title: Modified Method for Trapping and Analyzing15N in NO Released from Soils.
abstract: 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 measure15N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO2-) and nitrate (NO3-) have been proposed for15N 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 NO2by 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 subsequent15N analysis. NO trapping efficiencies of >85% were obtained with 50 ppb NO in a 0.5 L min-1air stream bubbling through a solution of 1.2 M H2O2and 0.5 M NaOH. In a laboratory test with distinct15NO 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 and15N labeling experiments. In a soil incubation experiment with15NH4NO3, NH415NO3, or Na15NO2amendments, we found distinct15N abundances in NO, indicating that our method is well suited to investigate NO sources in soils.
authors.affiliation: Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences , N-1432, Aas, Norway.
authors.forename: Ronghua
authors.identifier: 0000-0003-2047-445X
authors.initials: R
authors.lastname: Kang
authors.affiliation: Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences , N-1432, Aas, Norway.
authors.forename: Jan
authors.initials: J
authors.lastname: Mulder
authors.affiliation: Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences , N-1432, Aas, Norway.
authors.forename: Peter
authors.initials: P
authors.lastname: Dörsch