Suppression of Methane Production via the Promotion of Fe2+ Oxidation in Paddy Fields

Abstract

ABSTRACT Methane is a greenhouse gas, mainly generated from paddy fields and lakes by methanogens using hydrogen and acetic acid as substrates. In anaerobic environments with adequate Fe3+, iron-reducing microorganisms utilize these substrates, thus suppressing methane generation. We promoted Fe2+ oxidation to Fe3+ by physically stirring paddy soil and using a chelating agent (nitrilotriacetic acid; NTA) to evaluate the feasibility of the suppression of methane generation using Fe3+-reducing bacteria. Under anaerobic conditions, Fe3+ reduction to Fe2+ began immediately in the slurry made by adding water into air-dried paddy soil. Methane generation began on the 6th day when most Fe3+ was reduced. Under anaerobic conditions for 10 days followed by aerobic conditions, Fe2+ oxidation hardly progressed under static conditions. On stirring the slurry, Fe2+ oxidation progressed over 12 h (67% Fe2+ oxidized to Fe3+). When NTA was added under anaerobic conditions followed by stirring under aerobic conditions, Fe2+ oxidation was promoted further. The idea of physical stirring of paddy soil in the actual environment was derived from the effects of paddy soil stirring by ducks in interrelated rice–duck farming. In such farming, paddy soil contains more Fe3+ in its surface water compared with normal farming, resulting in suppressed methane generation.