Nitrogen-15 and Oxygen-18 Natural Abundance of Potassium Chloride Extractable Soil Nitrate Using the Denitrifier Method
2007
Rock, Luc | Ellert, Benjamin H.
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 ⁻— δ¹⁵N and δ¹⁸O 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 δ¹⁵N and 0.3‰ for δ¹⁸O values. After appropriate corrections, differences between our measured and consensus δ¹⁵N and δ¹⁸O values of standard NO₃ ⁻ generally were within the standard deviations given for the consensus values. Both δ¹⁵N and δ¹⁸O 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 δ¹⁵N and δ¹⁸O 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 ⁻
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