# Estimation of isotope variation of N 2O during denitrification by Pseudomonas aureofaciens and Pseudomonas chlororaphis: implications for N 2O source apportionment

Soil microbial processes, stimulated by agricultural fertilization, account for 90 % of anthropogenic nitrous oxide (inline-formulaN2O), the leading source of ozone depletion and a potent greenhouse gas. Efforts to reduce inline-formulaN2O flux commonly focus on reducing fertilization rates. Management of microbial processes responsible for inline-formulaN2O production may also be used to reduce inline-formulaN2O emissions, but this requires knowledge of the prevailing process. To this end, stable isotopes of inline-formulaN2O have been applied to differentiate inline-formulaN2O produced by nitrification and denitrification. To better understand the factors contributing to isotopic variation during denitrification, we characterized the inline-formulaδ15N, inline-formulaδ18O and site preference (SP; the intramolecular distribution of inline-formula15N in inline-formulaN2O) of inline-formulaN2O produced during inline-formula $M15inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgecc3e6dd5af0ffb1da8bfbfcb16b8e8b bg-15-3873-2018-ie00001.svg25pt16ptbg-15-3873-2018-ie00001.png reduction by Pseudomonas chlororaphis subsp. aureofaciens and P. c. subsp. chlororaphis. The terminal product of denitrification for these two species is inline-formulaN2O because they lack the gene nitrous oxide reductase, which is responsible for the reduction of inline-formulaN2O to Ninline-formula2. In addition to species, treatments included electron donor (citrate and succinate) and electron donor concentration (0.01, 0.1, 1 and 10 mM) as factors. In contrast to the expectation of a Rayleigh model, all treatments exhibited curvilinear behaviour between inline-formulaδ15N or inline-formulaδ18O and the extent of the reaction. The curvilinear behaviour indicates that the fractionation factor changed over the course of the reaction, something that is not unexpected for a multi-step process such as denitrification. Using the derivative of the equation, we estimated that the net isotope effects (inline-formulaη) vary by as much as 100 ‰ over the course of a single reaction, presenting challenges for using inline-formulaδ15N and inline-formulaδ18O as apportionment tools. In contrast, SP for denitrification was not affected by the extent of the reaction, the electron donor source or concentration, although the mean SP of inline-formulaN2O produced by each species differed. Therefore, SP remains a robust indicator of the origin of inline-formulaN2O. To improve apportionment estimates with SP, future studies could evaluate other factors that contribute to the variation in SP.

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Haslun, Joshua A. / Ostrom, Nathaniel E. / Hegg, Eric L. / et al: Estimation of isotope variation of N2O during denitrification by Pseudomonas aureofaciens and Pseudomonas chlororaphis: implications for N2O source apportionment. 2018. Copernicus Publications.

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