Nitrogen-15 Evaluation of Fall-Applied Anhydrous Ammonia: I. Efficiency of Nitrogen Uptake by Corn

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.