HR: 0830h
AN: B31E-0358    [PDF]
TI: Methane Production Pathways in a California Rice Paddy: Isotopic Evidence for Substantial CO$_{2}$ Reduction as Cause for Isotopically Light Emitted CH$_{4}$ Carbon
AU: * Tyler, S C
EM: styler@uci.edu
AF: University of California, Irvine, Earth System Science Dept., Irvine, CA 92697-3100 United States
AU: McMillan, A M
EM: mcmillan@essgrad.ps.uci.edu
AF: University of California, Irvine, Earth System Science Dept., Irvine, CA 92697-3100 United States
AU: Bearden, K
AF: University of California, Irvine, Earth System Science Dept., Irvine, CA 92697-3100 United States
AU: Chidthaisong, A
EM: amnat_c@jgsee.kmutt.ac.th
AF: King Mongkut's University of Technology Thonburi, The Joint Graduate School of Energy and Environment, Bangkok, 10140 Thailand
AU: Macalady, J
EM: jmacalad@carleton.edu
AF: Carleton College, Geology Department, Northfield, MN 55057 United States
AB: We report measurements of $\delta$$^{13}$C of emitted CH$_{4}$ and sediment CH$_{4}$ and CO$_{2}$ during the 1999 rice-growing season near Maxwell, CA. Two treatments, one with rice straw incorporated from the previous season and one without rice straw were studied. The $\delta$$^{13}$C value of emitted CH$_{4}$ was consistently lighter isotopically (-67$\permil$ to -83$\permil$ throughout the season) in both straw incorporated and straw removed (burned) plots than in fields we have studied in Texas, Kenya, and Japan. Measured isotopic values of the production zone CH$_{4}$ were compared to a two-point mixing curve representative of isotopic CH$_{4}$ produced from either pure methyl-group fermentation or CO$_{2}$ reduction pathways to partition the production pathways and to track seasonal changes in the production processes. Our sediment CH$_{4}$ and CO$_{2}$ isotope data indicate that fermentation was rarely the dominant methanogenic pathway - on the contrary CO$_{2}$ reduction with H$_{2}$ was more prevalent than fermentation methanogenesis throughout most of the season. The relatively isotopically light CH$_{4}$ emitted by the paddy fields is also a product of oxidation and stem-transport processes which have isotopic effects of their own. These effects are discussed in context with the methanogenic isotope effects to provide a complete picture of the paddy field CH$_{4}$ carbon isotope system.
DE: 0315 Biosphere/atmosphere interactions
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 4840 Microbiology
SC: Biogeosciences [B]
MN: 2003 Fall Meeting