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

Wetlands are important sources of methane (CH4) and sinks of carbon dioxide (CO2). However, little is known about CH4 and CO2 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 fluxe...

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Bibliographic Details
Published in:	Global change biology Vol. 25; no. 6; pp. 1967 - 1981
Main Authors:	Dalmagro, Higo J., Zanella de Arruda, Paulo H., Vourlitis, George L., Lathuillière, Michael J., de S. Nogueira, José, Couto, Eduardo G., Johnson, Mark S.
Format:	Journal Article
Language:	English
Published:	England Blackwell Publishing Ltd 01-06-2019
Subjects:	Carbon dioxide CH4 and CO2 fluxes Climate change Confidence intervals Covariance Dissolved organic carbon Duration Ecosystems Eddy‐covariance Floods Floodwater Fluxes Forests global change Global warming greenhouse gas balance Hydrologic data hyperseasonal savanna Methane net ecosystem carbon balance Offsets Radiative forcing Rainy season Recession Soil Soil conditions Soil water Soils Tropical climate Tropical forests tropical wetlands Uptake Wet season Eddy-covariance net ecosystem carbon balance greenhouse gas balance tropical wetlands global change hyperseasonal savanna CH4 and CO2 fluxes
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Summary:	Wetlands are important sources of methane (CH4) and sinks of carbon dioxide (CO2). However, little is known about CH4 and CO2 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 CH4 and CO2 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 CO2 but strong sources of CH4, particularly during inundation when reducing conditions in soils increase CH4 production and limit CO2 release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH4‐C m−2 d−1 and absorbed 1.6 ± 0.2 g CO2‐C m−2 d−1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH4 emissions decreased significantly (0.002 ± 0.001 g CH4‐C m−2 d−1) but remained a net source, while the net CO2 flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH4 fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO2 and CH4 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 CH4‐C + CO2‐C) in anaerobic phases and emitting 76 g C m−2in aerobic phases), high CH4 effluxes during the anaerobic flooded phase and modest CH4 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. Research in a seasonally flooded forest ecosystem in the Pantanal wetlands of South America showed rapid changes in CO2 and CH4 fluxes that resulted from alternating inundation and dry phases of the annual hydrologic cycle. Although the system was a net carbon sink, large methane fluxes result in the ecosystem increasing the net radiative forcing of greenhouse gases in the atmosphere. DOC was a minor component of the net ecosystem carbon balance; variability in the duration of annual flooding was responsible for most of the interannual variability in seasonal CO2 and CH4 fluxes.
ISSN:	1354-1013 1365-2486
DOI:	10.1111/gcb.14615