BACTERIAL OXIDATION OF METHYL-BROMIDE IN FUMIGATED AGRICULTURAL SOILS

The oxidation of [C-14]methyl bromide ([C-14]MeBr) to (CO2)-C-14 was m easured in field experiments with soils collected from two strawberry plots fumigated with mixtures of MeBr and chloropicrin (CCl3NO2), Alth ough these fumigants are considered potent biocides, we found that the highest rates of MeBr oxidation occurred 1 to 2 days after injection when the fields were tarped, rather than before or several days after injection, No oxidation of MeBr occurred in heat-killed soils, indicat ing that microbes were the causative agents of the oxidation. Degradat ion of MeBr by chemical and/or biological processes accounted for 20 t o 50% of the loss of MeBr during fumigation, with evasion to the atmos phere inferred to comprise the remainder, In laboratory incubations, c omplete removal of [C-14]MeBr occurred within a few days, with 47 to 6 7% of the added MeBr oxidized to (CO2)-C-14 and the remainder of count s associated with the solid phase. Chloropicrin inhibited the oxidatio n of MeBr, implying that use of this substance constrains the extent o f microbial degradation of MeBr during fumigation, Oxidation was by di rect bacterial attack of MeBr and not of methanol, a product of the ch emical hydrolysis of MeBr. Neither nitrifying nor methane-oxidizing ba cteria were sufficiently active in these soils to account for the obse rved oxidation of MeBr, nor could the microbial degradation of MeBr be linked to cooxidation with exogenously supplied electron donors, Howe ver, repeated addition of MeBr to live soils resulted in higher rates of its removal, suggesting that soil bacteria used MeBr as an electron donor for growth, To support this interpretation, we isolated a gram- negative, aerobic bacterium from these soils which grew with MeBr as a sole source of carbon and energy.