Anaerobic decomposition of tropical soils and plant material: Implication for the CO2 and CH4 budget of the Petit Saut Reservoir

Tropical hydroelectric reservoirs contribute significantly to atmospheric CH4 and CO2 emissions. To evaluate the contribution of the mineralization of the flooded soils and biomass to these atmospheric gas emissions, field and laboratory experiments were conducted. Cores were retrieved inform the littoral zone of the Petit Saut Reservoir (French Guiana), flooded 10 a prior to sampling, and different soils and plant material in the tropical forest surrounding the reservoir. All the samples were flooded and incubated in anoxic conditions in the dark at 30 °C. The potential CH4 and CO2 production rates were determined. Soils and plant material from the tropical forest were incubated over one year and the production measurements were performed at a frequency of 1–5 months. Methane and CO2 production rates of soils and littoral sediments were linearly correlated to the organic C (OC) content of the slurries. The slopes of the relationships were 2.6×102±5.6×101nmol(CH4)gOC(dry)-1h-1 and 3.5×102±6.9×101nmol(CO2)gOC(dry)-1h-1. For plant material, no relationship between the production rates and the OC content was found and on average over the year of incubation, production rates were 2.4×103±1×103nmol(CH4)gOC(dry)-1h-1 and 3.9×103±5×103nmol(CO2)gOC(dry)-1h-1, which is one order of magnitude higher than the mineralization of the soils and sediments. Extrapolated at the scale of the Petit Saut Reservoir over 10 a, these results show that the mineralization of the soil and the plant biomass initially flooded contributed to 75–95% of the total C emissions to the atmosphere since the flooding of the reservoir. Methane: CO2 molar ratios were 3 times higher for anaerobic decomposition than atmospheric emissions, quantitatively consistent with aerobic CH4 oxidation.