OXYGEN PROFILES AND METHANE TURNOVER IN A FLOODED RICE MICROCOSM

Summary. Dissolved 02 Was depleted within the top 3.5-mm surface layer of flooded rice soil microcosms without plants. In planted microcosms, however, O2 was detectable down to at least 40 mm in depth. O2 concentrations in the uppermost soil layers of microcosms with rice plants were higher in the light than in the dark, indicating O2 production by photosynthesis. The CH4 emission rates were nearly identical for illuminated and for darkened microcosms, demonstrating that the photosynthetically produced O2 did not increase CH4 oxidation in the rhizosphere. In contrast, CH4 emission rates increased when the microcosms were incubated under an N2 atmosphere, indicating that transport Of O2 from the atmosphere into the rhizosphere was important for CH4 oxidation. CH4 emission under air accounted for only 10%-20% of the cumulative CH4 production determined in cores taken from the microcosms. Apparently, 80%-90% of the CH4 produced was oxidized in the rhizosphere and thus was not emitted.