Carbon isotope fractionation of methyl bromideduring agricultural soil fumigations

The isotopic composition ofmethyl bromide (CH₃Br) has been suggestedto be a potentially useful tracer forconstraining the global CH₃Br budget. Inorder to determine the carbon isotopiccomposition of CH₃Br emitted from the mostsignificant anthropogenic application(pre-plant fumigation) we directly measured the¹³C of CH₃Br released duringcommercial fumigation. We also measured theisotopic fractionation associated withdegradation in agricultural soil under typicalfield fumigation conditions. The isotopiccomposition of CH₃Br collected in soilseveral hours after injection of the fumigantwas –44.5 and this value increased to –20.7over the following three days. The mean kineticisotope effect (KIE) associated withdegradation of CH₃Br in agricultural soil(12) was smaller than the reported value formethylotrophic bacterial strain IMB-1, isolatedfrom previously fumigated agricultural soil,but was similar to methylotrophic bacterialstrain CC495, isolated from a pristine forestlitter zone. Using this fractionationassociated with the degradation of CH₃Brin agricultural soil and the mean¹³C of the industriallymanufactured CH₃Br (–54.4), we calculatethat the agricultural soil fumigation sourcehas a carbon isotope signature that ranges from–52.8 to –42.0. Roughly 65% ofindustrially manufactured CH₃Br is usedfor field fumigations. The remaining 35% isused for structural and post-harvestfumigations with a minor amount used duringindustrial chemical manufacturing. Assumingthat the structural and post-harvest fumigationsources of CH₃Br are emitted withoutsubstantial fractionation, we calculate thatthe ¹³C of anthropogenicallyemitted CH₃Br ranges from –53.2 to –47.5.