Assessing nitrous oxide (N2O) isotopic analyzer performance for in-field use

Abstract

Analysis of N2O isotope ratios is a way to apportion N2O production in soils among source pathways and develop mitigation strategies. Measurement systems employing a field-deployable LAS instrument coupled with automatic and dynamic soil flux chambers, where the soil emitted gas continuously circulates between the analyzer and the chamber can provide direct in situ N2O flux and N2O isotope ratio measurements with high temporal resolution. Reliable use of these measurements, however, requires quantification of measurement accuracy at high analyzer sampling rates - 2 seconds. We evaluated the variability of the measurements of an LAS isotope N2O analyzer – the OA-ICOS (Isotopic N2O Analyzer model 914–0027, Los Gatos Research Inc, Los Gatos, CA), over N2O concentrations observed in field experiments. We calculated the uncertainty of isotope ratios estimated using the Keeling plot method for representative soil N2O fluxes and chamber closure times. The variability of OA-ICOS isotopic ratio measurements is inversely related to N2O concentration leading to high uncertainty in soil emitted N2O isotope ratios estimated using the Keeling plot method in which low concentration measurements have the largest influence. Instrument precision evaluated using reference gasses was propagated through Monte Carlo simulation to predict system performance under a range of conditions. Model predictions of in situ measurement accuracy were evaluated with soil flux simulations using soil emitted gasses. Isotope ratios derived using OA-ICOS measurements during soil flux simulations deviated markedly from IRMS measurements. System performance was limited by the concentration dependence of N2O isotope ratio measurements and the suspected presence of interferents in soil emitted gasses. These characteristics of the OA-ICOS isotopic analyzer make it poorly suited for in situ source partitioning of soil emitted N2O. Until we overcome these limitations in situ N2O isotopic ratio measurements by OA-ICOS and automatic flux chambers are best used qualitatively.