IMPACT OF REVERSING TILLAGE PRACTICES ON MOVEMENT AND DISSIPATION OF ATRAZINE IN SOIL

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Abstract
Field studies comparing the fate of herbicides under various tillage practices have attributed the observed differences in herbicide leaching to tillage effects. It is not clear whether observed herbicide behavior is caused by tillage practice alone rather than inherent variations associated with individual tillage treatment plots. Therefore, the objective of this field study was to evaluate the effect of reversing the tillage of 7-year-old no-till (NT) and conventional-till (CT) field plots on the leaching patterns and dissipation of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] in soil. No-till and CT field plots, established in 1986, were reversed in 1993 prior to corn planting. Atrazine concentrations were determined in the crop residue and in the top 50 cm of soil from both tillage systems 0, 2, 4, and 8 weeks after application in 1992 (before tillage reversal) and in 1993 and 1994 (after tillage reversal). An average of 1.5 to 2 times more atrazine was recovered in the surface 10 cm of soil under CT than under NT in all 3 years. This difference was attributable to the interception of atrazine by crop residue in the NT plots, regardless of tillage reversal (73, 44, and 58% intercepted in 1992, 1993, and 1994, respectively). The continuation of trends for more atrazine in the topsoil of CT plots than in NT plots after tillage reversal indicated the importance of crop residue on interception of the atrazine spray. Differences in atrazine means between NT and CT plots in the 0 to 10-cm soil depth 2 weeks after application were significant only in 1993 and 1994 at the 80% confidence intervals. This was primarily caused by a decrease in the variability of atrazine residue levels in the new CT plots (after tillage reversal) rather than to an increase in the magnitude of the differences between the means. Tillage probably resulted in both a more homogeneous distribution of organic matter within the top soil profile and disruption of the macropores in the new CT plots, resulting in a more uniform distribution of atrazine residues.