Nitrous Oxide and Dissolved Inorganic Nitrogen Dynamics in Scheldt River using Nitrogen Stable Isotope Composition.

Supervisors: Alberto Borges, Cedric Morana (University of Liège)
Nitrous oxide (N2O) is an important greenhouse gas, which not only works in medicine field and food industry, but more importantly, has 265 times higher global warming potential than CO2, plus plays a significant role in the stratospheric ozone depletion. Due to the enhancing human use of the river, the availability of nitrogen in aquatic systems has increased, thus intensifying N2O emissions of aquatic ecosystems. Therefore, better understanding of N2O biogeochemistry can help better constraining the N2O budget and informing mitigation strategies. Bacterial nitrification, denitrification and dissimilatory nitrate reduction (DNRA) are regarded as the major microbial N2O production processes, especially the first two. These biologically distinct origins of N2O can be identified based on their different stable isotopic signatures (δ15Nbulk, δ18O, δ15Nα, δ15Nβ, and site preference (SP)=δ15Nα−δ15Nβ). The later one, site preference, has proven to be a useful tool that revealing N2O biogeochemistry details regardless of substrate’s isotopic signature. Here in this thesis, we present in this work the N2O and DIN species observations along the Scheldt river and its major tributaries in June 2019 and February 2020. General N2O oversaturation with respect to the atmosphere was observed through our investigation, varied from 19 to 274noml/L. In addition, summer Scheldt emitted more N2O than winter. Upper Scheldt river from Tournai to Gent was the major contributor of N2O emitted from Scheldt all year around, followed by tidal Scheldt and Scheldt estuary. Nitrification was the dominant microbial N2O production processes supported by highest ammonium concentration. However, anoxic sedimentary denitrification was associated with N2O emission of the lower stream due to the ammonium limitation at these sites. Nitrate through our investigation is the dominant DIN species, ranging from 69 to 290 μmol/L across entire Scheldt, then ammonium, and nitrite.Intramolecular site preference in N2O isotopomer was diverse in terms of season and sites. In summer, it ranged from 12 to 24‰ for upper Scheldt and -19‰ to 55‰ for lower Scheldt. However, winter site preference values were all negative, ranged from -2.8 to 69‰, including tributaries and mainstream.