HR: 17:30h
AN: B14A-06    [Abstracts]
TI: Bacterial and Chemical Degradation of Methyl Bromide and Methyl Chloride in Soils: Too Much of a Good Thing.
AU: * Miller, L G
EM: lgmiller@usgs.gov
AF: U.S. Geological Survey, Mail Stop 465 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Baesman, S M
EM: sbaesman@usgs.gov
AF: U.S. Geological Survey, Mail Stop 480 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Oremland, R S
EM: roremlan@usgs.gov
AF: U.S. Geological Survey, Mail Stop 480 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: Soils are the second largest sink for atmospheric methyl bromide (CH₃Br) and methyl chloride (CH₃Cl). The largest sink, reaction with OH in the troposphere, is only two-fold greater. Soils, however, comprise a very thin (1-2 m) zone compared with the troposphere (10-18 km) and are therefore an environment of relatively intense biological activity. Soils influence the composition of the troposphere as well as the flux of ozone-depleting compounds, such as methyl halides, to the stratosphere. Soil bacteria have been shown to degrade tropospheric levels of CH₃Br (~10 pptv) and are likely responsible for removing CH₃Cl (~550 pptv) from the atmosphere. In fact, the current budgets for atmospheric CH₃Br and CH₃Cl are imbalanced by as much as 40%, primarily because there are as yet no identified sources to match the bacterial soil sink. The soil microorganisms responsible for degrading CH₃Br and CH₃Cl at fumigation (%) levels have been identified using stable isotope probing (SIP) and functional gene probing. Not surprisingly, there is a greater diversity of methyl halide degraders present and functioning in soil than has previously been determined using culture techniques. However, the identity of microorganisms responsible for degradation of methyl halides at tropospheric (pptv) levels remains to be determined. Chemical reactions, including hydrolysis and halide exchange, also remove CH₃Br but are not as important for CH₃Cl. The chemical reactions that operate at low concentrations (e.g. tropospheric levels) also operate at elevated concentrations, such as those achieved during soil fumigation with CH₃Br, and we have demonstrated large carbon kinetic isotope effects (KIEs) for these reactions. Similarly, we have reported large carbon KIEs for bacterial degradation of CH₃Br and CH₃Cl in both soil microcosms and pure cultures of methylotrophic bacteria. The KIEs of these sink reactions are used to constrain the atmospheric budgets and we present the current stable isotope mass balances for each compound.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0322 Constituent sources and sinks
DE: 0490 Trace gases
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
SC: Biogeosciences [B]
MN: Fall Meeting 2005