Surface nitrous oxide (N2O) concentrations and fluxes from different rivers draining contrasting landscapes: Spatio-temporal variability, controls, and implications based on IPCC emission factor

Increasing indirect nitrous oxide (N2O) emission from river networks as a result of enhanced human activities on landscapes has become a global issue, as N2O has been widely recognized as an important ozone-depleting greenhouse gas. However, indirect N2O emissions from different rivers, particularly for those that drain completely different landscapes, are poorly understood. Here, we investigated the spatial-temporal variability of N2O emissions among the different rivers in the Chaohu Lake Basin of Eastern China. Our results showed that river reaches in urban watersheds are the hotspots of N2O production, with a mean N2O concentration of ∼410 nmol L−1, which is 9–18 times greater than those mainly draining forested (23 nmol L−1), agricultural (42 nmol L−1) and mixed (45 nmol L−1) landscapes. Riverine dissolved N2O was generally supersaturated with respect to the atmosphere. Such N2O saturation can best be explained by nitrogen availability, except for those in the forested watersheds, where dissolved oxygen is thought to be the primary predictor. The estimated N2O fluxes in urban rivers reached ∼471 μmol m−2 d−1, a value of ∼22, 13, and 11 times that in forested, agricultural and mixed watersheds, respectively. Averaged riverine N2O emission factors (EF5r) of the forested, agricultural, urban and mixed watersheds were 0.066%, 0.12%, 0.95% and 0.16%, respectively, showing different deviations from the default EF5r that released by IPCC in 2019. This points to a need for more field measurements with wider spatial coverage and finer frequency to further refine the EF5r and to better reveal the mechanisms behind indirect N2O emissions as influenced by watershed landscapes. [Display omitted] •N2O fluxes from rivers of the Chaohu Lake Basin varied seasonally and spatially.•Main processes contributing to riverine N2O production may vary across watersheds.•Urban rivers are hotspots of N2O emission.•Separate emission factors for different watersheds are recommended. Main findings: (i) N2O concentration and fluxes from river networks in the Chaohu Lake Basin varied seasonally and spatially; (ii) Main controls and associated processes contributing to riverine N2O production may vary across watersheds; (iii) Urban rivers are hotspots of N2O emission; (iv) future refinements of IPCC’s EF5r should consider the main landscapes drained by rivers.