Early initiation of methane production in anoxic rice soil despite the presence of oxidants

Methane production was observed during the initial phase of anoxia in rice soil slurries despite a high redox potential and the presence of oxidants (NO3-, Fe(III), SO42-). Such early methane production was investigated by measuring the concentrations reductants (H-2 and acetate) and of oxidants (NO3-, Fe(III), SO42-) and by calculating the thermodynamic conditions for methanogenesis. During the early phase of anoxia in soil slurries, conditions for methanogenesis were favorable. Methanogenesis from H-2/CO2 only became unfavorable several days later when H-2 partial pressures transiently decreased to < 1 Pa still during the phase of Fe(III) and sulfate reduction. Addition of rice straw to the soil prevented this transient decrease of H-2 and suppression of CH4 production. The effect of chloroform and bromoethanesulfonic acid (BES) on the early methane production was also investigated. Since chloroform and BES completely suppressed the early methane production, we concluded that methanogenic archaea were responsible for this methane production during the early phase of anoxia in rice soil slurries. The lack of inhibition by methyl fluoride of the early methane production suggested that most of the CH4 production at the beginning of the incubation was caused by hydrogenotrophic methanogens. After 3 days of incubation, a shift from hydrogenotrophic to acetoclastic methanogenesis as the dominant source of methane occurred.