Assessing Nitrogen Use Efficiency and Nitrate Leaching of Maize Grown on Irrigated Sandy Soils.

Irrigated sands are some of the most productive and environmentally sensitive areas in Minnesota. Reducing nitrate leaching is critical for corn (Zea mays L.) production as ground water is a major source of drinking water in these soils. A randomized complete block design with four replications was established in Dakota County, MN with continuous corn (C-C) and in Pope County, MN with corn after soybeans (C-S), soybean after corn (S-C) and C-C with urea broadcast at rates of 0, 45, 90, 135, 180, 225, 270, and 315 kg N  ha^-1 as a split applications, half of the rate at pre-plant and half at V4 development stage. Two additional nitrogen sources Super U at 180 kg N ha^-1 and ESN at 180 and 225 kg N ha^-1, were applied at planting. Crop canopy was sensed with SPAD meter, GreenSeeker, and Crop Circle technologies. Whole plant dry biomass and nitrogen concentrations were measured at the V8 and V12 development stages. Grain nitrogen content and yield were determined at harvest. Soil water below the root zone was collected from suction cup lysimeters weekly throughout the growing season for nitrate (NO₃-N) analysis. Yield was effectively predicted (R² between 0.59 and 0.89) with the use of SPAD meter and Crop Circle. The Maximum Return to Nitrogen (MTRN) rate ranged from 240 to 270 kg N ha^-1 between the two locations. In Pope County in 2013 season-long mean water NO₃-N concentrations in mg L^-1 and load (between parenthesis in kg N ha^-1) were 36.7 (52) for C-S, 16.6 (27) for S-C, and 23.8 (32) for C-C. In C-C increasing N rate from 135 to 270 kg N ha^-1 increased NO₃-N load from 29 to 33 kg N ha^-1 while yield increased from 11.9 to 12.9 Mg ha^-1. No concentration or load differences occurred with different N sources.