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Abstract High rates of N fertilizer in the production of continuous corn ( Zea mays L.) have resulted in excessive nitrate N (NO 3 ‐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 NO 3 ‐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 NH 4 NO 3 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 NO 3 ‐N concentrations and the most transport occurred during the winter/spring soil moisture recharge period (November through April). Concentrations of NO 3 ‐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 NO 3 ‐N MCL may still be exceeded. For a given year, weather can impact percolation and leaching more than the current crop.
Owens et al. (Wed,) studied this question.