Reassessment of the NH4NO3thermal decomposition technique for calibration of the N2O isotopic composition.
BACKGROUND
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 the15N site-specific (SP ≡ δ15Nα- δ15Nβ) and bulk (δ15Nbulk= (δ15Nα+ δ15Nβ)/2) isotopic composition of N2O against the international standard for the15N/14N isotope ratio (AIR-N2). Within the current project15N fractionation effects during thermal decomposition of NH4NO3on the N2O site preference were studied using static and dynamic decomposition techniques.


RESULTS
The validity of the NH4NO3decomposition technique to link NH4+and NO3-moiety-specific δ15N analysis by IRMS to the site-specific nitrogen isotopic composition of N2O was confirmed. However, the accuracy of this approach for the calibration of δ15Nαand δ15Nβvalues was found to be limited by non-quantitative NH4NO3decomposition 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 NH4NO3decomposition reaction currently confine the anchoring of N2O site-specific isotopic composition to the international isotope ratio scale AIR-N2. The authors suggest establishing a set of N2O isotope reference materials with appropriate site-specific isotopic composition, as community standards, to improve inter-laboratory compatibility. Copyright © 2016 John Wiley & Sons, Ltd.
Laboratory for Air Pollution & Environmental Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129,, CH-8600, Dübendorf, Switzerland.
Joachim
J
Mohn
Laboratory for Air Pollution & Environmental Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129,, CH-8600, Dübendorf, Switzerland.
Wilhelm
W
Gutjahr
Department of Chemical Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan.
Sakae
S
Toyoda
Laboratory for Air Pollution & Environmental Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129,, CH-8600, Dübendorf, Switzerland.
Eliza
E
Harris
Laboratory for Air Pollution & Environmental Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129,, CH-8600, Dübendorf, Switzerland.
Erkan
E
Ibraim
Stable Isotope Laboratory (IsoLab), Max-Planck-Institute for Biogeochemistry (MPI-BGC), Hans-Knöll-Str. 10,, D-07745, Jena, Germany.
Heike
H
Geilmann
Forest Soils and Biogeochemistry, WSL, Zürcherstrasse 111,, CH-8903, Birmensdorf, Switzerland.
Patrick
P
Schleppi
Biogeochemistry, University of Basel, Bernoullistrasse 30,, CH-4056, Basel, Switzerland.
Thomas
T
Kuhn
Biogeochemistry, University of Basel, Bernoullistrasse 30,, CH-4056, Basel, Switzerland.
Moritz F
MF
Lehmann
Sustainable Agroecosystems, ETH Zürich, Tannenstrasse 1,, CH-8092, Zürich, Switzerland.
Charlotte
C
Decock
Institute of Agricultural Sciences, ETH Zürich, Universitätstrasse 2, CH-8092, Zürich, Switzerland.
Roland A
RA
Werner
Department of Chemical Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan.|Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan.
Naohiro
N
Yoshida
Stable Isotope Laboratory (IsoLab), Max-Planck-Institute for Biogeochemistry (MPI-BGC), Hans-Knöll-Str. 10,, D-07745, Jena, Germany.
Willi A
WA
Brand