Comparison of the difference and delta (15)nitrogen approaches for evaluating liquid urea ammonium nitrate utilization by maize

Isotopic nitrogen (N) research techniques may be required in watershed studies to determine the impact of landscape position on fertilizer efficiency and the soil supplying power. However, traditional approaches using N-15 labeled fertilizer may not be suitable when farmer equipment is used. The delta(15)N natural abundance isotopic approach has been used to evaluate N cycling in watersheds. The objectives of this study were to measure the precision of the delta(15)N measurement by the Europa 20-20 ratio mass spectrometer (Europa Scientific Ltd, UK), and to compare the difference and delta(15)N approaches for measuring fertilizer use by maize (Zea mays). A replicated field study containing two different N rates (0 and 15.7 g N m(-2)) were used for the study. Maize samples were collected at the 8th-leaf, silking, and plant maturity in 1992 and 1993. Samples were dried (80 degrees C), ground (<1-mm), weighed (stover 12 mg and grain 3 mg), and analyzed for total N and delta(15)N. Fertilizer utilization at the three growth stages was determined using the natural abundance delta(15)N and nonisotopic difference (fertilizer-control) techniques. During the study, the Europa 20-20 ratio mass spectrometer (Europa Scientific Ltd, UK) analyzed over a 100 samples a day and had consumable costs of less than $2.00 per sample. The standard deviations of the mean were less than 0.11 and 0.21 parts per thousand in 51 and 77% of the stover samples, respectively. In 1992, grain yields were not influenced by N fertilizer additions, while in 1993 grain yields were increased by N fertilizer. The difference method estimated that in 1992, 16% of the N fertilizer was utilized by the crop, while the natural abundance delta(15)N approach estimated that 36% of the fertilizer N was used by the crop. Differences between calculated values by the two techniques resulted from the difference method calculating net fertilizer use, while the delta(15)N approach calculated fertilizer contained in the plant. Because the delta(15)N approach estimates fertilizer use, this approach can be used to calculate soil N contained in the plant. In watershed studies, this information may provide the tools needed to evaluate N use in responsive and nonresponsive sites within a field. This research shows that the delta(15)N method compliments the difference method, can be used to measure actual fertilizer use when farmer equipment is used, and that the Europa 20-20 ratio mass spectre-meter (Europa Scientific Ltd, UK) has acceptable precision for the delta(15)N natural abundance approach.