Leaching of Nitrate, Atrazine, and Metribuzin from Sugarcane in Southern Louisiana

We have studied the leaching losses of NO 3 , atrazine [6-chloro-Nethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one] applied to sugarcane (Saccharum officinarum L.) planted in Mississippi River alluvial soil in southern Louisiana. Nitrogen (122 kg/ha) and atrazine (2.24 kg/ha) were applied in June, and atrazine (2.24 kg/ha) and metribuzin (1.12 kg/ha) were applied in December ; losses through a Sharkey clay (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquepts) into subsurface drains (5.5- and 10.9-m spacing) were measured for about 100 d in both seasons. Five days after application NO 3 -N appeared in its highest concentrations (5-11 mg/L) in the drain water ; after this first event, concentrations remained below 10 mg/L throughout the summer season. After application in the summer atrazine appeared in the subsurface drains at its highest seasonal concentrations (114-144 μg/L) on the day of application ; after 4 to 7 wk these concentrations remained below 3.0 μg/L. Total losses in the summer amounted to 3 to 8% of the NO 3 application and 0.6 to 1.2% of the atrazine application. Almost 50% of the NO 3 leaching into the subsurface drains occurred after Day 76, whereas 82% of the atrazine leached into the drains by Day 8. After the winter application, high concentrations of atrazine (67-81 μg/L) and metribuzin (52-94 μg/L) were measured within 8 d. Similarly, large concentrations of atrazine occurred in the drain water throughout the winter season. The much higher concentrations of atrazine during the winter study, compared with the summer, coincided with soil surface concentrations that were 3 to 10 times those of the summer. Total losses during the winter were 0.4 to 2.0% (atrazine) and 0.4 to 1.7% (metribuzin) of the applications. Evidence for preferential flow into the drains of the NO 3 and the herbicides is presented.