Nitrous oxide (N2O) is a strong greenhouse gas and ozone-destroying agent, with increasing atmospheric mixing ratios over the last few decades. The contribution of lakes to global N2O emissions is uncertain, in parts due to the lack of a better understanding of the environmental controls on N2O production and consumption processes. In this study, we investigated N2O dynamics in the seasonally stratified South Basin of eutrophic Lake Lugano (Switzerland) over one complete annual cycle by analysing the concentrations and stable isotopic composition of N2O, including N2O site preference (SP), and performing tracer incubation experiments with ¹⁵N-labelled nitrogen compounds. Strong accumulation of N2O (up to 100 nmol/L) was observed in the anoxic benthic nepheloid layer in near-bottom waters during summer stratification. It was accompanied by a significant decline in δ¹⁵N^bulk to -12 ‰ and an increase in SP  to +42 %permil;. Incomplete heterotrophic denitrification is usually the main production mechanism for N2O in low-oxygen environments. However, N2O produced through denitrification is expected to yield a low SP, whereas high SP values rather point to an oxidative N2O production pathway. The combined isotopic constraints on fractional N2O reduction remained ambivalent. N2O in the bottom water is potentially produced by an alternative denitrification pathway, i.e. chemolithotrophic denitrification, or its production may be coupled to an oxidative process, which will be investigated in future experiments.

Primary Presenter: Teresa Einzmann, University of Basel (teresa.einzmann@unibas.ch)

Authors:

Teresa Einzmann, University of Basel  (teresa.einzmann@unibas.ch)

Moritz Lehmann, University of Basel  ()

Annie Bourbonnais, University of South Carolina  ()

Jakob Zopfi, University of Basel  ()

Claudia Frey, University of Basel  ()