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

Determining 15 N in NO − 3 or NO − 2 is important when studying soil N transformations directly. Analysis of 15 N in NO − 3 by standard methods is time consuming and subject to errors such as cross‐contamination. The 15 N 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 15 N in NO − 2 has a poor detection limit. We developed new direct methods for determining 15 N in NO − 3 or NO − 2 in 2 M KCl soil extracts, based on the production of N 2 O from NO − 2 + NH 2 OH. For NO − 2 , N 2 O was produced by adding NH 2 OH at pH 1.7. For NO − 3 , NO − 2 and NH 2 OH were produced as intermediates during the reduction of NO − 3 by Cd‐Cu at pH 4.7. The 15 N content of the N 2 O was determined by automated continuous flow isotope‐ratio mass spectrometry. Measured values of 15 N in N 2 O were closely related to the theoretical values for 15 N in NO − 2 or NO − 3 . Five micromoles of NO − 3 or 0.5 µmol of NO − 2 at 0.1 atom % excess 15 N could be determined with coefficients of variation of <4%. The new methods are fast, simple, and not subject to contamination from the atmosphere or from other samples. They should facilitate studies on the simultaneous measurement of gross rates of mineralization, immobilization, nitrification, and NO − 3 reduction.