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Abstract

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.