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SOIL BIOLOGY & BIOCHEMISTRY

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

Agriculture & Agri-Food Canada, Lethbridge Research Centre, 5403 1st Ave. South, P.O. Box 3000, Lethbridge, AB, T1J 4B1, Canada

^* Corresponding author (lrock{at}ucalgary.ca).

In agroecosystems, most isotopic investigations of NO₃^– involve the use of tracers that are artificially enriched in ¹⁵N. Although the dual isotope composition of NO₃^–— ¹⁵N and ¹⁸O is especially beneficial for understanding the origin and fate of NO₃^–, its use for KCl-extractable soil NO₃^– has been hampered by the lack of a suitable analytical technique. Our objective was to test whether the denitrifier method, whereby NO₃^– is reduced to N₂O before mass spectrometric analysis, can be used to determine the N and O isotopic composition of NO₃^– from 2 M KCl soil extracts. Several internationally accepted NO₃^–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 NO₃^– standards analyzed did not exceed 0.2 for ¹⁵N and 0.3 for ¹⁸O values. After appropriate corrections, differences between our measured and consensus ¹⁵N and ¹⁸O values of standard NO₃^– generally were within the standard deviations given for the consensus values. Both ¹⁵N and ¹⁸O 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 ¹⁵N and ¹⁸O values, which were attributed to amendment with NH₄NO₃ fertilizer. Hence, our data indicate that the denitrifier method provides a fast, reliable, precise, and accurate way of simultaneously analyzing the natural abundances of ¹⁵N and ¹⁸O in KCl-extractable soil NO₃^–.