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 level (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 rye (Secale cereale L.) winter cover crop was used following soybean. In the spring of each corn year, 140 kg N/ha as NH4NO3 was surface applied to two large, undisturbed monolith lysimeters (8.1 m2 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‐soybean 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 year, weather can impact percolation and leaching more than the current crop.