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Core IdeasA manifold is described for preparing kilogram quantities of 15NH3.The 15NH3is 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 maysL.) 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 NH3from 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 NH3prepared 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.

Conventional steam‐distillation techniques for fractionating the N in soil hydrolysates have generally indicated little variation in the chemical distribution of soil organic N, regardless of soil type, cropping, cultivation, or N‐fertilization history. Nitrogen‐15 tracer studies to evaluate these techniques showed that determinations of amino sugar–N are subject to serious underestimation, and that analyses for amino acid–N are vitiated by incomplete conversion of amino acid–N to NH4–N following incomplete removal of hydrolyzable NH4and amino sugar–N. Diffusion methods were developed for fractionating the N in soil hydrolysates that are far more accurate and specific than steam distillation, while also being much simpler and more convenient. In these methods, total hydrolyzable N is measured by Kjeldahl digestion of the hydrolysate and diffusion of the digest with NaOH; diffusion is performed with MgO to determine hydrolyzable NH4–N; (NH4+ amino sugar)–N is recovered by diffusion with NaOH, after which amino acid–N is liberated by ninhydrin oxidation at pH <1.8 and recovered by diffusion with NaOH. Analytical accuracy and specificity were evaluated using a wide variety of purified organic‐N compounds and by checking the recovery of 15N added to soil hydrolysates as (NH4)2SO4, glucosamine, or glycine. Studies using a diverse set of soils showed that distillation and diffusion usually agreed to within 10% for quantitative analysis of total hydrolyzable N, NH4–N, and amino acid–N, whereas analyses of amino sugar–N were 74 to 317% greater by diffusion than by distillation.

When denitrification is measured directly using 15N, the emission of labeled N2and N2O is calculated from the volume of atmosphere confined within a closed chamber. This volume is readily estimated by measuring chamber height with a ruler; however, underestimation will occur with unsaturated soil because of air‐filled porosity. A much greater complication arises when plants are present, due to the volume occupied by the aboveground biomass and the internal volume available for gas exchange. A method was developed to measure atmospheric volume from dilution of a standard addition of Ne introduced prior to circulation of air within a closed‐chamber system. Ion‐current measurements to determine Ne, from which the volume was obtained by regression, were performed during mass spectrometric analysis for 15N‐labeled N2Volume measurements by this method were accurate to within 3%, as compared with gravimetric measurements of air‐filled porosity for bare soil varying in moisture content. When introduction of Ne into a darkened chamber was delayed to ensure stomatal closure, volume measurements for a turfgrass system were accurate to within 2.5%, based on the difference obtained before and after connecting a mason jar to provide a known volume. A more accurate method of measuring atmospheric volume will improve the accuracy achieved in direct measurement of denitrification.

Research was conducted to determine the extent to which the method of calculation affects estimates of fertilizer N efficiency using 15N as a tracer. Corn (Zea maysL.) was grown at three locations in Illinois that varied widely in soil type. Three early‐season moisture regimes (ambient, ambient + 100 mm of excess water, and ambient + 150 mm of excess water) were established to obtain different fertilizer N efficiencies on each soil. Application of 15N‐enriched KNO3(168 kg N ha−1) was made to a Drummer silty clay loam (fine‐silty, mixed, mesic Typic Haplaquoll) at DeKalb, a Cisne silt loam (fine, montmorillonitic, mesic Mollic Albaqualf) at Brownstown, and a Plainfield sand (mixed, mesic Typic Udipsamment) at Havana, IL. Fertilizer N efficiency was calculated from the difference between N uptake by fertilized versus unfertilized plants, from the amount of 15N recovered in the plant, and from the recovery of 15N in both the plant and soil. For the Drummer and Cisne soils, the three calculation procedures gave different percent fertilizer N efficiencies when averaged across moisture treatments, and different trends among moisture treatments. For the Plainfield soil, percent fertilizer N efficiency values were lower than those obtained for the Drummer or Cisne soil, and the method of calculation had very little effect. The results indicate that, for most soils, N fertilizer efficiency ratings vary with the method by which they are calculated. The present study demonstrates a need for standard terminology to help identify parameters used to define fertilizer efficiency.