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 NO₃-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 NO₃-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, NO₃-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 NO₃-N concentration and load in tile drainage water.