DETERMINING N-15 IN NITRITE OR NITRATE BY PRODUCING NITROUS-OXIDE

Determining N-15 in NO3- or NO2- is important when studying soil N transformations directly. Analysis of N-15 in No3- by standard methods is time consuming and subject to errors such as cross-contamination. The N-15 content of NO2- is determined conventionally as the difference between NO3- + NO2- and NO3- alone. Since NO3-concentrations are usually much greater than NO2- concentrations in soil, determination of N-15 in NO2- has a poor detection limit. We developed new direct methods for determining N-15 in NO3- or NO2-in 2 M KCl soil extracts, based on the production of N2O from NO2- + NH2OH. For NO2-, N2O was produced by adding NH2OH at pH 1.7. For NO3-, NO2- and NH2OH were produced as intermediates during the reduction of NO3- by Cd-Cu at pH 4.7. The N-15 content of the N2O was determined by automated continuous flow isotope-ratio mass spectrometry. Measured values of N-15 in N2O were closely related to the theoretical values for N-15 in NO2- or NO3-. Five micromoles of NO3- or 0.5 mumol of NO2- at 0.1 atom % excess N-15 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 NO3- reduction.