Surface nitrous oxide (NO) concentrations and fluxes from different rivers draining contrasting landscapes: Spatio-temporal variability, controls, and implications based on IPCC emission factor.

Increasing indirect nitrous oxide (NO) emission from river networks as a result of enhanced human activities on landscapes has become a global issue, as NO has been widely recognized as an important ozone-depleting greenhouse gas. However, indirect NO emissions from different rivers, particularly for those that drain completely different landscapes, are poorly understood. Here, we investigated the spatial-temporal variability of NO 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 NO production, with a mean NO concentration of ∼410 nmol L, which is 9-18 times greater than those mainly draining forested (23 nmol L), agricultural (42 nmol L) and mixed (45 nmol L) landscapes. Riverine dissolved NO was generally supersaturated with respect to the atmosphere. Such NO 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 NO fluxes in urban rivers reached ∼471 μmol m d, a value of ∼22, 13, and 11 times that in forested, agricultural and mixed watersheds, respectively. Averaged riverine NO emission factors (EF) 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 EF 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 EF and to better reveal the mechanisms behind indirect NO emissions as influenced by watershed landscapes.