CO2, H2, AND CH4 PRODUCTION IN RICE RHIZOSPHERE

Biochemical reactions occurring in the rice rhizosphere were examined in detail with emphasis placed on the competition between CH4 production and sulfate reduction. We measured CO2, H-2, and CH4 production activities in the rice rhizosphere at three growth stages (tillering stage, panicle initiation stage, and 10 days before the flowering stage). H-2 was produced by microorganisms on and/or inside roots. CO2 production was associated with the respiration of roots and rhizosphere microorganisms. CH4 was mainly produced in the rhizosphere soil. Not only root exudates but also H-2 and CO2 thus produced contributed to CH4 production in the rhizosphere. Root exudates and H-2 were the substrates for sulfate reduction in the rhizosphere, and there was a competition for these substances between the CH4-producing bacteria and sulfate-reducing bacteria. In the rice rhizosphere, sulfate reduction was dominant over CH4 production.