Evaluation of nitrogen-15 tracer techniques for direct measurement of denitrification in soil: IV. Field Studies

Field studies were conducted to determine whether underestimation occurs in measuring evolution of nitrous oxide (N₂O) using an ¹⁵N-tracer technique based on the assumption of isotopic uniformity. Microplots of 58, 553, or 1684 cm² in area were treated with ¹⁵N-labeled KNO₃ (68.5 atom % ¹⁵N) at the rate of 100 kg N ha^-1. Isotope-ratio analyses of the NO₃^- in soil cores collected from one plot of each size showed considerable spatial variability in ¹⁵N enrichment, but measurements of N₂O evolution from these plots by gas chromatography (GC) and mass spectrometry (MS) usually did not differ significantly. Replicate atmospheric samples were collected from three plots of each size following rainfall or irrigation for analysis of N₂O by GC and MS. The measurements differed significantly for approximately 60% of the collections that were made; however, the magnitude of the difference was usually small compared to spatial variability in N₂O evolution. Significant differences occurred more often with medium or large plots than with small plots, and the greater value was usually obtained by GC. Additional study results are discussed.