METHANE EMISSION FROM RICE - STABLE ISOTOPES, DIURNAL-VARIATIONS, ANDCO2 EXCHANGE

The importance of vegetation in supporting methane production and emis sion within flooded rice fields was demonstrated. Methane emission fro m 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 plo ts maintained free of vegetation. The diurnal maximum in the rate of m ethane emission was coincident with the release of the most C-13-enric hed methane and a maximum in transpiration rate rather than stomatal c onductance, suggesting that diurnal variations in methane emission rat e are linked with transpiration, in addition to temperature. Results o f isotopic measurements of belowground, lacunal, and emitted methane i ndicate that methane is transported from rice predominantly via molecu lar diffusion with a small component due to transpiration-induced bulk flow. Samples of methane collected from air-filled internal spaces wi thin the rice culm were C-13-enriched (-53.1 +/- 0.3 parts per thousan d) relative to emitted (-64.5 +/- 1.0 parts per thousand) and belowgro und methane (-59 +/- 1.0 parts per thousand). Reproduction of these ob served C-13 values with a numerical model required isotopic fractionat ion effects associated With transport of methane into and from rice pl ants. The model could not conclusively confirm rhizospheric methane ox idation. However, C-13-enriched methane was observed in the floodwater overlying the flooded soil (-44.4 +/- 2.2 parts per thousand), consis tent with the oxidation of substantial quantities of methane as it dif fused across the soil-water interface.