Modified Method for Trapping and Analyzing ¹⁵N in NO Released from Soils

¹⁵N isotope tracing is an effective and direct approach to investigate sources of nitric oxide (NO) formed in soils. However, NO is highly reactive and rapidly converted to nitrogen dioxide (NO₂) in the presence of ozone, making it impossible to directly measure ¹⁵N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO₂^-) and nitrate (NO₃^-) have been proposed for ¹⁵N analysis in high-concentration NO sources, such as combustion processes. In contrast, NO concentrations in the soil surface-near air are usually small (ppbv-range), posing major challenges to conversion efficiency and blank correction. Here, we present a modified method in which NO is oxidized quantitatively to NO₂ by chromium trioxide (CrO₃), before conversion to NO₂^- and NO₃^- in an alkaline hydrogen peroxide (H₂O₂) solution. A denitrifier method was used to reduce NO₂^- and NO₃^- in the trapping solution quantitatively to nitrous oxide (N₂O) for subsequent ¹⁵N analysis. NO trapping efficiencies of >85% were obtained with 50 ppb NO in a 0.5 L min^-1 air stream bubbling through a solution of 1.2 M H₂O₂ and 0.5 M NaOH. In a laboratory test with distinct ¹⁵NO abundances, the overall precision was 0.29‰ (δ-values) for natural abundance NO and 0.13 atom % for labeled NO, suggesting that our method can be used for both natural abundance studies and ¹⁵N labeling experiments. In a soil incubation experiment with ¹⁵NH₄NO₃, NH₄¹⁵NO₃, or Na¹⁵NO₂ amendments, we found distinct ¹⁵N abundances in NO, indicating that our method is well suited to investigate NO sources in soils.