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

Determining N-15 in NO3- or NO2- is important when studying soil N tra nsformations 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 b etween NO3- + NO2- and NO3- alone. Since NO3-concentrations are usuall y much greater than NO2- concentrations in soil, determination of N-15 in NO2- has a poor detection limit. We developed new direct methods f or 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 int ermediates during the reduction of NO3- by Cd-Cu at pH 4.7. The N-15 c ontent of the N2O was determined by automated continuous flow isotope- ratio mass spectrometry. Measured values of N-15 in N2O were closely r elated to the theoretical values for N-15 in NO2- or NO3-. Five microm oles of NO3- or 0.5 mumol of NO2- at 0.1 atom % excess N-15 could be d etermined 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, nitrifi cation, and NO3- reduction.