Evaluation of 15N Methods for the measurement of Denitrification in Soils

The precision of the ¹⁵N-emission and that of the ¹⁵N-balance methods was evaluated and both methods were compared in a denitrification experiment. ¹⁵N-analysis was performed with an isotope ratio mass spectrometer which was coupled to an elemental analyzer. The measuring sensitivity in soil and gas analysis was tested by analyzing ¹⁵N-standards. The detection limit for gas samples with two different procedures of ¹⁵N-gas analysis was δ¹⁵N = (4.5 ± 1.0)‰ and (0.5 ± 0.05)‰, respectively. The error in measurement was 19% and 12% respectively. ¹⁵N-analysis of a ¹⁵N-labelled soil (4.15 ppm ¹⁵N) resulted in a CV of 1.32%. The measurements had to be calibrated with soil standards because the ¹⁵N-values showed a continuous downward fluctuation in a range of 10–20% within several days, when only acetanilid was used for calibration. Mean ¹⁵N-losses which were determined with both methods during the denitrification experiment were in good agreement. The precision of the ¹⁵N-emission method was adequate in all variants of the experiment. The precision of the ¹⁵N-balance method however was unsatisfactory at low ¹⁵N-losses (0.2–2% of added ¹⁵N).