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Title: DISSIPATION OF THE HERBICIDE (14C) DIMETHENAMID UNDER ANAEROBIC AQUATIC CONDITIONS IN FLOODED SOIL MICROCOSMS

Author
item CRAWFORD, JENNIFER - UNIV OF ILLINOIS
item SIMS, GERALD
item SIMMONS, FREDERICK - UNIV OF ILLINOIS
item WAX, LOYD
item FREEDMAN, DAVID - UNIV OF ILLINOIS

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: The fate of pesticides in anaerobic (absence of oxygen) ecosystems is of key importance in the movement of these substances through the environment. Pesticide registration involves experiments designed to address anaerobic fate. Recent research has shown that not all anaerobic environments are alike, and the fate of pesticides may depend on the specific conditions present in any particular anaerobic environment. In this study, we examined the fate of the herbicide dimethenamid in two commonly occurring anaerobic regimes (nitrate reducing and sulfate reducing) and compared this with the fate of the compound using the standard approach employed for pesticide registration studies (unspecified anaerobic conditions). The results showed that the anaerobic fate of the compound was dependent upon the specific conditions present. Since the standard approach used in pesticide registration studies does not specify any particular anaerobic regime, nor does it involve the rigorous characterization of what conditions are present, it is difficult to apply the results of such registration studies to the anaerobic fate of pesticides in a variety of environments. The impact of this study is the potential for application of the approach employed for pesticide registration or other studies used to characterize or predict the fate of pesticides in the environment. Potential benefits which will be useful to public and private sector scientists, include the possibility of better prediction of pesticide fate and thus enhancement of our ability to manage agricultural systems to reduce contamination of natural waters.

Technical Abstract: Microbial degradation is believed to be the primary means of dimethenamid degradation in aerobic soil; however, the impact of anaerobic microbial activity on dissipation is not known. The objective of the current research was to compare 2 anaerobic metabolism protocols with the protocol recommended by the EPA in order to compare methods for establishing anaerobic conditions in flooded soil and the corresponding effect on 14C-dimethenamid dissipation and biodegradation as related to microbial activity. Sediment-water biometers were prepared with a saturated Sawmill silty clay loam (Cumulic Haplaquoll) and made anaerobic by either glucose pretreatment or N2-sparging. Sparged treatments included No3-+SO42 amendment, unamended, and autoclaved control. Biometers were dosed with radiolabeled and unlabeled dimethensmid and sacrificed in triplicate periodically during a 142-day incubation. Volatile, aqueous, extractable, and bound (unextractable) 14C-residues were characterized using liquid scintillation counting, thin-layer radiochromatography, and soil combustion. Eh, pH, NO3-, SO42-, Fe2+, FeS2, and CH4 were monitored. The most significant terminal fate of dimethenamid-derived 14C was bound residue formation (>50% of applied in all treatments). The activities of denitrifying and sulfate-reducing bacteria resulted in most extensive transformation of dimethenamid, evident by metabolite formation, compared with glucose-treated and unamended biometers. Dissipation and metabolite trends were nearly identical in the 2 treatments that were not amended with terminal electron acceptors. Results demonstrated that in flooded soil, anaerobic microbial activity was largely responsible for dimethenamid degradation and bound residue formation.