Methane fluxes from seasonally flooded forests in the Amazon basin

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Methane (CH4) is a strong greenhouse gas and its atmospheric concentration has nearly tripled since pre-industrial times. Natural wetlands are a major source of methane, which is produced by the anaerobic decomposition of organic material. The Amazon basin in South America is largely covered by wetlands and is a key region for tropical CH4 emissions. Despite its importance for the global methane budget, the overall CH4 emission from the Amazon basin is not well quantified and the individual CH4 sources are not well understood. The Amazon basin's 'bottom-up' (upscaling of individual sources) CH4 emission estimate differs by a factor 2 from the 'top-down' (inversion of atmospheric concentration measurements) emission estimate. Wetlands, such as the seasonally flooded forests, are key regions for methane emissions, but measurements are sparse. The overall objective of this proposal is to understand the role of seasonally flooded forests for the Amazonian CH4 budget. Above-canopy CH4 flux measurements in a seasonally flooded forest will be performed over at least one year. Measurements of this important spatial scale do currently not exist, but allow linking emissions of the individual sources used in 'bottom-up' studies with 'top-down' estimates, which provide the integrated flux for a region. The above-canopy measurements will be complemented by individual source measurements within its footprint. This combination of measurements allows linking the emissions of the individual sources with the fluxes measured above the canopy; hence the CH4 budget will be closed for the investigated area. This is important, because some sources are very uncertain, e.g. very recently, emissions from flooded tree stems have been suggested as a large hitherto disregarded CH4 source. Furthermore the above-canopy CH4 flux measurements over a full annual cycle provide an integrated flux for seasonally flooded forests that can be upscaled for comparison with the 'top-down' estimates. The measurements are proposed to take place at the K34 tower, which is located in a seasonally flooded forest close to Manaus (Brazil). Full access has been offered to install our instrumentation at this unique infrastructure, which contains not only the tower, but also air-conditioned housing for the instrumentation. For the flux measurements we will use a Relaxed Eddy Accumulation (REA) technique coupled to an FTIR-spectrometer. The setup of this FTIR-REA flux measurement system has been developed within the EU-project INGOS. This system is able to measure the fluxes of CH4, CO2, N2O, CO and del13CO2 simultaneously. The individual emission sources within the footprint of the flux measurements will be measured on campaign basis using a portable gas analyzer.