Competition for electron donors among nitrate reducers, ferric iron reducers, sulfate reducers, and methanogens in anoxic paddy soil

Abstract

Slurries of anoxic paddy soil were either freshly prepared or were partially depleted in endogenous electron donors by prolonged incubation under anaerobic conditions. Endogenous NO ⁻₃ was reduced within 4 h, followed by reduction of Fe³⁺ and SO ²⁻₄ , and later by production of CH₄. Addition of NO ⁻₃ slightly inhibited the production of Fe²⁺ in the depleted but not in the fresh paddy soil. Inhibition was overcome by the addition of H₂, acetate, or a mixture of fatty acids (and other compounds), indicating that these compounds served as electron donors for the bacteria reducing NO ⁻₃ and/or ferric iron. Addition on NO ⁻₃ also inhibited the reduction of SO ²⁻₄ in the depleted paddy soil. This inhibition was only overcome by H₂, but not by acetate or a mixture of compounds, indicating that H₂ was the predominant electron donor for the bacteria involved in NO ⁻₃ and/or SO ²⁻₄ reduction. SO ²⁻₄ reduction was also inhibited by exogenous Fe³⁺, but only in the depleted paddy soil. This inhibition was overcome by either H₂, acetate, or a mixture of compounds, suggesting that they served as electron donors for reduction of Fe³⁺ and/or SO ²⁺₄ . CH₄ production was inhibited by NO ⁻₃ both in depleted and in fresh paddy soil. Fe³⁺ and SO ²⁻₄ also inhibited methanogenesis, but the inhibition was stronger in the depleted than in the fresh paddy soil. Inhibition of CH₄ production was paralleled by a decrease in the steady state concentration of H₂ to a level which provided a free enthalpy of less than ΔG=−17 kJ mol^-1 CH₄ compared to more than ΔG=−32 kJ mol^-1 CH₄ in the control. The results indicate that in the presence of exogenous fe³⁺ or SO ²⁺₄ , methanogenic bacteria were outcompeted for H₂ by bacteria reducing Fe³⁺ or SO ²⁺₄ .