Greenhouse gas fluxes and microbiome in a peatland forest and peaty savanna in Epena, Republic of the Congo

The carbon and nitrogen budgets, as well as greenhouse gas (GHG) fluxes, in peatlands of the Congo Basin – most likely the largest tropical peatland area – remain almost unexplored. In March 2024 (dry season) and November 2024 (wet season), we conducted measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes, along with peat chemistry, in a hardwood peat swamp forest and peaty savanna in Epena, Republic of the Congo. Manual soil and tree stem chambers were used to collect gas for further analysis using a Shimadzu gas chromatographer. Soil temperature, pH, O₂, water content, and groundwater table (GWT) levels were measured using portable analysers. Additionally, soil, trunk wood and tree leaves were sampled for subsequent physico-chemical and microbiome analysis. All the laboratory analyses were carried out at the University of Tartu. Four sampling sessions were conducted during both the dry and wet season, providing data across typical seasonal variation for the Congolese peat swamp ecosystem.
In the dry season (GWT −0.5 to −2.0 m), respiration of CO₂ from peat was relatively high (160–280 mg C m^–2 h^–1 as site averages) while the peat consumed 0.045…0.055 mg CH4-C m^–2 h^–1 as site averages. N₂O fluxes were very low while only the forest peat showed a significant N₂O emission of 96 μg N m^–2 h^–1. We explain the pattern with severe heat (up to 42^oC in topsoil) and dryness (down to 0.36 m³ water m^–3 topsoil). CH₄ was consumed in the peat while low CH₄ emissions were observed from Raphia sp. palm stems. Soil abundances of control genes associated with the N₂O and CH₄ cycles were low, likely due to the heat and dryness. In the dry season, preliminary gene analysis showed presence of archaeal nitrifiers and denitrifiers in the forest peat, with absence of bacterial nitrifiers. Fungal denitrifiers were also present in the peat of both ecosystems.
In the wet season (GWT –0.3 to +1.5 m), respiration of CO₂ from peat was expectedly low (54–81 mg C m^–2 h^–1 as site averages). Both the forest and savanna emitted CH₄ (0.12…1.0 and 0.033…0.18 mg C m^–2 h^–1, respectively, as site averages) and no significant amount of N₂O, as expected from a waterlogged ecosystem. The modest CH₄ emissions were explained by variability of O₂ with the uneven ground surface, the mobile groundwater and the flow connection to Likouala riverwater.