Methane production as a function of anaerobic carbon mineralization: A process model

Anaerobic carbon mineralization is a major regulator of soil methane production, but the relationship between these processes is variable. To explain the dynamics of this relationship a model was developed, which comprises the dynamics of alternative electron-acceptors, of acetate and of methanogenic biomass. Major assumptions are: (i) alternative electron-acceptors suppress methanogenesis and (ii) the rate of electron-acceptor reduction is controlled by anaerobic carbon mineralization. The model was applied to anaerobic incubation experiments with slurried soil samples from a drained and an undrained peat soil in the Netherlands to test the model and to further interpret the data. Three parameters were fitted with a Monte Carlo method, using experimentally determined time series of methane, carbon dioxide and acetate. The fitted parameters were the initial concentration of electron-acceptors, the initial concentration of methanogenic biomass and the maximum relative growth rate of methanogenic biomass. Simulated and measured time courses of methane corresponded reasonably well. The model as such stresses the importance of alternative electron-acceptors. At the drained site initial alternative electron-acceptor concentrations were between 0.3 and 0.8 mol electron equivalents (el. eqv.) kg−1 dw soil, whereas at the undrained site they were between 0.0 and 0.3 mol el. eqv. kg−1 dw soil, depending on the experimental treatments. The sum of measured NO3− and SO42− concentrations and estimated maximum Fe3+ and Mn4+ concentrations was much lower than the fitted concentrations of alternative electron-acceptors. Apparently, reduction of unknown electron-acceptors consumed a large part of anaerobically-mineralized carbon which, therefore, was not available for methanogenesis.