Continuous and unattended measurements of the site preference of nitrous oxide emitted from an agricultural soil using quantum cascade laser spectrometry with intercomparison with isotope ratio mass spectrometry.

Rationale: The difference between the (15)N natural abundance of (14)N-(15)N-O and (15)N-(14)N-O (site preference; SP) is used to understand the mechanisms underlying N2O emissions from soils. We investigated the use of quantum cascade laser (QCL) absorption spectrometry for continuous and precise analysis of the SP of N2O emitted from a field soil at atmospheric mixing ratios.

Methods: A QCL-based spectrometer was used to determine the SP of soil-emitted N2O accumulated in a closed chamber system without preconcentration. N2O standards (<2500 ppbv) were used to evaluate the precision of the QCL spectrometry (QCLS) system. CO2 and H2O were removed from the gas samples. Intercomparison measurements of QCLS and isotope ratio mass spectrometry (IRMS) were performed on N2O calibration gases at different mixing ratios. The observed dependency of the QCLS result on the N2O mixing ratio was corrected.

Results: Measurement of SP of N2O emitted from the field suggested that the SP of N2O varied from 0 to 40‰ over a period of 1 month. The precisions of the SP measurements (300-2500 ppbv) were <1.9‰ for δ(15)N(α) values, <2.6‰ for δ(15)N(β) values, <2.1‰ for δ(15)N(bulk) values, and <2.1‰ for the SP (1 min averaging time) obtained on a once-an-hour calibrated QCLS system, with a cell temperature control precision of ±0.01 K.

Conclusions: Continuous and unattended measurements of the SP of N2O emitted from soils were achieved at low N2O mixing ratios. The accuracy of the QCLS measurements for the SP of N2O was significantly improved by precisely controlling the temperature of the system and by correcting for the concentration dependency of the raw data through an intercomparison with IRMS measurements.