Carbon isotope fractionation of methyl bromide during agricultural soil fumigations

The isotopic composition of methyl bromide (CH3Br) has been suggested to be a potentially useful tracer for constraining the global CH3Br budget. In order to determine the carbon isotopic composition of CH3Br emitted from the most significant anthropogenic application (pre-plant fumigation) we directly measured the delta(13)C of CH3Br released during commercial fumigation. We also measured the isotopic fractionation associated with degradation in agricultural soil under typical field fumigation conditions. The isotopic composition of CH3Br collected in soil several hours after injection of the fumigant was -44.5parts per thousand and this value increased to -20.7parts per thousand over the following three days. The mean kinetic isotope effect (KIE) associated with degradation of CH3Br in agricultural soil (12parts per thousand) was smaller than the reported value for methylotrophic bacterial strain IMB-1, isolated from previously fumigated agricultural soil, but was similar to methylotrophic bacterial strain CC495, isolated from a pristine forest litter zone. Using this fractionation associated with the degradation of CH3Br in agricultural soil and the mean delta(13)C of the industrially manufactured CH3Br (-54.4parts per thousand), we calculate that the agricultural soil fumigation source has a carbon isotope signature that ranges from -52.8parts per thousand to -42.0parts per thousand. Roughly 65% of industrially manufactured CH3Br is used for field fumigations. The remaining 35% is used for structural and post-harvest fumigations with a minor amount used during industrial chemical manufacturing. Assuming that the structural and post-harvest fumigation sources of CH3Br are emitted without substantial fractionation, we calculate that the delta(13)C of anthropogenically emitted CH3Br ranges from -53.2parts per thousand to -47.5parts per thousand.