Differences in CH4 oxidation and pathways of production between rice cultivars deduced from measurements of CH4 flux and delta C-13 of CH4 and CO2

We report measurements of CH4 flux and delta(13)C and delta D values of emi tted CH4 and sediment CH4 and CO2 during the 1995 rice growing season in Be aumont, Texas. Four rice plant cultivars, Lemont, Mars, Cypress, and Della, and an unplanted plot were studied to provide possible explanations for th e differences in CH4 emissions between cultivars. Using the measured isotop e values, along with data of CH4 and CO2 concentrations and other ecosystem data, we determined differences between cultivars in the processes of oxid ation and production throughout the growing season. For instance, rhizosphe ric CH4 oxidation increased as the season progressed in both Mars and Lemon t cultivars. Late in the season, however, 71+/-10% of CH4 produced in the M ars plot was oxidized compared to only 39+/-10% in the Lemont plot. The con tribution of acetate fermentation to methanogenesis at specific times durin g the season was calculated using measured isotopic values and assuming ide ntical isotopic fractionation factors in methanogenic pathways for the cult ivars. In these calculations a range of values for the contribution to CH4 production from acetate fermentation and CO2 reduction with H-2 was estimat ed by considering different fractionation factors for the methanogenic CO2 reduction pathway and the possibility of a 10% contribution to CH4 producti on from acetate produced by homoacetogenesis. In general, a steady increase in the CH4 portion produced by acetate fermentation was noted in the Lemon t cultivar, while an increase followed by a decrease near the end of the se ason was observed for the Mars cultivar.