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

Field studies comparing the fate of herbicides under various tillage p ractices have attributed the observed differences in herbicide leachin g to tillage effects, It is not clear whether observed herbicide behav ior is caused by tillage practice alone rather than inherent variation s associated with individual tillage treatment plots, Therefore, the o bjective of this field study was to evaluate the effect of reversing t he tillage of 7-year-old no-till (NT) and conventional-till (CT) field plots on the leaching patterns and dissipation of atrazine hyl-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 plant ing, Atrazine concentrations were determined in the crop residue and i n 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 1 994 (after tillage reversal), An average of 1.5 to 2 times more atrazi ne was recovered in the surface 10 cm of soil under CT than under NT i n all 3 years, This difference was attributable to the interception of atrazine by crop residue in the NT plots, regardless of tillage rever sal (73, 44, and 58% intercepted in 1992, 1993, and 1994, respectively ), The continuation of trends for more atrazine in the topsoil of CT p lots 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 th e variability of atrazine residue levels in the new CT plots (after ti llage reversal) rather than to an increase in the magnitude of the dif ferences between the means, Tillage probably resulted in both a more h omogeneous distribution of organic matter within the top soil profile and disruption of the macropores in the new CT plots, resulting in a m ore uniform distribution of atrazine residues.