Estimation of isotope variation of N₂O during denitrification by <i>Pseudomonas aureofaciens</i> and <i>Pseudomonas chlororaphis</i>: implications for N₂O source apportionment

• Main Authors: J. A. Haslun, N. E. Ostrom, E. L. Hegg, P. H. Ostrom
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-06-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/15/3873/2018/bg-15-3873-2018.pdf

Soil microbial processes, stimulated by agricultural fertilization, account for 90 % of anthropogenic nitrous oxide (N₂O), the leading source of ozone depletion and a potent greenhouse gas. Efforts to reduce N₂O flux commonly focus on reducing fertilization rates. Management of microbial processes responsible for N₂O production may also be used to reduce N₂O emissions, but this requires knowledge of the prevailing process. To this end, stable isotopes of N₂O have been applied to differentiate N₂O produced by nitrification and denitrification. To better understand the factors contributing to isotopic variation during denitrification, we characterized the <i>δ</i>¹⁵N, <i>δ</i>¹⁸O and site preference (SP; the intramolecular distribution of ¹⁵N in N₂O) of N₂O produced during NO₃⁻ reduction by <i>Pseudomonas chlororaphis subsp. aureofaciens</i> and <i>P. c. subsp. chlororaphis</i>. The terminal product of denitrification for these two species is N₂O because they lack the gene nitrous oxide reductase, which is responsible for the reduction of N₂O to N₂. 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 <i>δ</i>¹⁵N or <i>δ</i>¹⁸O 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 (<i>η</i>) vary by as much as 100 ‰ over the course of a single reaction, presenting challenges for using <i>δ</i>¹⁵N and <i>δ</i>¹⁸O 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 N₂O produced by each species differed. Therefore, SP remains a robust indicator of the origin of N₂O. To improve apportionment estimates with SP, future studies could evaluate other factors that contribute to the variation in SP.