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

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 H2 and acetate during this first phase allowed exergonic methanogenesis with Gibbs free energies of <−30 kJ mol−1 CH4. In most soils, the accumulation of CH4 slowed down and came to a halt after about 4 d. At this time CH4 partial pressures were still relatively low (about 10–100 Pa). In this second phase, Gibbs free energies of H2-dependent methanogenesis increased up to −10 kJ mol−1 CH4, i.e. values that were no longer permissive for CH4 production. This phase was dominated by sulfate reduction or reduction of Fe(III) instead of CH4 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 CH4 and resumption of CH4 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 H2 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 CH4 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 CH4 production (first phase) was not interrupted by an intermediate increase of the Gibbs free energy so that these soils released CH4 right from the beginning of submergence until the end. These soils also showed the highest maximum CH4 production rates.