Lowering the detection limit for dinitrogen using the enrichment of nitrous oxide

The flux of N2 can be measured directly by uniformly enriching the denitrifying pool of nitrate in soil with 15N and allowing the evolved N2 to enrich the normal atmosphere in 15N within an enclosure. The enrichment of the source of the labelled N2 molecules (15XN) is calculated from the differences between normal and enriched atmospheres for 29N2/28N2 (Δ29R) and for 30N2/28N2 (Δ30R). Then the fraction (d) of the headspace N2 derived from the labelled nitrate pool is calculated from 15XN and Δ30R. The accuracy of measurement of 15XN is limited at present by the precision with which continuous-flow, isotope-ratio mass spectrometers can measure Δ29R. During denitrification N2O is produced from the same labelled pool of NO3− as N2. Measuring the molecular ratios for N2O of 45R (45N2O/44N2O) and 46R (46N2O/44N2O) enables the enrichment of the source of the labelled N2O to be calculated. We present performance data for the measurement of 15N in N2O by continuous-flow isotope-ratio mass spectrometry. Estimating 15XN from N2O enables the limit of detection for d to be lowered from 16×10−6 using Δ29R and Δ30R to 1×10−6 when the denitrifying pool is about 0.60 atom fraction in 15N. Thus, the detection limit for N2 can be lowered to 7.5 g N2-N ha−1 d−1 for an enclosure with a volume to surface area ratio of 5: 1.