Nitrogen‐15 and Oxygen‐18 Natural Abundance of Potassium Chloride Extractable Soil Nitrate Using the Denitrifier Method

In agroecosystems, most isotopic investigations of NO 3 − involve the use of tracers that are artificially enriched in 15 N. Although the dual isotope composition of NO 3 − — δ 15 N and δ 18 O is especially beneficial for understanding the origin and fate of NO 3 − , its use for KCl‐extractable soil NO 3 − has been hampered by the lack of a suitable analytical technique. Our objective was to test whether the denitrifier method, whereby NO 3 − is reduced to N 2 O before mass spectrometric analysis, can be used to determine the N and O isotopic composition of NO 3 − from 2 M KCl soil extracts. Several internationally accepted NO 3 − standards were dissolved in 2 M KCl, the conventional extractant for soil inorganic N, and inoculated with the bacterial strain Pseudomonas aureofaciens (ATCC no. 13985). The standard deviation of the NO 3 − standards analyzed did not exceed 0.2‰ for δ 15 N and 0.3‰ for δ 18 O values. After appropriate corrections, differences between our measured and consensus δ 15 N and δ 18 O values of standard NO 3 − generally were within the standard deviations given for the consensus values. Both δ 15 N and δ 18 O values were reproducible among separate analytical runs. The method was also tested on genuine 2 M KCl extracts from unfertilized and fertilized soils. Depending on N fertilization, the soils had distinct δ 15 N and δ 18 O values, which were attributed to amendment with NH 4 NO 3 fertilizer. Hence, our data indicate that the denitrifier method provides a fast, reliable, precise, and accurate way of simultaneously analyzing the natural abundances of 15 N and 18 O in KCl‐extractable soil NO 3 −