Methane fluxes from soil and tree stem surfaces in flooded and non-flooded forests in the Central Amazon basin.

Methane (CH4) is a greenhouse gas with 35 times the warming potential of carbon dioxide. In the last 15 years, the concentration of atmospheric CH4 has sharply increased and the signature of carbon stable isotope in CH4 has become more negative suggesting biotic sources, such as tropical wetlands, might be partly responsible of the current atmospheric methane budget. Floodplains in the Brazilian Amazon have been found to release vast amounts of CH4 but the methane dynamics in upland forests are not very well studied. We assessed the magnitude of CH4 fluxes from soil and tree stem surfaces across dry and wet seasons in two contrasting ecosystems in the Central Amazon basin: the seasonally flooded varzea and the upland terra firme forest. Likewise, some potential drivers of such fluxes were assessed: tree diameter, stem height of measurement, tree species, water table depth, and air temperature. Methane fluxes were measured using chamber-based techniques in the period 2022-2023. Overall, greater fluxes were released from the trees stems of the varzea forest during the first half of the wet season (June-August). On the other hand, the stem surface of upland trees emitted very low CH4 fluxes (< 1 mg m-2 h-1). Methane fluxes of most trees from the flooded forests decreased with stem height, a pattern not shown by tree fluxes in the upland forest. The fluxes from tree stem emissions varied by tree species in both forest types: Munguba tree (Pachira aquatica) and Jarana tree emitted more CH4 fluxes than other species in varzea and upland forests, respectively. The next step of our research will be the assessment of the microbial role in the methane cycle of both forest types using a combination of isotopic and -omic techniques.