Effects of alternative electron acceptors and temperature on methanogenesis in rice paddy soils

Soil slurries collected from rice paddies were incubated anaerobically at different temperatures. Changes in the composition of electron acceptors and electron donors in the slurries were monitored daily. During the incubation NO3− was reduced first, followed by Fe3+ and SO42− reduction and methane production. The first two steps in the reduction sequence were exclusive, while competition occurred between sulfate reducers and methanogens. The temperature dependence of the reduction processes was expressed in a Q10 value, which was 2.4 for iron reduction, 1.6 for sulfate reduction and 4.6 for methane production. This means that from 14 °C to 30 °C methane formation rates increased 5.4 times as fast as sulfate reduction rates. This was explained by a change in the competition for acetate and can be due to different temperature responses for acetoclastic methanogens in Qmax or KM than for sulfate reducers. Temperature thus not only influenced conversion rates, but also the competitiveness of organisms. CO2 release during the incubation could be explained with the measured inorganic electron acceptors. Organic electron acceptors do thus not seem important in rice paddies.