Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2008
Publication Date: 3/13/2009
Citation: Zablotowicz, R.M., Krutz, L.J., Accinelli, C., Reddy, K.N. 2009. Bromoxynil Degradation in a Mississippi Silt Loam Soil. Pest Management Science 65:658-664.
Interpretive Summary: There is limited data on the environmental fate of the herbicide bromoxynil, thus a series of laboratory studies were conducted to assess the degradation and sorption of this herbicide and explore mechanisms for its dissipation. Bromoxynil is moderately sorbed to soil with the greatest amount binding to surface soils from no-till plots. The half-life of bromoxynil is less than 2 d in these studies, with its major sink as incorporation into soil organic matter, mediated by microbial metabolism, with a temporary accumulation of metabolites. Under recommended application rates bromoxynil should have minimal risk to the soil or water environment.
Technical Abstract: There is a paucity of data on bromoxynil fate in agricultural soils. The objective of these laboratory experiments were to 1) assess bromoxynil sorption, mineralization, bound residue formation, and persistence of extractable residues in native (i.e., non-autoclaved) Dundee silt loam soil collected from 0- to 2- cm (A) and 2- to 10- cm (B) depths from plots under continuous conventional-tillage (CT) and no-tillage (NT); 2) compare bromoxynil mineralization, bound residue formation, and persistence of extractible residues between native and autoclaved CT-A soils; 3) determine the partitioning of non-extractible residues between the humic and fulvic acid fractions in native CT-A soil; and 4) ascertain the effects of bromoxynil concentration (2 or 10 mg kg-1) on extractible residues, bound residue formation and metabolite formation in native CT-A soil. Bromoxynil Kd values ranged from 0.7 to 1.4 L kg-1 and were positively correlated with soil organic carbon. Cumulative mineralization (38.5±1.5), bound residue formation (46.5±0.5), and persistence of extractable residues ( T1/2 < 1 d) in native soils were independent of tillage and depth. Autoclaving CT-A decreased mineralization and bound residue formation 257-fold and 6.0-fold, respectively. Bromoxynil persistence in CT-A soil was rate independent (T 1/2 < 1d), and the majority of non-extractable residues (87%) was associated with the humic acid fraction of soil organic matter. Independent of tillage or depth, bromoxynil half-life values in native soil is less than 1 day due to rapid incorporation of the herbicide into non-extractable residues. Bound residue formation is governed principally by a biochemical mechanism(s) predicated upon metabolite formation. Non-extractable residues are associated primarily with the humic acid fraction of soil organic matter and are moderately bio-available as evidenced by the mineralization data. Collectively, these data indicate that the risk for off-site transport of bromoxynil residues is low due to rapid incorporation into non-extractable residues.