Nitrogen Management for Corn and Groundwater Quality in Upper Midwest Irrigated Sands

Groundwater contamination from NO3–N leaching in corn (Zea mays L.) production with coarse‐textured soils poses an environmental concern. Our objectives were to evaluate NO3–N leaching in continuous corn (CC), corn after soybean (Glycine max L.) (CSb), and soybean after corn (SbC) in irrigated sandy soils in Minnesota related to (i) N rate using best management practices of split‐N application, (ii) a split‐N application and single preplant applications of enhanced‐efficiency fertilizers (EEF), and (iii) residual N treatment in SbC. Urea (0–315 kg N ha−1 in 45‐kg increments) was broadcast as a split application (half at preplant and half at the V4 development stage) and polymer‐coated urea (ESN), ESN/urea, and SuperU at preplant at a rate of 180 kg N ha−1 on an Arvilla sandy loam soil. In May and June, 75% of the total drainage and 73% of the total NO3–N leached occurred. At the economic optimum N rate (EONR), season‐long NO3–N leaching rates were 86 and 106 kg NO3–N ha−1 for CC and CSb, respectively. In CC, reducing the EONR by 20% reduced grain yield by 4% and NO3–N leached by 9%, and a 25% reduction in EONR resulted in an additional 2% reduction for both, whereas no significant reductions occurred for CSb. Similar NO3–N leaching occurred with EEFs and the split‐N application. After 4 yr of no N application, we measured 9 to 20 mg NO3–N L−1 and leaching of 21 to 51 kg NO3–N ha−1, highlighting the difficulty of meeting drinking water quality standards in corn cropping systems. Core Ideas Enhanced‐efficiency N fertilizers did not reduce nitrate amounts leached. Suboptimum N fertilizer rates reduce corn yield without enhancing water quality. Early spring precipitation, when crops are small, drives nitrate amounts leached.