DISSIPATION AND DISTRIBUTION OF HERBICIDES IN A FLUVENTIC HAPLUDOLL SOIL

Determination of dissipation rates and/or accumulation of herbicides f rom long-term field studies can increase our understanding of agricult ural 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 ro-4-(ethylamino)-6-isopropylamino-1,3,5-triazine] and 25 di ssipation curves for alachlor -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 dissipatio n 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 s tatistically significant difference in alachlor dissipation over the 5 -year period. Overall, there was not a measurable difference in dissip ation of either herbicide that could be attributed to management syste m. Dissipation of atrazine was modeled well with a first-order exponen tial decay rate constant of 0.02 d(-1). Accumulation of atrazine in th e 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 app lication; zero-order dissipation controlled by desorption for fall, wi nter, and spring; and the accumulation of 20 mu g kg(-1) alachlor per year in a desorption resistant soil fraction.