Amazon upland forest CH4 fluxes: A small sink or a small source? A case study

Methane (CH₄) is an important greenhouse gas whose natural sources are still poorly understood, and significant uncertainties in their quantification remain. Tropical upland forests generally present a small CH₄ sink with occasional local emission hotspots, making it challenging to determine the net ecosystem flux. This study employs two complementary micro-meteorological methods to estimate methane (CH₄) fluxes from a Terra Firme (upland forest) ecosystem in the central Amazon. At the Amazon Tall Tower Observatory (ATTO) field site, CH₄ and CO₂ concentrations are continuously monitored at five different heights above and below the canopy (79, 53, 38, 24, 4 meters). Additionally, Eddy Covariance measurements of CO₂ and H₂O fluxes, along with micro-meteorological observations, are conducted at various heights. To estimate ecosystem CH₄ fluxes, the Modified Bowen Ratio (MBR) technique was applied. This method uses the concentration gradient and the flux of a trace gas (in this study CO₂ and H₂O), to calculate the eddy diffusivity (k), which is then used to estimate the flux of another trace gas (in this study CH₄). MBR CH₄ fluxes were estimated for the period 2014-2021. Additionally, vertical concentration profiles were analyzed to gain further insights into the temporal patterns of CH₄ fluxes. The MBR flux estimates and vertical profile analyses revealed clear seasonal patterns. During the wet season, positive concentration gradients indicated net CH₄ emissions from the forest, while in the dry season, negative gradients suggested net CH₄ uptake. Ecosystem median CH₄ fluxes, estimated with the MBR technique, dominantly showed CH₄ uptake and ranged between -2 and 2 nmol m^-2 s^-1. With these results, we aim to highlight the seasonal and interannual patterns of upland forest CH₄ fluxes, which are essential for understanding the contribution of tropical upland forests to the Amazon's CH₄ budget.