Novel Method for Nitrogen Isotopic Analysis of Soil-Emitted Nitric Oxide

The global inventory of NOx (NOx = NO + NO2) emissions is poorly constrained, with a large portion of the uncertainty attributed to soil NO emissions that result from soil abiotic and microbial processes. While natural abundance stable N isotopes (delta N-15) in various soil N-containing compounds have proven to be a robust tracer of soil N cycling, soil delta N-15-NO is rarely quantified due to the measurement difficulties. Here, we present a new method that collects soil-emitted NO through NO conversion to NO2 in excess ozone (O-3) and subsequent NO2 collection in a 20% triethanolamine (TEA) solution as nitrite and nitrate for delta N-15 analysis using the denitrifier method. The precision and accuracy of the method were quantified through repeated collection of an analytical NO tank and intercalibration with a modified EPA NOx collection method. The results show that the efficiency of NO conversion to NO2 and subsequent NO2 collection in the TEA solution is >98% under a variety of controlled conditions. The method precision (1 sigma) and accuracy across the entire analytical procedure are +/- 1.1 parts per thousand. We report the first analyses of soil delta N-15-NO (-59.8 parts per thousand to -23.4 parts per thousand) from wetting-induced NO pulses at both laboratory and field scales that have important implications for understanding soil NO dynamics.