Effect of Nitrogen Management on Nitrate Leaching and Maize Nitrogen Use Efficiency on Highly Productive Irrigated Sandy Soils.

Groundwater nitrate (NO₃-N) concentrations in irrigated sandy soils  under corn (Zea mays L.) production in the Midwest are of increasing concern, and have prompted the need to identify new or enhanced nitrogen management practices in these areas. The objective of this study was to evaluate agricultural technologies that may improve nitrogen (N) management for profitable corn production and mitigate negative effects of NO₃-N in groundwater. From 2011 to 2014 corn was grown at two sites in Minnesota on sandy soils, Dakota County, MN with a continuous corn (C-C) rotation and Pope County, MN with a C-C, corn after soybeans (C-SB), and soybean after corn (Sb-C) rotations. Twelve treatments were applied including urea broadcast at rates of 0, 45, 90, 135, 180, 225, 270, and 315 kg N ha^-1 as a split application, half at pre-plant and half at the V4 development stage, pre-plant Super U at 180 kg N ha^-1, and pre-plant ESN at 180 and 225 kg N ha^-1. Canopy sensing with SPAD, GreenSeeker, and Crop Circle was done at V8 and V12 and NO₃-N basal stalk measurements at R6 development stage. Soil water NO₃-N samples were collected weekly throughout the growing season below the rooting zone using suction lysimeters. The mean Maximum Return to N (MRTN) was 237 kg ha^-1 and produced a mean-yield increase above the unfertilized check of 6.5 Mg ha^-1. Canopy sensors and plant measurements provided limited utility and under-predicted N needs. Nitrogen use efficiency and yields were increased with split-applied urea compared to all other pre-plant sources at 180 kg N ha^-1, but no reduction in NO₃-N leaching occurred. Season-long NO₃-N concentrations ranged from 10 to 46 mg L^-1 and overall annual loss was 27 to 41 kg NO₃-N ha^-1.  Reducing N rate below the MRTN substantially reduced yield without reducing NO₃-N leaching losses.