Stem and soil methane fluxes of different ecosystems in Central Amazon

Trees can significantly influence the net exchange of methane between forests and the atmosphere but what controls this behaviour in hyper-diverse ecosystems of the Central Amazon is not well defined. Variations in topography and rainfall cause predictable, but poorly documented, changes to the balance between methanotrophy and metanogenesis in soils and sediments across the landscape and between seasons. Trees can act as conduits for methane produced below-ground, however, the rate of such transport is mediated by inter and intra-species traits that need to be understood.

To better understand the relationships among methane exchange, topographic position, seasonal rainfall and tree species in forests of the Central Amazon, we are conducting two related studies within the Uatumã Sustainable Development Reserve and Amazon Tall Tower Observatory, Amazonas, Brazil. The first is measuring soil and stem fluxes in six plots (6 trees / plot) along a topographic transition from a well-drained plateau, through slopes to a waterlogged valley. The second study is focusing on stem fluxes from six different species (5 trees / species), with differences in wood density and phenology, growing at a similar elevation in an Igapó forest of the adjacent Uatumã river. These observations, starting in September 2023, are being made every 2-3 months to capture the influence of seasonal rainfall and inundation.

During the dry season (September, 2023), soils in the plateau (-1.4 ± 0.27 nmol m^-2 s^-1) and slope (-1.66 ± 0.11 to -1.20 ± 0.29 nmol m^-2 s^-1) plots acted as a sink for methane, whilst, those in the valley plot where a source (11.40 ± 2.56 nmol m^-2 s^-1 ). Reflecting this pattern, stem emissions were mostly observed in the valley (10.7 ± 4.71 nmol m^-2 s^-1) and in particular from the palm Mauritia flexuosa. Stem fluxes in the plateau and slope plots were marginal (0.0028 ± 0.0039 to 0.224 ± 0.0554 nmol m^-2 s^-1). In the Igapó (November, 2023), the exposed soil behaved as a sink for methane. Differences were observed among the species studied, with the largest emissions from Nectandra amazonum – low wood density group (0.64 ± 0.14 nmol m^-2 s^-1 ) and Inga sp. – high wood density group (0.39 ± 0.049 nmol m^-2 s^-1), while the other 4 species had lower emissions (0.0023 ± 0.01 to 0.10 ± 0.01 nmol m^-2 s^-1). Together these results support methane produced below-ground as the main source of tree emissions across this landscape and highlight the need to take species composition into account when considering the net exchange of methane from these ecosystems.