Isotopic comparison of ammonium and nitrate sources applied in-season to corn

Most synthetic nitrogen (N) fertilizers used for corn (Zea mays L.) production supply ammonium (NH₄⁺), but N availability can be reduced by volatilization or microbial immobilization. The former process is eliminated and the latter reduced by fertilizing with nitrate (NO₃⁻), which is best done in-season to promote crop uptake while minimizing N loss by leaching or denitrification. To compare ¹⁵N uptake for in-season surface applications of labeled potassium nitrate (KNO₃), urea-ammonium nitrate (UAN), and urea, field studies were conducted in two growing seasons on Mollisols and Alfisols located in production fields under second- or third-year corn or a corn–soybean (Glycine max L. Merr.) rotation. In contrast to dry matter production and total N uptake, the three ¹⁵N sources often differed significantly in N derived from the labeled fertilizer (NDF¹⁵F) and fertilizer ¹⁵N uptake efficiency (F¹⁵NUE), both of which decreased in the order: KNO₃ > UAN > urea. As evidenced by a laboratory incubation study using the same three ¹⁵N sources, this trend was attributed to differences in NH₃ volatilization and microbial immobilization that was greater for ammoniacal than NO₃⁻ fertilizers. When surface applying a sidedressing to corn in the Midwestern United States, N uptake efficiency can be increased by the use of NO₃⁻ fertilizers.