Assessment of nitrification and urease inhibitors on nitrate leaching in corn (Zea mays L.)

Agricultural ecosystems are one of the largest global contributors to nitrate (NO3-) contamination of surface- and groundwater through fertilizer application. Improved fertilizer practices are needed to manage crop nutrient supply in corn (Zea mays L.) while minimizing impacts to clean water reserves. The goal of this study was to compare current nitrogen (N) fertilizer practices (urea at planting) with "packages" of improved management practices (a combination of right timing and product) that farmers potentially use. We conducted measurements in a continuous corn system from November 2015 to May 2017 at a large field scale (four 4 ha plots). Nitrate concentration was measured below the root zone and drainage estimated using a soil water budget approach in which evapotranspiration was measured using the eddy covariance method. The objective was to compare NO3--N leaching from fields receiving urea vs. urea + combination of nitrification and urease inhibitors (NUI) fertilizer applications at planting, urea-ammonium nitrate (UAN) vs. UAN + NUI applied at sidedress, and a combination of these practices: urea + NUI at planting vs. UAN at sidedress. Drainage was only significant in the non-growing season. Neither fertilizer products applied with NUI at planting or sidedress proved to significantly reduce NO3--N leaching. The combination of delaying fertilization to sidedress and applying UAN significantly reduced the soil water NO3--N concentration compared with urea + NUI at planting (mean of 5.2 vs. 6.7 mg L-1) but only in 2015-2016. Based on these results, applying UAN at sidedress is recommended, although additional study years are needed to confirm those results.