Unravelling spatiotemporal N₂O dynamics in an urbanized estuary system using natural abundance isotopes

Estuaries are strong sources of N₂O to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of this powerful greenhouse gas. Here, we investigated the spatiotemporal dynamics of the N cycle in an estuary with a focus on the emission mechanisms and pathways of N₂O. By coupling N₂O isotopocule analysis and substrate NO₃^- isotope analysis, we found that nutrient availability, oxygen level, salinity gradient and temperature variation were major drivers of the N₂O emissions from the Scheldt Estuary. In winter, lower temperature and higher O₂ concentration diminished denitrification rates and reduction of N₂O to N₂, while both were enhanced in warmer summer, causing higher fraction of reduced N₂O. As a result, we found comparable N₂O fluxes and dissolved concentrations between the two seasons. Decrease in salinity level and increase in NO₃^- concentration accelerated N₂O production when moving upstream of the estuary where more urbanization and higher NO₃^- from wastewater discharges were found. However, these drivers had no significant effect on the fraction of N₂O derived by either denitrification or nitrification and/or fungal denitrification since the fractional proportion of these pathways showed no spatiotemporal variations, remaining around 89 % and 11 %, respectively. These findings challenge the conventional notion that N₂O fluxes are generally higher in summer because of higher denitrification rates while confirming that denitrification is the most important pathway of N₂O production in the estuaries. Furthermore, our study highlight the importance of combining various isotope analyses to gain in-depth understanding about N₂O emission pathways and N cycling in dynamic systems like estuaries.