Unravelling spatiotemporal NO dynamics in an urbanized estuary system using natural abundance isotopes.

Estuaries are strong sources of NO 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 NO. By coupling NO isotopocule analysis and substrate NO isotope analysis, we found that nutrient availability, oxygen level, salinity gradient and temperature variation were major drivers of the NO emissions from the Scheldt Estuary. In winter, lower temperature and higher O concentration diminished denitrification rates and reduction of NO to N, while both were enhanced in warmer summer, causing higher fraction of reduced NO. As a result, we found comparable NO fluxes and dissolved concentrations between the two seasons. Decrease in salinity level and increase in NO concentration accelerated NO production when moving upstream of the estuary where more urbanization and higher NO from wastewater discharges were found. However, these drivers had no significant effect on the fraction of NO 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 NO fluxes are generally higher in summer because of higher denitrification rates while confirming that denitrification is the most important pathway of NO production in the estuaries. Furthermore, our study highlight the importance of combining various isotope analyses to gain in-depth understanding about NO emission pathways and N cycling in dynamic systems like estuaries.