Impact of nitrogen fertilizer timing on nitrate loss and crop production in northwest Iowa

In the U.S. Midwest, nitrate in subsurface tile drainage from corn ( Zea mays L.)–soybean [ Glycine max (L.) Merr.] systems is detrimental to water quality at local and national scales. The objective of this replicated plot study in northwest Iowa, performed in 2015–2020, was to investigate the influence of nitrogen (N) fertilizer timing on crop production and NO 3 load in subsurface (tile) drainage discharge. Four treatments applied to corn included fall anhydrous ammonia with a nitrification inhibitor (F), spring anhydrous ammonia (S), split‐banded urea at planting and mid‐vegetative growth (SS), and no N fertilizer (0N). Across crops and years, NO 3 –N concentration in subsurface drainage discharge was the same at 11.7 mg L –1 for F and S applied anhydrous ammonia (AA). The NO 3 –N concentration was statistically lower with SS urea (10 mg L –1 ) than F and S, and 0N was lower than SS at 8.3 mg L –1 . Average annual NO 3 –N loads were not different between any treatments due to plot variability in drainage discharge. Corn responded to N application, with overall mean yield the same for F, S, and SS. There were no agronomic or water quality benefits for applying AA in spring compared with fall, where the F included a nitrification inhibitor and was applied to cold soils. Split‐applied urea had a small positive water quality impact but no crop yield enhancement. This study shows that there were improvements to NO 3 –N concentration in subsurface drainage discharge, but more nutrient reduction practices are needed than fertilizer N management alone to reduce nitrate load to surface water systems.