Importance of alternative denitrification pathways in a seasonally stratified lake basin

Nitrous oxide (N₂O) is a strong greenhouse gas and ozone-destroying compound, whose atmospheric concentrations have been increasing over the past decades. Lakes play a relatively uncertain role with regards to their contribution to the global natural N₂O emissions, and a better understanding of the environmental controls on lacustrine N₂O production and consumption is needed.

We investigated N₂O production pathways in the anoxic deep hypolimnion of the South Basin of eutrophic Lake Lugano (Switzerland) during summer stratification. Temporary high concentrations of N₂O (up to 3000 nmol/L) were observed in the oxygen-depleted near-bottom waters, accompanied by a site preference (SP, indicating the intramolecular ¹⁵N distribution) of +32 ‰. Incomplete heterotrophic denitrification is commonly thought to be the main N₂O production pathway in low-oxygen environments, but it is typically characterized by a low SP of -5-0 ‰ (Sutka et al. 2006). High SP values, as observed here, rather point to an oxidative N₂O production mechanism such as (micro-aerobic) nitrification, yet they may also be caused by partial reduction of N₂O to N₂ (Ostrom et al. 2007). We performed incubation experiments with ¹⁵N-labeled NH₄⁺ to investigate oxidative N₂O production through (micro-aerobic) nitrification, and ¹⁵N-labeled NO₃^- to investigate reductive N₂O production through denitrification. Our results point indeed to a reductive mode of N₂O production in the anoxic bottom water. Alternative denitrification pathways known to produce N₂O with potentially higher SP, such as fungal denitrification (SP = >30 ‰, Rohe et al. 2014) and chemo-denitrification (SP = 0-27 ‰, Li et al. 2022), were investigated in additional incubation experiments using bacterial and fungal inhibitors. These experiments revealed that bacterial denitrification contributes most to N₂O production in the sediment and the bottom water layer, while fungal- and chemo-denitrification were much less important.The elevated SP values in the bottom-water N₂O during summer stratification are most likely due to the fact that much of the produced N₂O has been reduced to N₂. Thus, N₂O reduction can completely mask primary N₂O isotopic source signatures in redox transition zones, complicating the use of N₂O stable isotope measurements to disentangle reductive and oxidative N₂O production and to reveal alternative denitrification pathways.

 

REFERENCES

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Rohe, L., Anderson, T.-H., Braker, G., Flessa, H., Giesemann, A., Lwicka-Szczebak, D., Wrage-Mönnig, N., & Well, R. 2014: Dual isotope and isotopomer signatures of nitrous oxide from fungal denitrification – a pure culture study, Rapid Communiciations in Mass Spectrometry, 28, 1893-1903.

Sutka, R. L., Ostrom, N. E., Ostrom, P. H., Breznak, J. A., Gandhi, H., Pitt, A. J., & Li, F. 2006: Distinguishing nitrous oxide production from nitrification and denitrification on the basis of isotopomer abundances, Applied and Environmental Microbiology, 72(1), 638–644.