Wetlands are the largest global source of atmospheric methane (CH4)1, a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain2,3, the largest natural geographic source of CH4 in the tropics, consistently underestimate the atmospheric burden of CH4 determined via remote sensing and inversion modelling4,5, pointing to a major gap in our understanding of the contribution of these ecosystems to CH4 emissions. Here we report CH4 fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH4 emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests6 and tropical peat swamp forests7, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δ13C) of −66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH4 a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a ‘top-down’ regional estimate of CH4 emissions of 42.7 ± 5.6 teragrams of CH4 a year for the Amazon basin, based on regular vertical lower-troposphere CH4 profiles covering the period 2010–2013. We find close agreement between our ‘top-down’ and combined ‘bottom-up’ estimates, indicating that large CH4 emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH4 budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH4 emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH4 source when trees are combined with other emission sources.
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The bottom-up measurement study was funded by grants from the following bodies: UK Natural Environment Research Council to V.G. (Principal Investigator, grant NE/J010928/1) and E.R.C.H. (co-investigator, NE/J009032/1); several CNPq (Brazil National Council of Research and Development) and FAPERJ (Science Foundation from the State of Rio de Janeiro) grants to A.E.-P.; Swedish Research Council, VR (grant 2012-00048) to D.B.; and the Swedish-Brazilian collaboration STINT/CAPES to D.B. and A.E.-P. (grant 2012-2085). Atmospheric CH4 vertical profile measurements were funded by the following agencies: NERC consortium AMAZONICA, which is led by E.G. (grant NE/F005806/1) and includes L.V.G. and J.B.M.; FAPESP (State of Sao Paulo Science Foundation) via the Carbon Tracker project and FAPESP-NERC via the ACO project (grants 08/58120-3, 11/51841-0); the EU via the 7th-Framework Programme project GEOCARBON (grant 283080); and CNPq (grant 403241/2012-3, awarded to L.V.G.). NASA, NOAA, IPEN and INPE contributed to the construction and maintenance of the GHG laboratory in Brazil. We also acknowledge support from the FAPESP-NERC ACO project for a workshop that brought bottom-up and top-down data together. V.G. and E.G. thank the NERC consortium MOYA (grant NE/N015606/1) and D.B. thanks the Swedish Research Council (VR) for salary contributions. V.G. and S.R.P. acknowledge support from the AXA Research Fund. We thank T. Silva for assistance with tree species identification and A. Breves, E. Cristina, F. M. Silva, J. Valle, J. P. Felizardo, L. Rodrigues, L. Valladares, P. Souto, R. Pollery and V. Figueiredo for field work assistance. We thank A. Dwarakanath for data analysis and P. Monaghan and A. McAleer for performing δ13C-CH4 analyses.
This file contains Supplementary Figures 1-2 and Supplementary Tables 1-5.