Thermodynamics of methane production in different rice paddy soils from China, the Philippines and Italy

Abstract

Methane production was measured in anoxic slurries of rice field soils that were collected from 16 different sites in China, the Philippines and Italy. The following general pattern was observed. Methane started to increase exponentially right from the beginning of anoxic incubation at positive redox potentials (360–510 mV). The concentrations of H 2 and acetate during this first phase allowed exergonic methanogenesis with Gibbs free energies of <−30 kJ mol −1 CH 4. In most soils, the accumulation of CH 4 slowed down and came to a halt after about 4 d. At this time CH 4 partial pressures were still relatively low (about 10–100 Pa). In this second phase, Gibbs free energies of H 2-dependent methanogenesis increased up to −10 kJ mol −1 CH 4, i.e. values that were no longer permissive for CH 4 production. This phase was dominated by sulfate reduction or reduction of Fe(III) instead of CH 4 production. The end of sulfate reduction and Fe(II) production usually coincided with a decrease of the Gibbs free energies to values of <−25 kJ mol −1 CH 4 and resumption of CH 4 production. Methane was then vigorously produced and eventually accumulated with a constant rate until the end of incubation (120 d). This third phase was also characterized by relatively constant H 2 partial pressures and acetate concentrations which were in the different soils at about 2–5 Pa and 30–120 μM, respectively, indicating that production and methanogenic consumption of these compounds was in steady state. The time when vigorous CH 4 production started and steady state was reached was different in the various soils tested and seemed to depend mainly on the availability of degradable organic substrates rather than the amount of reducible sulfate and ferric iron. In a few soils the initial CH 4 production (first phase) was not interrupted by an intermediate increase of the Gibbs free energy so that these soils released CH 4 right from the beginning of submergence until the end. These soils also showed the highest maximum CH 4 production rates.