Production, oxidation and emission of methane in rice paddies

Production and emission of methane from submerged paddy soil was studied in laboratory rice cultures and in Italian paddy fields. Up to 80% of the CH4 produced in the paddy soil did not reach the atmosphere but was apparently oxidized in the rhizosphere. CH4 emission through the rice plants was inhibited by an atmosphere of pure O2 but was stimulated by an atmosphere of pure N2 or an atmosphere containing 5% acetylene. Gas bubbles taken from the submerged soil contained up to 60% CH4 but only smaller than 1% CH4 after the bubbles had passed the soil-water interface or had entered the intercellular gas space system of the rice plants. CH4 oxidation activities were detected in the oxic surface layer of the submerged paddy soil. Flooding the paddy soil with water containing greater than 0.15% sea salt (0.01% sulfate) resulted in a strong inhibition of the rates of methanogenesis and a decrease in the rates of CH4 emission. This result explains the observation of relatively low CH4 emissio n rates in rice paddy areas flooded with brackish water. (IFU