TILLAGE EFFECTS ON DRAINAGE FLUXES AND NITRATE LEACHING THROUGH UNSATURATED ZONE UNDER IRRIGATED CORN-SOYBEAN ROTATION

High levels of nitrate-nitrogen (NO3-N) in the nation's groundwater are a significant environmental concern. To date no studies have yet evaluated the effects of various tillage practices on percolated drainage and NO3-N fluxes below the rootzone of cropping systems in the northern Great Plains. A field study was conducted to examine and compare the effect of no-tillage (NT) and conventional tillage (CT) practices on seasonal drainage fluxes and NO3-N leaching losses below the rootzone in irrigated corn (Zea mays L.) and soybean (Glycine max L.) on a Lihen sandy loam soil. Sixteen passive capillary lysimeters, PCAPs (75 cm long polyvinyl chloride pipe with a collecting surface area of 0.1 m(2)) were placed 90 cm below the soil surface to quantify percolated drainage water below the rootzone of corn-soybean rotation under NT and CT practices. The study was designed as a randomized complete block with four replications. Drainage and NO3-N fluxes were not significantly affected by the tillage in 2014, 2015, 2016, and 2017 due to substantial spatial variations among lysimeters within each treatment. Average cumulative seasonal drainage depths across 4 years were 22.26 and 34.46 mm for corn and 24.95 and 28.16 mm for soybean under NT and CT, respectively. Averaged 4-yr cumulative NO3-N losses were 17.61 and 26.74 kg ha(-1) for corn and 25.47 and 23.56 kg ha(-1) for soybean under NT and CT, respectively. Flow-weighted NO3-N concentrations over 4 years were 57.9 and 65.7 mg L-1 for corn while those for soybean were 74.8 and 67.0 mg L-1 under NT and CT, respectively. Nitrate-nitrogen concentrations generally exceed the safe drinking water standard of 10 mg L-1. Reducing N inputs in well-drained soils and using site specific N and irrigation management practices are required to lower input expenses, reduce N leaching losses and sustain environmental quality.