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

N 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 (NO) in the presence of ozone, making it impossible to directly measure N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO) and nitrate (NO) have been proposed for N 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 NO by chromium trioxide (CrO), before conversion to NO and NO in an alkaline hydrogen peroxide (HO) solution. A denitrifier method was used to reduce NO and NO in the trapping solution quantitatively to nitrous oxide (NO) for subsequent N analysis. NO trapping efficiencies of >85% were obtained with 50 ppb NO in a 0.5 L min air stream bubbling through a solution of 1.2 M HO and 0.5 M NaOH. In a laboratory test with distinct NO 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 N labeling experiments. In a soil incubation experiment with NHNO, NHNO, or NaNO amendments, we found distinct N abundances in NO, indicating that our method is well suited to investigate NO sources in soils.