Bacterial and Chemical Degradation of Methyl Bromide and Methyl Chloride in Soils: Too Much of a Good Thing.
Abstract
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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.B14A..06M
- Keywords:
-
- 0315 Biosphere/atmosphere interactions (0426;
- 1610);
- 0322 Constituent sources and sinks;
- 0490 Trace gases;
- 1041 Stable isotope geochemistry (0454;
- 4870);
- 1615 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 4805;
- 4912)