ATRAZINE DISSIPATION, STUBBLE INTERCEPTION AND RETENTION IN SOILS UNDER VARIOUS TILLAGE SYSTEMS

Dissipation of atrazine {2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine} was studied in conventional till and no-till sorghum {Sorghum bicolor (L.) Moench} plots in a winter wheat (Triticum aestivum L.)-sorghum-fallow rotation in 1979, 1980, and 1981 at North Platte, Nebraska. In 1979, atrazine levels decreased sharply in both tillage systems 26 days after application. In 1980, most of the atrazine had dissipated from the top 10 cm of soil after 80 days in both systems. Atrazine levels in the surface 10 cm of soil in both systems decreased 74 and 59%, respectively, 20 days after application in 1981. Soil incubation research showed that initial atrazine dissipation was similar in conventional and no-till soils and was not altered by soil sterilization. Twenty days after application, atrazine levels decreased by approximately 50% in both soils. These observations support those obtained from field studies in which sharp decreases in atrazine levels occurred within 40 days after application and no difference in dissipation was found in conventional till and no-till plots. Atrazine retention by wheat stubble in the no-till rotation was determined on standing and flat stubble and in underlying soils at 3, 6, 9, 12, and 15 weeks after application. After application of 1.7 kg/ha atrazine to wheat stubble (approximately 6400 kg/ha), 60% of the herbicide was intercepted by the stubble and 40% reached the soil. Three weeks after application 91 and 64% of the initially retained atrazine was lost from standing and flat stubble, respectively. By the fifteenth week no atrazine was found on the stubble and only 13% of the initially applied atrazine was present in the underlying surface soil. Ring labeled (\u2714)C-atrazine was applied to wheat stubble in the greenhouse and the effect of successive, alternate day application of 12.5, 25, or 50 mm water on atrazine retention was determined. Atrazine loss from stubble was greatest with the first water application, but the relationship between water application rate and herbicide retention was not linear. Atrazine residues after the last water application were highest on stubble which had received the least amount of water. Dissipation of (\u2714)C-atrazine was determined in irrigated chisel, disk, and reduced tillage field plots using 25 cm aluminum columns. Tillage treatments had been applied for 2 years prior to the start of this study. No difference in percent of total column (\u2714)C activity was found among any of the tillage treatments at a given depth. In each case, (\u2714)C activity was greatest in the top 2.5 cm of soil