Nitrate Loss in Subsurface Drainage from a Corn–Soybean Rotation as Affected by Nitrogen Rate and Nitrapyrin

Successful N management practices for the US Midwest must optimize crop production and minimize NO 3 –N losses from subsurface tile drainage. The objective of this study was to measure the effects of N rate, N application timing, and nitrapyrin [2‐chloro‐6‐(trichlormethyl) pyridine] on corn ( Zea mays L.) production and NO 3 –N in tile drainage water in a corn–soybean [ Glycine max (L.) Merr.] rotation in Minnesota. Anhydrous ammonia was applied at 90 and 179 kg ha −1 with nitrapyrin in the fall and at 134 kg ha −1 with and without nitrapyrin in fall and spring. However, drainage water monitoring was only conducted on fall treatments. Over a 5‐yr period, 71% of drainage occurred in April through June and <1% occurred from November through March due to frozen soil. Averaged across N treatments and crops, annual drainage ranged from 69 to 380 mm among years. From 2001 through 2003, NO 3 –N concentrations averaged 13.8, 15.6, and 20.0 mg L −1 in corn and 7.3, 8.2, and 12.6 mg L −1 in soybean when 90, 134, and 179 kg N ha −1 was fall applied with nitrapyrin to corn, respectively. Corn grain yields were greater with spring‐applied N at 134 kg ha −1 (11.3 Mg ha −1 ) than with fall‐applied N at 134 and 179 kg ha −1 with nitrapyrin (10.5 and 10.8 Mg ha −1 , respectively), and nitrapyrin did not affect corn production or water quality. Fall application of N is common on cold soils in Minnesota. These data showed that fall application required a greater rate of N to optimize yield than spring and that greater fall rate often increased NO 3 –N concentration and load in tile drainage water. Core Ideas Nitrate concentration and load in tile drainage water increased as N rate increased. Concentrations and loads were 44 and 33% less in soybean than in corn, respectively. Nitrapyrin did not affect nitrate in drainage water or corn yield.