Exploring the Potential of an Isotopic N2O Analyzer to Observe Soil Biogeochemical Processes in Real-time

We evaluated the potential applications of an Isotopic Nitrous Oxide (N₂O) Analyzer (Los Gatos Research), which can measure isotopic values of N₂O (δ¹⁵N^α, δ¹⁵N^β, δ¹⁵N and δ¹⁸O) as well as [N₂O] at real-time. The analyzer can provide continuous and precise measurements of the isotopic values with a quantum cascade laser along with cavity enhanced laser absorption spectroscopy technique. We evaluated the analyzer by quantifying N₂O produced from soils and sediments, N₂O dissolved in water, and a N₂O standard for δ¹⁵N and δ¹⁸O. In quantifying N₂O production from soils, we used four soils collected from diverse grassland sites across the North American Great Plains. In this lobe of the study, we investigated the relationship between N₂O production rates and soil water content (SWC) by manipulating soil water levels in a lab setting. We hypothesized that N₂O production rates would be positively correlated with SWC because we expected denitrification to be a more important N₂O source than nitrification. Consistent with this hypothesis, we observed that soils adjusted at high SWC (20 to 50%) produced significant N₂O. Parallel experiments using ¹⁵N labeling (¹⁵NH₄⁺ and ¹⁵NO₃^-) suggested that N₂O from the soils with high SWC was produced mostly via denitrification. Most of the soils adjusted with low SWC (10 to 20%) produced little N₂O. When we observed measurable N₂O from one soil with low SWC, the ¹⁵N labeling experiment suggested that N₂O was produced via nitrification. Our measurements demonstrate that the Isotopic N₂O Analyzer can be a powerful tool to investigate N₂O dynamics in various materials, such as soils and water, in different environmental settings.