Methane oxidation and pathways of production in a Texas paddy field deduced from measurements of flux, δl3C, and δD of CH4

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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 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 (∼46.5 g m−2 clayey and ∼43 g m−2 sandy) and average δ13CH4 (seasonal averages of −56.11±1.21‰ clayey and −53.57±0.97‰ 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 (α) associated with the CO2/H2 reduction pathway for CH4 formation. Using a range of reasonable values for α, 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 δ13CH4 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±l‰, was used to calculate oxidation percent using belowground and emitted δ13CH4 values. In Texas, oxidation of CH4 in the soil increased from ∼20 to ∼60% over the 6 week period just prior to harvest.