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 in itiated, however, the relationship between production, oxidation, and emission of CH4 and the processes controlling them must be better know n. To that end we have made measurements of concentration and stable c arbon and hydrogen isotopes of CH4 and CO2 in paddy fields along the G ulf 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 a verage 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 emitt ed CH4 were observed in two plots with different soil textures, by mak ing additional measurements of belowground CH4 and CO2 we learned much about processes occurring in the paddy field. We estimated that rough ly 98% of the CH4 released was transported through the plant and that residence times for belowground CH4 were from about 1 to 5 hours durin g most of the season, indicating fast processing of both organic carbo n and current photosynthesized carbon to make CH4. The percentage of C H4 made from acetate fermentation calculated using isotope data was st rongly dependent on the value of the fractionation factor (alpha) asso ciated with the CO2/H-2 reduction pathway for CH4 formation. Using a r ange of reasonable values for alpha, we calculated that acetate fermen tation was from 67 to 80% early in the season to 29 to 60% late in the season (generally decreasing as the season progressed). Most importan tly, 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 pr oduced from field measurements of delta(13)CH(4) under natural conditi ons. Because most emitted CH4 is transported by the rice plant, it was necessary to determine the isotopic fractionation CH4 underwent durin g its transport through the plant. This value, 12+/-1 parts per thousa nd, was used to calculate oxidation percent using belowground and emit ted delta(13)CH(4) values. In Texas, oxidation of CH4 in the soil incr eased from similar to 20 to similar to 60% over the 6 week period just prior to harvest.