Mechanism of degradation of methyl bromide and propargyl bromide in soil

The degradation of methyl bromide (MB) and propargyl bromide (PB) was inves tigated in soil and water to obtain information on the mechanism of degrada tion. It has been suggested that primary alkyl halides (including MB and th e potential alternatives PB and methyl iodide) can undergo S(N)2 nucleophil ic substitution with nucleophilic sites on soil organic matter (i.e., -NH2, -NH, -OH, -SH), The pattern of product formation observed in this study pr ovides more direct evidence that fumigants that are primary alkyl halides c an alkylate soil organic matter and that this may be a significant mechanis m of degradation in soil. Degradation in water samples (hydrolysis) formed Br- and the corresponding alcohol (propargyl alcohol from PB, methanol from MB) in equimolar amounts. The rate of hydrolysis was not significantly dif ferent from the rate of Br- formation for both MB and PB, Degradation in tw o soils resulted in the formation of Br-, but very little production of the corresponding alcohol, indicating that some mechanism other than hydrolysi s must be occurring in the soil. Degradation of MB and PB was much more rap id in the higher-organic-matter day loam soil than in the sandy loam soil. Spiking C-14-labeled MB to soil resulted in the formation of nonextractable (soil-bound) C-14, which increased as the extractable C-14 decreased, Micr obial oxidation was not significant in these soil samples, which were steri lized through autoclaving and/or treatment with high concentrations of fumi gants, These results provide further experimental evidence that MB, PB, and similar compounds can alkylate soil organic matter.