1 Laboratory for Air Pollution & Environmental Technology, Swiss Federal Laboratories for Materials Science and Technology (EMPA), Überlandstr. 129, CH-8600 Dübendorf, Switzerland
2 Department of Chemical Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
3 Stable Isotope Laboratory, Max-Planck-Institute for Biogeochemistry (MPI-BGC), Hans-Knöll-Str. 10, DE-07745 Jena, Germany
4 Forest Soils and Biogeochemistry, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
5 Biogeochemistry, University of Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland
6 Sustainable Agroecosystems, ETH Zürich, Tannenstrasse 1, CH-8092 Zürich, Switzerland
7 Institute of Agricultural Sciences, ETH Zürich, Universitätstrasse 2, CH-8092 Zürich, Switzerland
8 Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Rationale: In the last few years, the study of N2O site-specific nitrogen isotope composition has been established as a powerful technique to disentangle N2O emission pathways. This trend has been accelerated by significant analytical progress in the field of isotope-ratio mass-spectrometry (IRMS) and more recently quantum cascade laser absorption spectroscopy (QCLAS).
Methods: The ammonium nitrate (NH4NO3) decomposition technique provides a strategy to scale the 15N site-specific (SP ≡ δ15Nα – δ15Nβ) and bulk (δ15Nbulk = (δ15Nα + δ15Nβ) / 2) isotopic composition of N2O against the international standard for the 15N/14N isotope ratio (AIR-N2). Within the current project 15N fractionation effects during thermal decomposition of NH4NO3 on the N2O site preference were studied using static and dynamic decomposition techniques.
Results: The validity of the NH4NO3 decomposition technique to link NH4+ and NO3- moiety-specific δ15N analysis by IRMS to site-specific nitrogen isotopic composition of N2O was confirmed. However, the accuracy of this approach for calibration of δ15Nα and δ15Nβ was found to be limited by non-quantitative NH4NO3 decomposition in combination with substantially different isotope enrichment factors for the conversion of the NO3- or NH4+ nitrogen atom into the α or β position of the N2O molecule.
Conclusions: The study reveals that the completeness and reproducibility of the NH4NO3 decomposition reaction currently confines the anchoring of N2O site specific isotopic composition to the international isotope ratio scale AIR-N2. The authors suggest to establish a set of N2O isotope reference materials with appropriate site-specific isotopic composition, as community standards, to improve inter-laboratory compatibility.
Keywords: nitrous oxide, isotopic composition, site-preference, NH4NO3 thermal decomposition, isotope fractionation