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.

Core Ideas Existing HF methods are labor‐intensive and do not recover recently fixed NH4+‐N. Diffusion techniques were developed that are simpler and more quantitative. These techniques permit 15N analyses of fixed NH4+‐N in tracer studies. The determination of fixed ammonium (NH4+) normally involves hydrofluoric acid (HF) digestion to decompose clay minerals following a rigorous pretreatment to control interference by organic nitrogen (N); however, the methods are tedious and time‐consuming, and recoveries can be incomplete if the soil under analysis was recently fertilized with ammoniacal N. Diffusion methods were developed to simplify and improve this determination, which utilize a potassium chloride (KCl) pretreatment to clearly differentiate exchangeable from nonexchangeable NH4+. In these methods, the HF digest is treated with magnesium carbonate (MgCO3) to neutralize residual acidity and precipitate magnesium fluoride (MgF2), and fixed NH4+‐N is then liberated with magnesium oxide (MgO) as diffusion is performed on an orbital shaker for 3 h at 45 to 50°C (Method A) or for 3 d at 20 to 25°C (Method B). When averaged across 16 diverse soils, both diffusion methods were in close agreement with the Silva‐Bremner technique using a potassium hypobromite (KOBr) pretreatment, although significant discrepancies were more common for Method A than B, indicating greater potential for organic interferences. In a study to compare recoveries of recently fixed 15NH4+‐N, Method A was significantly higher than the procedure of Silva and Bremner. The latter finding leaves no doubt about the need to avoid the use of KOBr for measuring fixed NH4+ in soils recently fertilized with ammoniacal N, and demonstrates that the methods described will be advantageous for tracer research involving 15N.