Aquatic and Soil CO₂ Emissions from forested wetlands of Congo's Cuvette Centrale

Abstract. Within tropical forest ecosystems, wetlands such as swamp forests are an important interface between the terrestrial and aquatic landscape. Despite this assumed importance, there is a paucity of carbon flux data from wetlands in tropical Africa. Therefore, the magnitude and source of CO₂ fluxes, carbon isotopic ratios, and environmental conditions were measured for three years between 2019 to 2022 in a seasonally flooded forest and a perennially flooded forest in the Cuvette Centrale of the Congo Basin. The mean surface fluxes for the seasonally flooded site and the perennially flooded site were 2.36 ± 0.51 µmol m^-2 s^-1 and 4.38 ± 0.64 µmol m^-2 s^-1 respectively. The time series data revealed no discernible seasonal pattern in CO₂ fluxes. As for the environmental drivers, the fluxes at the seasonally flooded site exhibited a positive correlation with soil temperature and soil moisture. Additionally, the water table depth appeared to be a significant factor, demonstrating a quadratic relationship with the soil fluxes at the seasonally flooded site. δ¹³C values showed a progressive increase across the carbon pools, from above-ground biomass, then leaf litter, to soil organic carbon (SOC). However, there was no significant difference in δ¹³C enrichment between SOC and soil respired CO₂.

An in-situ derived gas transfer velocity (k₆₀₀ = 2.95 cm h^-1) was used to calculate the aquatic CO₂ fluxes at the perennially flooded site. Despite the low k_600, relatively high CO₂ surface fluxes were found due to very high dissolved pCO₂ values measured in the flooding waters. Overall, these results offer a quantification of the CO₂ fluxes from forested wetlands and provide an insight of the temporal variability of these fluxes as well as their sensitivity to environmental drivers.