide and methane fromthese habitats, diffusive and ebullitive emissions andtransport through plant aerenchyma were measuredmonthly during 2005 in permanently and seasonallyflooded areas. CO2 emissions averaged2193 mg C m-2 day-1. Methane was consumed inunflooded environments and emitted in floodedenvironments with average values of -4.8 and60 mg C m-2 day-1, respectively. Bubbles were emittedprimarily during falling water periods when hydrostaticpressure at the sediment-water interface declined.CO2 and CH4 emissions increased when dissolved O2decreased and vegetation was more abundant. Totalarea and seasonally varying flooded areas for twowetlands, located north and south of the Negro River,were determined through analysis of synthetic apertureradar and optical remotely sensed data. The combinedareas of these two wetlands (3000 km2) emitted1147 Gg C year-1 as CO2 and 31 Gg C year-1 asCH4. If these rates are extrapolated to the area occupiedby hydromorphic soils in the upper Negro basin,63 Tg C year-1 of CO2 and 1.7 Tg C year-1 as CH4are estimated as the regional evasion to the atmosphere.

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