A new mechanistic model of δ18O-N2O formation by denitrification

Abstract

Anaerobic incubations of flooded and non-flooded agricultural soils and stream sediment were conducted with 18O-labelled water to investigate the stable isotope ratios (δ15N and δ18O) of nitrous oxide (N2O) produced from denitrification. The rates of N2O production and δ15N- and δ18O-N2O values were measured. The amount of oxygen exchange (O-exchange) with water, and the nitrogen and oxygen isotope effects (ε) were calculated. The net 15N isotope effect (NO3−→N2O) for denitrification in this study varied from −30‰ to −9‰. The net 18O isotope effect ranged between +32‰ and +60‰ and was negatively correlated to the total fraction of O-exchange, which varied between 0.40 and 0.94.This manuscript describes a new, comprehensive set of mathematical expressions that can be used to model δ18O values of N2O formed by denitrification and calculate the magnitude of O-exchange and the net 18O isotope effect for denitrification. This mathematical approach is compared to another method of approximating O-exchange (Snider et al., 2009), and we show that this older method provides a minimum estimate of O-exchange. Using this mechanistic model, we discuss how N2O consumption, open/closed systems, and variations in the N2O:N2 ratio can influence the observed δ18O-N2O.The net 18O isotope effect for denitrification in this study was partially controlled by the fractions of O-exchange and N2O reduction, which were likely influenced by the actively denitrifying microbial community and the soil moisture. We conclude that δ18O-N2O values are, in many cases, useful tracers of N2O production because they are often higher than the majority of nitrification-derived δ18O-N2O values. The usefulness of δ18O values to apportion sources must be determined on a case-by-case basis.