Evaluation of Nitrogen-15 Tracer Techniques for Direct Measurement of Denitrification in Soil: III. Laboratory Studies
Laboratory studies were conducted to evaluate the assumption of isotopic uniformity that underlies mass spectrometric determination of dinitrogen (N2) and nitrous oxide (N2O) evolved during denitrification in soil treated with 15N-labeled fertilizer. In these studies, a gas chromatograph (GC) and mass spectrometer (MS) were used to analyze replicate atmospheric samples containing N2O generated through oxidation of 15N-labeled NH2OH, evolved from 15N-treated soil slurries incubated at constant moisture content (1 kg H2O kg−1), or evolved from 15N-treated soil subjected to wetting and drying cycles. Both instruments gave almost identical values in analysis of N2O derived from a single pool of 15N-labeled NH2OH or from two pools having similar, but not necessarily the same, enrichments. The MS gave lower values for N2O derived from two pools differing markedly in their enrichment of 15N. The values measured by GC and MS differed significantly (0.05 level) for about 50% of the soil atmospheric samples analyzed; however, the differences usually did not exceed 10% and probably resulted from analytical error rather than lack of isotopic uniformity in the NO−3 denitrified, because in about half of these cases, the greater value was obtained by MS.
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