EVALUATION OF N-15 METHODS FOR THE MEASUREMENT OF DENITRIFICATION IN SOILS

The precision of the N-15-emission and that of the N-15-balance method s was evaluated and both methods were compared in a denitrification ex periment. N-15-analysis was performed with an isotope ratio mass spect rometer which was coupled to an elemental analyzer. The measuring sens itivity in soil and gas analysis was tested by analyzing N-15-standard s. The detection limit for gas samples with two different procedures o f N-15-gas analysis was delta(15)N = (4.5 +/- 10)parts per thousand an d (0.5 +/- 0.05)parts per thousand, respectively. The error in measure ment was 19%and 12%respectively. N-15-analysis of a N-15-labelled soil (4.15 ppm N-15) resulted in a CV of 1.32% The measurements had to be calibrated with soil standards because the N-15-values showed a contin uous downward fluctuation in a range of 10-20% within several days, wh en only acetanilid was used for calibration. Mean N-15-losses which we re determined with both methods during the denitrification experiment were in good agreement. The precision of the N-15-emission method was adequate in all variants of the experiment. The precision of the N-15- balance method however was unsatisfactory at low N-15-losses (0.2-2% o f added N-15).