Lysimeter study of nitrate leaching from a corn-soybean rotation

High rates of N fertilizer in the production of continuous corn (Zea mays L,) have resulted in excessive nitrate N(NO3-N) leaching, with concentrations in ground water frequently exceeding the maximum contaminant Ic, cl (MCL) of 10 mg/L. This study was conducted to determine whether NO3-N leaching would be reduced by allowing for a legume N credit for soybean [Glycine max (L.) Merr.], and applying less N fertilizer to corn in a corn-soybean rotation than would be applied to continuous corn. A rge (Secale cereale L,) winter cover crop was used following soybean. In the spring of each corn gear, 140 kg Nma as NH4NO3 was surface applied to two large, undisturbed monolith lysimeters (8.1 m(2) surface area, 2.4 m deep), and 196 kg N/ha was applied to two other lysimeters. Prior to 6 yr of this treatment, there was a 6 yr period during which lysimeters received 224 kg N/ha in the spring of the corn year. The highest NO3-N concentrations and the most transport occurred during the winter/spring soil moisture recharge period (November through April). Concentrations of NO3-N in the percolate from all four lysimeters were similar, with a 6-yr, flow-weighted average of 9.9 +/- 2.5 mg/L. Although reducing N fertilizer inputs in a corn-soybenn rotation to allow for a legume N credit may lower N leaching amounts and concentrations, the NO3-N MCL may still be exceeded. For a given gear, weather can impart percolation and leaching more than the current crop.