Fall application of anhydrous ammonia (NH3) is a common practice for corn (Zea mays L.) production in the midwestern United States, but evaluations to date have relied entirely on yield comparisons that provide no means of distinguishing fertilizer from soil N uptake. To quantify fertilizer N uptake efficiency (FNUE) when using this practice, field trials using 15NH3 were conducted between 2016 and 2018 at four sites in a corn–soybean (Glycine max L. Merr.) rotation and at two sites under continuous corn. At each site, 224 kg N ha−1 was applied with and without the use of nitrapyrin (NP) to inhibit nitrification. Relative to grain yields without fertilizer N, response to fall N fertilization occurred at four of the six sites studied, averaging 53% (4.0 Mg ha−1) in two growing seasons with below‐normal rainfall. The use of NP was beneficial at only one site for increasing total N uptake, but resulted in a decrease at another, along with a significant reduction in grain yield. Isotopic estimates of FNUE (i.e., F15NUE) ranged from 12 to 34% (21% on average) for grain and from 16 to 42% (28% on average) for total aboveground biomass, and N derived from fertilizer ranged from 20 to 46% (32% on average). Both isotopic parameters were highest for the site lowest in native N availability, demonstrating the potential of site‐specific N management to improve fall NH3 fertilization by accounting for soil N mineralization.

A common strategy for improving fertilizer N uptake efficiency by corn (Zea mays L.) is to synchronize application with crop N demand during the growing season, which can be done using the Y Drop system recently developed for surface dribble placement. A 2‐yr field study was conducted using dual‐labeled urea‐ammonium nitrate (UAN) solution to compare fertilizer 15N uptake efficiency (F15NUE) for surface and subsurface sidedress applications to soils of contrasting fertility under either second‐year corn or a corn–soybean [Glycine max (L.) Merr.] rotation. Besides placement, treatments were designed to allow comparison of 15N uptake from UAN applications made at planting vs. at the V9 growth stage. The importance of soil N supply was demonstrated by estimates of N derived from fertilizer that ranged from 17 to 47% in total aboveground biomass and never exceeded N derived from soil. Of the fertilizer N recovered in the crop at harvest, the majority originated from the 15N applied at sidedressing rather than at planting. The range in F15NUE was from 12 to 42% (26% on average) for grain and from 14 to 51% (31% on average) for total aboveground biomass, the only significant difference occurring when the subsurface treatment outperformed Y Drop placement under conditions conducive to urea N loss through NH3 volatilization. Both methods of application offer the same fundamental advantage, in that fertilizer N is supplied during the period of maximal crop uptake.

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.

A manifold is described for preparing kilogram quantities of 15NH3. The 15NH3 is condensed into a collection tank that allows the use of a nitrification inhibitor. The collection tank connects to a tractor‐mounted toolbar for knifed applications to field plots. Although direct application of anhydrous ammonia (NH3) is a common practice for Midwestern corn (Zea mays L.) production in the USA, field studies utilizing 15N to quantify nitrogen (N) uptake efficiency have long been impeded by the difficulties and safety hazards inherent to a liquified gas that must be handled and applied under pressure. A manifold system is described for transferring known quantities of NH3 from labeled and unlabeled sources to obtain a desired 15N enrichment, and for collecting the mixture cryogenically in a tank specifically configured for knifed applications using a tractor‐mounted tool bar. Enrichments of 1.2 to 1.5 atom% 15N were obtained for 3 kg of NH3 prepared within a normal working day by a 15‐fold dilution with 10 atom% 15N as the starting label. A collection capacity of such magnitude represents a 3000% upscaling over systems previously described for this purpose, providing an essential prerequisite for field plot research to realistically assess the fate and fertilizer value of anhydrous NH3.

Numerous studies have been reported to ascertain the benefits of starter fertilization for corn (Zea mays L.) production, but most rely on data collected by measuring early growth or grain yield at harvest. As a more conclusive evaluation, five field trials were conducted using 15 N-labeled urea-ammonium nitrate (UAN) solution (90 kg N ha −1 ) to compare single and dual subsurface band placement, as well as surface dribble and broadcasting with incorporation. Significant differences in dry matter production and total N uptake occurred between fertilized and unfertilized treatments, but not among the different placements studied. In contrast, fertilizer 15 N uptake efficiency (F 15 NUE) tended to be greater for either type of subsurface banding than for the dribble treatment, demonstrating the added sensitivity that can be achieved through isotopic measurements. More importantly, band placement consistently outperformed broadcasting, according to estimates of F 15 NUE for total aboveground biomass that ranged from 11% to 46% when UAN was banded and from 7% to 34% for broadcast applications.