Methane oxidation and pathways of production in a Texas paddy field deduced from measurements of flux, delta C-13, and delta D of CH4

Irrigated rice paddies are one of the few methane (CH4) sources where the management of its emissions may be possible. Before that can he initiated, however, the relationship between production, oxidation, and emission of CH4 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 CH4 and CO2 in paddy fields along the Gulf Coast of Texas. Although only small differences in total CH4 flux (similar to 46.5 g m(-2) clayey and similar to 43 g m(-2) sandy) and average delta(13)CH(4) (seasonal averages of -56.11 +/- 1.21 parts per thousand clayey and -53.57+/-0.97 parts per thousand sandy) from emitted CH4 were observed in two plots with different soil textures, by making additional measurements of belowground CH4 and CO2 we learned much about processes occurring in the paddy field. We estimated that roughly 98% of the CH4 released was transported through the plant and that residence times for belowground CH4 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 (alpha) associated with the CO2/H-2 reduction pathway for CH4 formation. Using a range of reasonable values for alpha, 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 CH4 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 CH4 produced from field measurements of delta(13)CH(4) under natural conditions. Because most emitted CH4 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 parts per thousand, was used to calculate oxidation percent using belowground and emitted delta(13)CH(4) values. In Texas, oxidation of CH4 in the soil increased from similar to 20 to similar to 60% over the 6 week period just prior to harvest.