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

摘要

The global inventory of NO_x (NO_x = NO + NO_2) 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 (δ~(15)N) in various soil N-containing compounds have proven to be a robust tracer of soil N cycling, soil δ~(15)N-NO is rarely quantified due to the measurement difficulties. Here, we present a new method that collects soil-emitted NO through NO conversion to NO_2 in excess ozone (O_3) and subsequent NO_2 collection in a 20% triethanolamine (TEA) solution as nitrite and nitrate for δ~(15)N 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 NO_x collection method. The results show that the efficiency of NO conversion to NO_2 and subsequent NO_2 collection in the TEA solution is >98% under a variety of controlled conditions. The method precision (1σ) and accuracy across the entire analytical procedure are ±1.1‰. We report the first analyses of soil δ~(15)N-NO (— 59.8‰ to -23.4‰) from wetting-induced NO pulses at both laboratory and field scales that have important implications for understanding soil NO dynamics.