Temperature control on wastewater and downstream nitrous oxide emissions in an urbanized river system

Although eutrophic urban rivers receiving loads of wastewater represent an important anthropogenic source of N₂O, little is known as to how temperature and other environmental factors affect temporal variations in N₂O emissions from wastewater treatment plants (WWTPs) and downstream rivers. Two-year monitoring at a WWTP and five river sites was complemented with available water quality data, laboratory incubations, and stable isotopes in N₂O and NO₃^- to explore how wastewater effluents interact with seasonal changes in environmental conditions to affect downstream metabolic processes and N₂O emissions from the lower Han River traversing the megacity Seoul. Water quality data from four WWTPs revealed significant inverse relationships between water temperature and the concentrations or fluxes of total N (TN) in effluents. Increased TN fluxes at low temperatures concurred with N₂O surges in WWTP effluents and downstream rivers, counteracting the long-term decline in TN fluxes resulting from enhanced wastewater treatments. Incubation experiments with river water and sediment, in isolation or combined, implied the hypoxic winter sediment as a large source of N₂O, whereas the anoxic summer sediment produced a smaller amount of N₂O only when it was added with oxic water. For both WWTP effluents and downstream rivers, bulk isotope ratios and intramolecular distribution of ¹⁵N in N₂O distinctly differed between summer and winter, indicating incomplete denitrification in the hypoxic sediment at low temperatures as a primary downstream source adding to WWTP-derived N₂O. Winter surges in wastewater TN and sediment N₂O release highlight temperature variability as an underappreciated control over anthropogenic N₂O emissions from increasingly urbanized river systems worldwide.