Methane and Nitrous Oxide Fluxes in Soil and Stems of Malaysian Tropical Peat Swamp Forest

Tropical peat swamp forests are crucial global carbon (C) reserves. Prevailing waterlogged conditions in peat soils prevent the complete decomposition of dead plant material. As a result, more organic matter is produced than decomposed, leading to the gradual accumulation of peat. However, the destabilisation of tropical peatlands through climate warming, droughts, and changes in land use threaten this C sink capacity. Anaerobic conditions in peat soils lead to methane (CH₄) production through decomposition and nitrous oxide (N₂O) production under moderate levels of soil oxygen content. Earlier evidence suggests that tree stems in tropical peat swamp forests are significant sources of CH₄; however, little information is available on their exchange of N₂O.
This study investigated CH₄ and N₂O exchange of soil and stems of Combretocarpus rotunditus and Shorea albida trees in a peat swamp forest in Sarawak, Malaysia, from September 2022 to September 2023. To describe the temporal dynamics of greenhouse gas (GHG) exchange, we measured gas fluxes from the soil and stems at different heights (10, 80 and 170 cm from the tree's base) using the manual static chamber method and spectroscopic gas analysis. The chemical composition of the soil was analysed and several environmental parameters, including groundwater level, soil moisture content, soil and air temperature, were simultaneously measured with the GHG fluxes to determine the relationships between the fluxes and environmental factors.
Soil CH₄ emissions varied between 52.3 and 807 μg C m⁻² h⁻¹, with higher values observed during the wet season in conjunction with higher groundwater levels. On the other hand, the soil N₂O fluxes were relatively low and did not show a distinct seasonal pattern, ranging from -1.33 to 3.54 μg N m⁻² h⁻¹. Annual average soil CH₄ and N₂O emissions were 392 μg C m⁻² h⁻¹ and 0.65 μg N m⁻² h⁻¹, respectively. The highest average stem CH₄ emissions (1.48 μg C m⁻² h⁻¹) were recorded at the lowest parts of trees, with a vertical decrease in emissions and an overall uptake observed at the highest measurement point. In contrast, stem N₂O emissions were small, with no clear trend with measurement height.

In summary, we observed moderate and variable soil CH₄ emissions with limited generalisable relation to measured environmental parameters. Soil and stem N₂O emissions were relatively small. These results indicate the need for further comprehensive soil and stem GHG analyses in tropical peat swamp systems to better understand the GHG dynamics of this critical ecosystem.