Dissipation and distribution of herbicides in a Fluventic Hapludoll soil

Determination of dissipation rates and/or accumulation of herbicides from long-term field studies can increase our understanding of agricultural chemical distribution within the environment. Unfortunately, the cost of sample collection and analysis has limited the development of significant, long-term data sets describing chemical dissipation under natural climatic conditions. In this study, 15 dissipation curves for atrazine [2-chloro-4- (ethylamino)-6-isopropylamino-1,3,5-triazine] and 25 dissipation curves for alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] were developed by using data collected over a 5-year period in a Fluventic Hapludoll. Three agricultural management systems that included differences in tillage and crop rotations were imposed on the surface. Statistically significant differences in atrazine dissipation curves occurred among treatments in the fall and winter of the first year of the experiment. Only two of the 39 sampling events showed a statistically significant difference in alachlor dissipation over the 5-year period. Overall, there was not a measurable difference in dissipation of either herbicide that could be attributed to management system. Dissipation of atrazine was modeled well with a first-order exponential decay rate constant of 0.02 d^-1. Accumulation of atrazine in the soil profile did not occur. Alachlor was initially modeled well with a rate constant of 0.04 d^-1. Alachlor behavior in the soil could be described by first-order dissipation for the two months following application; zero-order dissipation controlled by desorption for fall, winter, and spring; and the accumulation of 20 μg kg^-1 alachlor per year in a desorption resistant soil fraction.