Importance of alternative denitrification pathways in a seasonally stratified lake basin

Abstract

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 Li, S., Wang, S., Pang, Y., & Ji, G. 2022: Influence of electron donors (Fe, C, S) on N2O production during nitrate reduction in lake sediments: Evidence from isotopes and functional genes, ACS ES&T Water, 2(7), 1254–1264. Ostrom, N. E., A. Pitt, R. Sutka, P. H. Ostrom, A. S. Grandy, K. M. Huizinga, and G. P. Robertson (2007), Isotopologue effects during N2O reduction in soils and in pure cultures of denitrifiers, J. Geophys. Res., 112, G02005. 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.