Soybean (Glycine max) influences metolachlor mobility in soil

This study was conducted to evaluate the mobility of 14 C-metolachlor over 1 yr for three seasons when applied preemergent to undisturbed field lysimeters with and without soybean representing cropped and noncropped zones, respectively. Leachate was collected weekly and analyzed for total 14 C, metolachlor, and metabolites. Lysimeters were removed, sectioned, and analyzed for 14 C. Sixty and 90 days after treatment (DAT), there was less soil water in lysimeters with soybean. Recovery of 14 C in lysimeters decreased with time and ranged from 54 to 74% 30 DAT followed by a slower rate of loss with 35 to 49% remaining 365 DAT. Comparable amounts of total 14 C were observed in soybean lysimeters as in fallow lysimeters 30, 60, and 90 DAT. 14 C distribution in the lysimeters, however, was quite different. Sixty and 90 DAT, 14 C mobility in soybean lysimeters was less than in fallow lysimeters. Also, less leachate was collected from soybean lysimeters, which resulted in later appearances and lesser amounts of 14 C in the leachate. Cumulative leachate from lysimeters with and without soybean 365 DAT contained 2% and 10 to 18% of the applied 14 C, respectively. Peak concentrations of 14 C in leachate from fallow columns occurred about 90 DAT and were two to 19 times higher than 14 C concentrations in leachate from soybean lysimeters. Metolachlor concentrations in leachate were well below the National Health Advisory level for drinking water in all cases. Apparent volatilization losses of 14 C amounted to 26 to 46% of the applied 14 C-metolachlor 30 DAT. These results suggest that herbicide mobility is different in cropped vs. fallow sites and possibly in intra- and interrow crop positions.