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 nitrogen (N) management practices on corn applied at 135 lb N/A (150 kg N/ha), and a no N control: 1) fall-applied anhydrous ammonia with nitrification inhibitor (Nitrapyrin); 2) spring-applied anhydrous ammonia (no inhibitor); and 3) split with 40 lb urea N/A (45 kg/ha) added at planting plus variable N at sidedress added as urease inhibitor-treated urea (Agrotain®). 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 leaching losses through water flowing in tile drains, 2) determine the effects of N fertilizer application timing on crop yield; and 3) disseminate project findings to scientific and farmer groups.

In 2017, corn yield increased by more than 75 bu/A with treatments 1 and 2 compared to the unfertilized control. In 2017 there was a lower corn yield with the split N application compared to fall and spring ammonia timing; likely due to the limited precipitation after the sidedress N application and dry summer conditions which limited N movement into the corn root zone. Soybean yields in 2017 were greater than 60 bu/A. Soybean yield differences following the previous corn N treatments occurred in 2017, but were inconsistent and not explainable by corn N fertilizer treatment.

For the corn phase, there was no significant difference in the nitrate concentration of water draining from the N-fertilized or the non-fertilized plots.

For the soybean phase, the treatment where no N was applied to the previous 2016 corn crop had a statistically significant lower nitrate concentration than the treatment that received N fertilizer immediately prior to planting in 2016.

For both corn and soybean phases, the nitrate concentration was the same for the control and the split N application in 2017. Of note is that the quantity of drainage was much lower than in previous years, with average drainage of about 3.5 inches compared to 10 inches in 2015 and 17 inches in 2016.

Overall, the total phosphorus (TP) and total reactive phosphorus (TRP) concentrations in the tile flow are small, and there were no statistically significant differences in TP or TRP concentrations between treatments. Based on comparing concentrations of TP and TRP, it is evident most of the TP is in the TRP form.