Riparian zone denitrification affects nitrogen flux through a tidal freshwater river

Tidal freshwater zones (TFZ) of coastal rivers link terrestrial watersheds to the ocean and are characterized by large, regularly inundated riparian zones. We investigated the effect of riparian denitrification on nitrogen flux in the TFZ Newport River, North Carolina (U.S.A.) by developing an empirical model of denitrification and parameterizing it using measured denitrification rates, sediment oxidation-reduction potential dynamics, and riparian topography. Denitrification rates were measured monthly in laboratory-incubated sediment cores by using a membrane inlet mass spectrometer to assess net water-borne N-2 flux from the cores. Annual average rates of denitrification in three intertidal riparian habitats, emergent marsh, mudflat, and hardwood forest, were 1864, 1956, and 2018 mu g m(-2) h(-1), respectively. Laboratory experiments and in-situ monitoring revealed that the temporal lag between tidal inundation and reduced, denitrifying conditions was 4-5 h. Field measurements and remotely sensed data showed that the inundated surface area during high tide was three times greater than that at low tide. By combining data on denitrification, oxidation-reduction potential, and topography, the model predicted that the daily denitrification flux constituted 2-15% of the daily riverine nitrate flux during most of the year and > 100% during low discharge periods. Current regional and global nitrogen budgets thus may overestimate nitrogen delivery to the ocean by not accounting for the TFZ denitrification.