Large emissions from floodplain trees close the Amazon methane budget

Abstract

Wetlands are the largest global source of atmospheric methane (CH₄), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH₄ in the tropics, consistently underestimate the atmospheric burden of CH₄ determined via remote sensing and inversion modelling, 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 forests and tropical peat swamp forests, 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.