Novel laser spectroscopic technique for continuous analysis of N ₂O isotopomers - application and intercomparison with isotope ratio mass spectrometry

RATIONALE: Nitrous oxide (N2O), a highly climate-relevant trace gas, is mainly derived from microbial denitrification and nitrification processes in soils. Apportioning N2O to these source processes is a challenging task, but better understanding of the processes is required to improve mitigation strategies. The N2O site-specific 15N signatures from denitrification and nitrification have been shown to be clearly different, making this signature a potential tool for N2O source identification. We have applied for the first time quantum cascade laser absorption spectroscopy (QCLAS) for the continuous analysis of the intramolecular 15N distribution of soil-derived N2O and compared this with state-of-the-art isotope ratio mass spectrometry (IRMS). METHODS: Soil was amended with nitrate and sucrose and incubated in a laboratory setup. The N2O release was quantified by FTIR spectroscopy, while the N2O intramolecular 15N distribution was continuously analyzed by online QCLAS at 1 Hz resolution. The QCLAS results on time-integrating flask samples were compared with those from the IRMS analysis. RESULTS: The analytical precision (2s) of QCLAS was around 0.3 % for the d15N bulk and the 15N site preference (SP) for 1-min average values. Comparing the two techniques on flask samples, excellent agreement (R2 = 0.99; offset of 1.2 %) was observed for the d15Nbulk values while for the SP values the correlation was less good (R2 = 0.76; offset of 0.9 %), presumably due to the lower precision of the IRMS SP measurements. CONCLUSIONS: These findings validate QCLAS as a viable alternative technique with even higher precision than state-of-the-art IRMS. Thus, laser spectroscopy has the potential to contribute significantly to a better understanding of N turnover in soils, which is crucial for advancing strategies to mitigate emissions of this efficient greenhouse gas.