EFFECTS OF CONTROLLED DRAINAGE ON NITRATE CONCENTRATIONS IN SUBSURFACE DRAIN DISCHARGE

A water table management field study was conducted on a Bainesville si lt loam soil during 1992 and 1993. The water table levels studied were conventional free outlet subsurface drainage (FD), and controlled wat er tables (CWT) of 0.50 and 0.25 m above the drain level. The three tr eatments were replicated thrice resulting in nine plots, each measurin g 115 m long by 18.69 m wide. A subsurface drain was installed 1.0 m d eep in the centre of each plot. Drain discharge, nitrate concentration s in drainage effluent, rainfall and water table elevations were measu red during the two growing seasons. The plots were cropped with grain corn (Zea mays L.) in 1992, and soybean (Glycine max (L.) Mill.) in 19 93. Controlled drainage had a significant effect on drain discharge qu antity and quality. In 1992, the 0.25 and 0.50 m CWT treatments reduce d drain flow by 58.7% and 65.3% respectively; and in 1993, by 40.9% an d 95%, respectively, compared with the FD treatment. In 1992, there wa s a 75.9% and 68.9% reduction of nitrate concentration in drain flow w ith the 0.25 and 0.50 m CWT, respectively, compared with FD. In 1993, the reductions were 62.3% and 95.7% for the 0.25 and 0.5 m CWT, respec tively. While it was impossible to maintain the water tables consisten tly at 0.5 and 0.25 m throughout the growing season, these results sho w that there are significant environmental benefits with controlled dr ainage.