Fertilizer Timing and Rate, and Cover Crop Effects on Tile Nitrate Loss.

Large-scale and long-term field experiments are needed to measure the effectiveness of suggested conservation practices to reduce nitrate losses from tile-drained corn and soybean fields in the Midwest, while allowing for increasing yields. We are evaluating fertilizer timing, rate, and application, as well as cover crops (6 different treatments with 3 replicates each, 180 kg N/ha full N rate following soybean) on nitrate concentrations and loads in tile drainage water using individual tile laterals spaced 30 m apart, each draining 1.8 ha. Half of the field was in corn and half in soybean each year, for a total of 36 laterals with continuous flow monitoring and ISCO automatic water samplers. Increases in tile nitrate concentrations could clearly be seen due to fall N applications (with inhibitor, but warm 2015/2016 winter), spring N, and side-dressed N, with a decrease in nitrate during the spring following corn with a cereal rye cover crop (flown on standing corn in September). Split applications (1/2 rate fall, ¼ rate spring, ¼ rate sidedress) had similar spring nitrate concentrations as the full rate in the fall. The treatment with a reduced fertilizer N rate (134 kg N/ha) had lower tile nitrate losses, but also decreased yields (approx. 10%). After the first two years of this long-term study, tile nitrate concentrations have been dependent on both: 1) the long-term field N balance controlling the background concentration of nitrate in tile water, about 10 mg N/L; and 2) rapid increases in soil nitrate from fertilizer additions or decreases in soil nitrate due to cover crop uptake. Initial results suggest 4R practices such as nitrification inhibitors with fall N or split fall/spring/sidedress applications did not reduce nitrate losses. However, additional field seasons are needed to verify these results.