Impacts of 4R Nitrogen Management on Crop Production and Nitrate-Nitrogen Loss in Tile Drainage

This three-year study near Sutherland, Iowa began in January 2015 to evaluate three N management practices in corn at 135 lb N/A (150 kg N/ha), and a no N control: 1) fall anhydrous ammonia with nitrification inhibitor (Nitrapyrin), 2) spring anhydrous ammonia (no inhibitor), and 3) split N application with variable N at sidedress (40 lb N/A (45 kg/ha) of urea 2 x 2 in. (5 x 5 cm) starter at planting plus in-season urease-inhibitor (Agrotain®) treated urea. Unfortunately, soil freezing occurred early in fall 2014 and prevented fall N application; necessitating N application in early spring 2015. Each treatment is replicated four times within a corn-soybean rotation, with each phase of the rotation present each year. The objectives are to: 1) determine the effects of N fertilizer application timing on nitrate-N leaching losses through tile flow in each crop phase; 2) determine the effects of N fertilizer application timing on crop yield; and 3) disseminate project findings through peer-reviewed journal articles, research farm reports, Extension and Outreach fact sheets and presentations, and other outlets as appropriate. Agronomic operations were completed in a timely manner in 2015. The 2015 year was characterized by 80% more precipitation in late summer and 225% more fall precipitation than the normal 30-yr average precipitation [30.7 inches/calendar year (78 cm/yr)].

There was a 40 bu/A (2,500 kg/ha) yield increase with the use of N in treatments 1-3 (average 223 bu/A, or 13,990 kg/ha) as compared to treatment 4 where no N was applied. Of note is that oats were uniformly grown on the site in 2013 and soybean in 2014. There were no statistical differences among the 2015 soybean yields (63 bu/A, or 3,375 kg/ha), which would be expected based on the uniform previous site history and no treatments applied to soybean. There were no statistically significant differences in flow-weighted nitrate-N concentrations in drainage water between treatments where soybean was grown in 2015 (12.7 mg/L nitrate-N), which would also be expected. In the corn phase, the treatment where no N was applied had statistically lower nitrate-N concentrations than treatments where N was applied to corn (14.6 vs. 9.1 mg/L nitrate-N). We will begin water quality monitoring in 2016 as soon as weather conditions allow. We are also working to summarize the crop sensing data, stalk nitrate concentrations, and soil nitrate data that were collected in 2015.