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

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Abstract
This study evaluated the performance of photo‐acoustic infra‐red spectroscopy (PAIRS) for measuring nitrous oxide (N2O) fluxes in the field, in comparison with long‐path infra‐red spectroscopy ('Hawk'), gas chromatography (GC), and continuous flow isotope‐ratio mass spectrometry (CF‐IRMS). The N2O flux measurements from fertilized and grazed grassland were made simultaneously by the different methods, before and after water application. Before irrigation, mean N2O fluxes ranged from 3 to 20 g N ha1 day1 for the PAIRS and GC measurements, but were undetectable with the Hawk. Within 2 hours of irrigation, mean fluxes increased to 740, 640, and 270 g N ha‘1day1, based on GC, PAIRS, and Hawk measurements, respectively. After about 24 hours, irrigation had reached its full effect and N2O fluxes had increased to 1,050,710, and 410 g N ha1 day1. The GC measurements were consistently higher than the PAIRS measurements. However, a second experiment, comparing the PAIRS analyzer with continuous flow isotope‐ratio mass spectrometry (CF‐IRMS), suggested that the former was negatively biased; the PAIRS response was about 33% lower than CF‐IRMS. When the resulting correction factor was applied to the results of the first experiment, there was very good agreement between the PAIRS and GC measurements. The Hawk measurements were 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 windy weather conditions. Windy conditions also resulted in an underestimation of the N2O flux by PAIRS compared to GC and CF‐IRMS analysis, which could not solely be attributed to a change in the analyzer sensitivity. There was no obvious explanation for this discrepancy and further investigations are needed to resolve this issue.