Nitrate Loss in Subsurface Drainage as Affected by Nitrogen Fertilizer Rate

The relationships between N fertilizer rate, yield, and NO 3 leaching need to be quantified to develop soil and crop management practices that are economically and environmentally sustainable. From 1996 through 1999, we measured yield and NO 3 loss from a subsurface drained field in central Iowa at three N fertilizer rates: a low (L) rate of 67 kg ha −1 in 1996 and 57 kg ha −1 in 1998, a medium (M) rate of 135 kg ha −1 in 1996 and 114 kg ha −1 in 1998, and a high (H) rate of 202 kg ha −1 in 1996 and 172 kg ha −1 in 1998. Corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] were grown in rotation with N fertilizer applied in the spring to corn only. For the L treatment, NO 3 concentrations in the drainage water exceeded the 10 mg N L −1 maximum contaminant level (MCL) established by the USEPA for drinking water only during the years that corn was grown. For the M and H treatments, NO 3 concentrations exceeded the MCL in all years, regardless of crop grown. For all years, the NO 3 mass loss in tile drainage water from the H treatment (48 kg N ha −1 ) was significantly greater than the mass losses from the M (35 kg N ha −1 ) and L (29 kg N ha −1 ) treatments, which were not significantly different. The economically optimum N fertilizer rate for corn was between 67 and 135 kg ha −1 in 1996 and 114 and 172 kg ha −1 in 1998, but the net N mass balance indicated that N was being mined from the soil at these N fertilizer levels and that the system would not be sustainable.