Fractionation of nitrogen and oxygen isotopic composition in N2O produced by bacterial denitrification

Abstract

The isotopic composition of nitrous oxide (N2O) reveals valuable information on the biological production sources that contribute to N2O accumulation in the atmosphere, i.e. denitrification, nitrifier-denitrification and nitrification1. Isotopic fingerprints for each of these microbial pathways have been identified in past work, however, overlapping signatures of co-occurring N2O production processes2, and limitations in the robustness of associated fractionation factors under varying growth/environmental conditions3 still pose significant challenges.We will present data from the initial phase of our project, where we study N2O production and associated N and O isotopic fractionation by the denitrifier Pseudomonas aureofaciens, grown in the laboratory under different growth conditions and thus different reaction kinetics. N2O production was quantified on-line by Fourier-Transformation IR-spectroscopy (FTIR), and the isotopic composition of produced N2O was determined by quantum cascade-laser-absorption spectroscopy (QCLAS). The combination of N2O production and isotope data (continuously measured) allowed us to elucidate changes in N and O isotope fractionation in response to changing reaction kinetics.In a later phase of the project, we will expand our isotope-analytical capability by including also the doubly-substituted molecules of N2O, 15N15N16O (556), 14N15N18O (458) and 15N14N18O (548). More specifically, we will interrogate the symmetry of N – N bond formation, verify combinatorial effects during N2O production, and we will test whether Δ556, Δ458, and Δ548 (and the preference for 15N substitution in the central/terminal N-position) can be applied as proxies for reaction kinetics4, 5.  1 Yu, L., Harris, E., Lewicka Szczebak, D., Barthel, M., Blomberg, M.R., Harris, S.J., Johnson, M.S., Lehmann, M.F., Liisberg, J., Müller, C. and Ostrom, N.E., 2020. What can we learn from N2O isotope data?–Analytics, processes and modelling. Rapid Communications in Mass Spectrometry, 34(20), p.e8858.2 Kantnerová, K., Tuzson, B., Emmenegger, L., Bernasconi, S.M. and Mohn, J., 2019. Quantifying isotopic signatures of N2O using quantum cascade laser absorption spectroscopy. Chimia, 73(4), pp.232-232.3 Haslun, J.A., Ostrom, N.E., Hegg, E.L. and Ostrom, P.H., 2018. Estimation of isotope variation of N2O during denitrification by Pseudomonas aureofaciens and Pseudomonas chlororaphis: implications for N2O source apportionment. Biogeosciences, 15(12), pp.3873-3882.4 Yeung, L.Y., 2016. Combinatorial effects on clumped isotopes and their significance in biogeochemistry. Geochimica et Cosmochimica Acta, 172, pp.22-38.5 Kantnerová, K., Hattori, S., Toyoda, S., Yoshida, N., Emmenegger, L., Bernasconi, S.M. and Mohn, J., 2022. Clumped isotope signatures of nitrous oxide formed by bacterial denitrification. Geochimica et Cosmochimica Acta, 328, pp.120-129.