Measurement of nitrous oxide emissions from grassland soil using photo-acoustic infra-red spectroscopy, long-path infrared spectroscopy, gas chromatography, and continuous flow isotope-ratio mass spectrometry

This study evaluated the performance of photo-acoustic infra-red spectrosco py (PAIRS) for measuring nitrous oxide (N2O) fluxes in the field, in compar ison with long-path infra-red spectroscopy ('Hawk'), gas chromatography (GC ), and continuous flow isotope-ratio mass spectrometry (CF-IRMS). The N2O f lux measurements from fertilized and grazed grassland were made simultaneou sly by the different methods, before and after water application. Before ir rigation, mean N2O fluxes ranged from 3 to 20 g N ha(-1) day(-1) for the PA IRS and GC measurements, but were undetectable with the Hawk. Within 2 hour s of irrigation, mean fluxes increased to 740, 640, and 270 g N ha(-1) day( -1), based on GC, PAIRS, and Hawk measurements, respectively. After about 2 4 hours, irrigation had reached its full effect and N2O fluxes had increase d to 1,050, 710, and 410 g N ha(-1) day(-1). The GC measurements were consi stently higher than the PAIRS measurements. However, a second experiment, c omparing the PAIRS analyzer with continuous flow isotope-ratio mass spectro metry (CF-IRMS), suggested that the former was negatively biased; the PAIRS response was about 33% lower than CF-IRMS. When the resulting correction f actor was applied to the results of the first experiment, there was very go od agreement between the PAIRS and GC measurements. The Hawk measurements w ere lower than PAIRS and GC, but a statistical comparison was not possible, due to the limited number of Hawk measurements that could be made in the w indy weather conditions. Windy conditions also resulted in an underestimati on of the N2O flux by PAIRS compared to GC and CF-IRMS analysis, which coul d not solely be attributed to a change in the analyzer sensitivity. There w as no obvious explanation for this discrepancy and further investigations a re needed to resolve this issue.