Rj. Stevens et Rj. Laughlin, DETERMINING N-15 IN NITRITE OR NITRATE BY PRODUCING NITROUS-OXIDE, Soil Science Society of America journal, 58(4), 1994, pp. 1108-1116
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.