Biodegradation of the herbicide bromoxynil and its plant cell wall bound residues in an agricultural soil

The mineralization and formation of metabolites and nonextractable residues of the herbicide [¹⁴C]bromoxyniloctanoate ([¹⁴C]3,5-dibromo-4-octanoylbenzonitrile) and the corresponding agent substance [¹⁴C]bromoxynil ([¹⁴C]3,5-dibromo-4-hydroxybenzonitrile) was investigated in a soil from an agricultural site in a model experiment. The mineralization of maize cell wall bound bromoxynil residues was also investigated in the agricultural soil material. The mineralization of [¹⁴C]bromoxynil and [¹⁴C]bromoxyniloctanoate in soil within 60 days amounted up to 42 and 49%, respectively. After the experiments, 52% of the originally applied [¹⁴C]bromoxynil and 44% of the [¹⁴C]bromoxyniloctanoate formed nonextractable residues in soil. Plant cell wall bound [¹⁴C]bromoxynil residues were also mineralized to an extent of about 21% within 70 days; the main portion of 76% persisted as nonextractable residues in the soil. In bacterial enrichment cultures and in soil two polar metabolites were observed; one of it could be identified as 3,5-dibromo-4-hydroxybenzoate and the other could be described tentatively as 3,5-dibromo-4-hydroxybenzamide.