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

Abstract

AbstractMost 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.