Dynamic isotope equations for 13CH4 and 13CO2 describing methane formation with a focus on the effect of anaerobic respiration in sediments of some tropical lakes

The production of CO2 during sediment decomposition in field and laboratory incubations is often reported to be higher than that of CH4. We present a mathematical description of cellulose conversion into methane and carbon dioxide in the anoxic sediments of tropical Ladario and Belmont Lakes, which were investigated before. The processes of anaerobic respiration and acidogenesis competing for cellulose monomer, as well as the processes of anaerobic respiration, acetoclastic methanogenesis, and syntrophic acetate oxidation, competing for acetate, are considered. The gaseous and dissolved H2, CO2, and CH4 are taken into account in dynamic equations for total 12C+13C and 13C carbon in CH4 and CO2. Considering the amounts of CH4 and CO2 produced during the batch tests and changes in their carbon isotope values, modelling based on balances of summary total 12C+13C and 13C carbon is used to adequately describe the experimental data on sediment methanization. The high CO2:CH4 ratio with a delay in methane production observed in the batch experiments is interpreted by the model as due to an initial Fe3+ reduction. The model is validated by describing the system's dynamics under strong inhibition of acetoclastic methanogenesis by methyl fluoride. For comparison, the model is used to show a standard pathway of cellulose methanization in the tropical Anzol de Ouro Lake with a 1:1 ratio of produced CO2 to CH4, in conditions in which anaerobic respiration was proved to be insignificant.