Biodegradability of atrazine, cyanazine, and dicamba in wetland soils

Abstract

Small amounts of applied pesticide chemicals eventually can end up in nontarget areas such as wetlands, sediments, and groundwater where anaerobic conditions often predominate. Runoff and leaching are major means by which pesticides move away from application sites. Pesticides also can find their way into nontarget areas as a result of inappropriate disposal and accidental spills. We evaluated the biodegradability of atrazine, cyanazine, and dicamba in wetland soils under nitratereducing and methanogenic conditions. Wetland soil samples were collected from three different sites in the Chesapeake Bay watershed region. These sites represented both tidal (Lawnes and Levy soils) and nontidal (Myatt soil) wetlands. Tidal wetlands are water-saturated throughout the year, whereas nontidal wetlands are saturated only during certain times of the year. Herbicide fate studies were conducted in wetland soil microcosms consisting of serum bottles filled with soil slurry and containing either atrazine, cyanazine, or dicamba.

Atrazine was extremely stable in wetland soil microcosms regardless of incubation temperature, redox status (nitrate-reducing versus methanogenic conditions), or soil type. Neither temperature nor redox status affected cyanazine stability in Myatt wetland soil microcosms. We observed a significant decrease in cyanazine concentration in Lawnes wetland microcosms incubated under methanogenic and nitratereducing conditions. Losses were more pronounced at 25° than at 15°C. Results from enrichment culture studies suggest that cyanazine was cometabolized (i.e., cyanazine could not be used as a carbon and energy source by the microorganisms) in Lawnes soil microcosms. Dicamba was readily biodegraded in the wetland soils tested, although total mineralization was not achieved.