The natural abundance of ¹⁵N and ¹⁸O isotopes in nitrous oxide (N₂O) as well as the ¹⁵N intramolecular distribution can be used to obtain important information on its geochemical cycle, because many biological and chemical processes lead to distinct isotopic signatures. N₂O is a linear, non-symmetric molecule (N–N–O), with the four main isotopic species: ¹⁴N¹⁴N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, and ¹⁴N¹⁴N¹⁸O. The nitrogen atom at the center (a site) and one at the end (b site) lead to two structural isomers, namely ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O, referred to as ¹⁵N^a and ¹⁵N^b, respectively.

Employing recently available quantum cascade lasers (QCL) we are able to perform continuous and precise analysis of the isotope ratios d¹⁵N and d¹⁸O-N₂O and the site-specific isotope ratios d¹⁵Na and d¹⁵Nb (e.g. Wächter et al. 2008). By coupling the QCL spectrometer to a fully-automated preconcentration unit we achieve quasi-continuous and high precision analysis of N₂O isotopic species at ambient mixing ratios (Mohn et al. 2010, 2012,). In an inter-laboratory campaign we demonstrated excellent compatibility of QCL-based N₂O isotopomer analysis with isotope-ratio mass-spectrometry (IRMS) for d¹⁵N and d¹⁸O and superior performance for d¹⁵Na and d¹⁵Nb (Mohn et al. 2014).

 

Applications include laboratory incubation experiments on soil, wastewater and aqueous solution to determine the site-specific isotopic signatures of distinct N₂O source and sink processes. (Köster et al. 2013, Wunderlin et al. 2013, Heil et al. 2014). These source signatures in combination with ambient N₂O isotopic time series are then used for source allocation and partitioning (Mohn et al. 2012, 2013, Harris et al. 2014). In a first long-term field campaign on an intensively managed grassland controls on N₂O source processes were investigated based on their N₂O isotopic composition (Wolf et al. 2014).

In an ongoing project (Trace N₂O) in cooperation with KIT (Germany) continuous N₂O isotope measurements  are combined with backward Lagrangian particle dispersion modeling (FLEXPART-COSMO), and an inversion system to determine the source strengths of total N₂O and its isotopic signatures in northern Switzerland. In a complementary “bottom-up” approach, a state of the art biogeochemical model (LandscapeDNDC) will be extended with a sub-module capable of simulating isotopic signatures of N₂O emitted from soils. This complementary approach will allow checking for consistency between biogeochemistry modeling and observations and identifying weaknesses in our current understanding of the underlying processes.