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

The flux of N-2 can be measured directly by uniformly enriching the denitri fying pool of nitrate in soil with N-15 and allowing the evolved N-2 to enr ich the normal atmosphere in N-15 within an enclosure. The enrichment of th e source of the labelled N-2 molecules (X-15(N)) is calculated from the dif ferences between normal and enriched atmospheres for N-29(2)/N-28(2) (Delta R-29) and for N-30(2)/N-28(2) (Delta R-30). Then the fraction (d) of the h eadspace N-2 derived from the labelled nitrate pool is calculated from X-15 (N) and Delta R-30. The accuracy of measurement of X-15(N) is limited at pr esent by the precision with which continuous-flow, isotope-ratio mass spect rometers can measure Delta R-29. During denitrification N2O is produced fro m the same labelled pool of NO3- as N-2. Measuring the molecular ratios for N2O of R-45 ((N2O)-N-45/(N2O)-N-44) and R-46 ((N2O)-N-46/(N2O)-N-44) enabl es the enrichment of the source of the labelled N2O to be calculated. We pr esent performance data for the measurement of N-15 in N2O by continuous-flo w isotope-ratio mass spectrometry. Estimating X-15(N) from N2O enables the limit of detection for d to be lowered from 16 x 10(-6) using Delta R-29 an d Delta R-30 to 1 x 10(-6) when the denitrifying pool is about 0.60 atom fr action in N-15. Thus, the detection limit for N-2 can be lowered to 7.5 g N -2-N ha(-1) d(-1) for an enclosure with a volume to surface area ratio of 5 : 1. (C) 2001 Elsevier Science Ltd. All rights reserved.