Production, oxidation and emission of methane in rice paddies

Abstract 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 < 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 > 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 emission rates in rice paddy areas flooded with brackish water.