Evaluation of Nitrogen-15 Tracer Techniques for Direct Measurement of Denitrification in Soil: IV. Field Studies
- R. L. Mulvaney * and
- R. M. Vanden Heuvel
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 NO−3 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 (0.05 level), demonstrating that appreciable error does not necessarily arise in the use of 15N-tracer methodology to measure denitrification of NO−3 that is not isotopically uniform. In a subsequent study, 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 (0.05 level) 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. The extent of the difference was not affected by increasing the period of enclosure used in collection of atmospheric samples, but it was affected by subsequent application of fertilizer (25 kg N ha−1), which led to serious underestimation by MS of N2O evolved from most of the medium and large plots.
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