Relationships between methane production and sulfate reduction in reclaimed rice field soils

The change in relationships between methane production and sulfate reduction was investigated in reclaimed rice field soils at different time points after reclamation of tidal flat in Korea. Sulfate concentrations of soils in the ca. 60‐year‐old and 26‐year‐old reclaimed rice fields were much lower than that in a natural tidal flat. During 60 d of anaerobic incubation, total methane production and sulfate consumption of the soil slurries were 7.0 μmol CH4/g and 8.2 μnol SO4 2‐/g in the 60‐year‐old rice field, 5.6 μmol CH4/g and 12.7 μmol SO4 2‐/g in the 26‐year‐old rice field, and ca. 0 μmol CH4/g and 22.4 μmol SO4 2‐/g in a natural tidal flat. Relative percent electron flow through sulfate reduction in the 60‐year‐old rice field was much lower (50.8%) compared with the 26‐year‐old rice field (69.3%) and the tidal flat (99.9%). The addition of an inhibitor of methanogenesis (2‐bromoethanesulfonate) had no effect on sulfate reduction in the soil slurries of the reclaimed rice fields. However, instant stimulation of methane production was achieved with addition of an inhibitor of sulfate reduction (molybdate) in the soil slurries from the 26‐year‐old reclaimed rice field. The specific inhibitor experiments suggest that the relationship of methanogenesis and sulfate reduction might become mutually exclusive or syntrophic depending on sulfate content in the soil after reclamation. Sulfate, thus sulfate reduction activity of sulfate‐reducing bacteria, would be an important environmental factor that inhibits methane production and determines the major pathway of electron and carbon flow in anaerobic carbon mineralization of reclaimed rice field soils.