Determining Nitrogen‐15 in Nitrite or Nitrate by Producing Nitrous Oxide

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Abstract
Determining 15N in NO−3 or NO−2 is important when studying soil N transformations directly. Analysis of 15N in NO−3 by standard methods is time consuming and subject to errors such as cross‐contamination. The 15N content of NO−2 is determined conventionally as the difference between NO−3 + NO−2 and NO−3 alone. Since NO−3 concentrations are usually much greater than NO−2 concentrations in soil, determination of 15N in NO−2 has a poor detection limit. We developed new direct methods for determining 15N in NO−3 or NO−2 in 2 M KCl soil extracts, based on the production of N2O from NO−2 + NH2OH. For NO−2, N2O was produced by adding NH2OH at pH 1.7. For NO−3, NO−2 and NH2OH were produced as intermediates during the reduction of NO−3 by Cd‐Cu at pH 4.7. The 15N content of the N2O was determined by automated continuous flow isotope‐ratio mass spectrometry. Measured values of 15N in N2O were closely related to the theoretical values for 15N in NO−2 or NO−3. Five micromoles of NO−3 or 0.5 µmol of NO−2 at 0.1 atom % excess 15N could be determined with coefficients of variation of −3 reduction.

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