Abstract. Irrigated rice paddies are one of the few methane (CH4) sources where the management of its emissions may be possible. Before that can be initiated, however, the relationship between production, oxidation, and emission of CH 4 and the processes controlling them must be better known. To that end we have made measurements of concentration and stable carbon and hydrogen isotopes of CH 4 and CO 2 in paddy fields along the Gulf Coast of Texas. Although only small differences in total CH4 flux (-46.5 2 3 2 13 g m-clayey and -4_ g m-sandy) and average/5 -CH 4 (seasonal averages of -56.11+1.21%o clayey and -53.57+0.97%o sandy) I¾om emitted CH4 were observed in two plots with different soil textures, by making additional measurements of belowground CH4 and CO 2 we learned much about processes occurring in the paddy field. We estimated that roughly 98% of the CH 4 released was transported through the plant and that residence times for belowground CH 4 were from about 1 to 5 hours during most of the season, indicating fast processing of both organic carbon and current photosynthesized carbon to make CH4. The percentage of CH4 made from acetate fermentation calculated using isotope data was strongly dependent on the value of the fractionation factor (or) associated with the CO2/H 2 reduction pathway for CH4 formation. Using a range of reasonable values for or, we calculated that acetate fermentation was from 67 to 80% early in the season to 29 to 60% late in the season (generally decreasing as the season progressed). Most importantly, we have strong evidence that rhizospheric CH 4 oxidation occurs in paddy fields. We have developed a semiempirical equation and used it to calculate the percent of CH4 oxidized as a function of total CH 4 produced from field measurements of 513CH4 under natural conditions. Because most emitted CH 4 is transported by the rice plant, it was necessary to determine the isotopic fractionation CH4 underwent during its transport through the plant. This value, 12_+1%o, was used to calculate oxidation percent using belowground and emitted/513CH4 values. In Texas, oxidation of CH4 in the soil increased from -20 to -60% over the 6 week period just prior to harvest.

Abstract. We report measurements of CH4 flux and fil 3C and fid values of emitted CH4 and sediment CH4 and CO2 during the 1995 rice growing season in Beaumont, Texas. Four rice plant cultivars, Lemont, Mars, Cypress, and Della, and an unplanted plot were studied to provide possible explanations for the differences in CH4 emissions between cultivars. Using the measured isotope values, along with data of CH4 and CO2 concentrations and other ecosystem data, we determined differences between cultivars in the processes of oxidation and production throughout the growing season. For instance, rhizospheric CH4 oxidation increased as the season progressed in both Mars and Lemont cultivars. Late in the season, however, 71_+10% of CH4 produced in the Mars plot was oxidized compared to only 39_+10% in the Lemont plot. The contribution of acetate fermentation to methanogenesis at specific times during the season was calculated using measured isotopic values and assuming identical isotopic fractionation factors in methanogenic pathways for the cultivars. In these calculations a range of values for the contribution to CH4 production from acetate fermentation and CO2 reduction with H2 was estimated by considering different fractionation factors for the methanogenic CO2 reduction pathway and the possibility of a 10% contribution to CH4 production from acetate produced by homoacetogenesis. In general, a steady increase in the CH4 portion produced by acetate fermentation was noted in the Lemont cultivar, while an increase followed by a decrease near the end of the season was observed for the Mars cultivar.

Abstract. We have conducted a series of experiments to determine the effects different diets, including the addition of unsaturated fatty acids, had on isotopic signatures of CH4 emissions from dairy cattle. Measurements of emitted 15•3CH4, 15•3CO2, and 15D-CH4 were made on gases collected over a 24-hour period from animal chambers containing individual Holstein cows. Some measurements of 15D-H20 from samples collected directly from the rumen were also made. We observed variation in •5•3CH4 values with time after feeding and a correlation in 15•3C between emitted gases and diet. The average 15•3CH4 value, which includes all samples of emitted gases, was -70.6+4.9%0 (n=57). Measurements of 15D-CH4 over the 24-hour sampling period had an average value of-357.8+15.0%0 (n=56). These 15D measurements are among the lightest reported for CH4 produced by cattle. Our cattle data indicate that hydrogen incorporated into the CH4 produced by CO2 reduction in high H2 concentration environments is fractionated to a greater degree than that incorporated in systems with relatively low H2 conditions, such as wetlands. Our results support bacterial studies that have demonstrated large hydrogen fractionation in high H2 concentration systems during methanogenesis.