, Volume 60, Issue 2, pp 181-190

Carbon isotope fractionation of methyl bromideduring agricultural soil fumigations

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Abstract

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 theδ13C 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 −20.7‰over 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 meanδ13C 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 δ13C of anthropogenicallyemitted CH3Br ranges from −53.2‰ to −47.5‰.