Effects of short-term drainage and aeration on the production of methane in submerged rice soil

Emission rates of CH4 were measured in microcosms of submerged soil which were planted with rice. Drainage of the rice microcosms for 48 h resulted in drastically decreased CH4 emission rates which only slowly recovered to the rates of the undrained controls. Drainage also resulted in drastically increased sulphate concentrations which only slowly decreased to nearly zero background values after the microcosms were submerged again. The mechanisms responsible for the decrease of CH4 production by aeration were investigated in slurries of a loamy and a sandy Italian rice soil. Incubation of the soil slurries under anoxic conditions resulted first in the reduction of nitrate, sulphate and ferric iron before CH4 production started. Incubation of the soil slurries for 48 h under air resulted in immediate and complete inhibition of CH4 production. Although the soil slurries were then again incubated under anoxic conditions (Np atmosphere), the inhibition of CH4 production persisted for more than 30 days. The redox potential of the soil increased after the aeration but returned within 15 days to the low values typical for CH4 production. However, the concentrations of sulphate and of ferric iron increased dramatically after the aeration and stayed at elevated levels for the period during which CH4 production was inhibited. These observations show that even brief exposure of the soil to O-2 allowed the production of sulphate and ferric iron from their reduced precursors. Elevated sulphate and ferric iron concentrations allowed sulphate-reducing and ferric iron-reducing bacteria to outcompete methanogenic bacteria on H-2 as common substrate. Indeed, concentrations of H-2 were decreased as long as sulphate and ferric iron were high so that the Gibbs free energy of CH4 production from H-2/CO2 was also increased (less exergonic). On the other hand, concentrations of acetate, the more important precursor for CH4, were not much affected by the short aeration of the soil slurries, and the Gibbs free energy of CH4 production from acetate was highly exergonic suggesting that acetotrophic methanogens were not outcompeted but were otherwise inhibited. Aeration also resulted in increased rates of CO2 production and in a short-term increase of N2O production. However, these increases were < 10% of the decreased production of CH4 and did not represent a trade-off in terms of CO2 equivalents. Hence, short-term drainage and aeration of submerged paddy fields may be a useful mitigation option for decreasing the emission of greenhouse gases.