Contrasting processes involving denitrification and biological N2-fixation drive N2O hotspots along two large lotic ecosystems in Germany

Lotic ecosystems traversing mixed land-use landscapes are sources of GHGs to the atmosphere, but their emission strength is uncertain due to longitudinal GHG heterogeneities. In this study, we quantified N₂O (as well as CO₂ and CH₄ concentrations) and N₂ concentrations and several water quality parameters along the Rhine river and the Mittelland canal, two critical inland waterways in Germany in the summer of 2023. Our main objectives were to compare N₂O concentrations along the two ecosystems and to identify the main drivers responsible for their longitudinal heterogeneities. The results indicated that N₂O concentrations in both ecosystems were oversaturated relative to equilibrium concentrations (116 – 782 % saturation), particularly in the Mittelland canal. We also found significant longitudinal variability in % N₂O saturation along the mainstems of both lotic ecosystems (CV = 43 – 68 %), with the highest variability in the Mittelland canal, suggesting that single N₂O measurements along large lotic ecosystems are not representative of the entire reach. Overall, these significant longitudinal N₂O heterogeneities were driven by differences in biogeochemical processes between the two lotic ecosystems. N₂O was strongly related to N₂ concentrations, with a negative relationship in the Rhine river and a positive relationship in the Mittelland canal. Based on these findings, we concluded that denitrification drives the N₂O hotspots in the Canal, while coupled biological N₂-fixation and nitrification accounted for N₂O hotspots in the Rhine. These findings also highlight the need to include N₂ concentration measurements in GHG sampling campaigns, as it has the potential to help better constrain nitrogen cycling in lotic ecosystems.