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

Fall application of anhydrous ammonia (NH 3 ) 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 15 NH 3 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., F 15 NUE) 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 NH 3 fertilization by accounting for soil N mineralization.