Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

Wetlands are important sources of methane (CH 4 ) and sinks of carbon dioxide (CO 2 ). However, little is known about CH 4 and CO 2 fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH 4 and CO 2 in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO 2 but strong sources of CH 4 , particularly during inundation when reducing conditions in soils increase CH 4 production and limit CO 2 release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH 4 ‐C m −2 d −1 and absorbed 1.6 ± 0.2 g CO 2 ‐C m −2 d −1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH 4 emissions decreased significantly (0.002 ± 0.001 g CH 4 ‐C m −2 d −1 ) but remained a net source, while the net CO 2 flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH 4 fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO 2 and CH 4 were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m −2 (as CH 4 ‐C + CO 2 ‐C) in anaerobic phases and emitting 76 g C m −2 in aerobic phases), high CH 4 effluxes during the anaerobic flooded phase and modest CH 4 effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.