Methane emission from rice: Stable isotopes, diurnal variations, and CO2 exchange

The importance of vegetation in supporting methane production and emission within flooded rice fields was demonstrated. Methane emission from Lousiana, United States, rice fields was correlated to the quantity of live aboveground biomass and the rate of CO, exchange. The quantity of belowground methane was greater in vegetated plots relative to plots maintained free of vegetation. The diurnal maximum in the rate of methane emission was coincident with the release of the most C-13-enriched methane and a maximum in transpiration rate rather than stomatal conductance, suggesting that diurnal variations in methane emission rate are linked with transpiration, in addition to temperature. Results of isotopic measurements of belowground, lacunal, and emitted methane indicate that methane is transported from rice predominantly via molecular diffusion with a small component due to transpiration-induced bulk flow. Samples of methane collected from air-filled internal spaces within the rice culm were C-13-enriched (-53.1 +/- 0.3 parts per thousand) relative to emitted (-64.5 +/- 1.0 parts per thousand) and belowground methane (-59 +/- 1.0 parts per thousand). Reproduction of these observed C-13 values with a numerical model required isotopic fractionation effects associated With transport of methane into and from rice plants. The model could not conclusively confirm rhizospheric methane oxidation. However, C-13-enriched methane was observed in the floodwater overlying the flooded soil (-44.4 +/- 2.2 parts per thousand), consistent with the oxidation of substantial quantities of methane as it diffused across the soil-water interface.