Tidal freshwater rivers (TFR) are an understudied link in the global nitrogen cycle that couple river basins with the ocean. Denitrification occurring in the broad riparian zones of TFRs decreases nitrogen flux to estuaries and oceans, though the magnitude of this flux relative to riverine nitrogen loads is unknown. This study developed an empirical model of riparian zone denitrification and parameterized it using measured denitrification rates, sediment redox dynamics, and riparian topography of a TFR in North Carolina, U.S.A. Denitrification rates were measured monthly in laboratory-incubated sediment cores by using a membrane inlet mass spectrometer to assess net water-borne N₂ flux from the cores. Sediment cores were instrumented with redox probes and manipulated in the laboratory to measure the onset of reduced conditions following inundation. Floodplain topography was modeled using a geographic information system to analyze high-accuracy elevation data.

Results/Conclusions

Annual average rates of denitrification in freshwater, intertidal wetland, mudflat, and riparian forest sediments were 1864, 1956, and 2018 mg m^-2 hr^-1, respectively. The temporal lag between tidal inundation and reduced, denitrifying conditions was 4-5 hr. Combining these experimental results with floodplain topography, the model predicted that the annual average daily denitrification flux constituted 20% of the daily riverine nitrate flux. However, riverine nitrate flux varied 17-fold over an annual period while riparian denitrification flux varied only 3-fold. While the TFR riparian zone was a significant sink for nitrogen, denitrification flux was relatively constant compared to riverine flux. The first implication of these results is that nitrogen loss within TFRs needs to be included in continental-to-ocean nitrogen budgets. Secondly, the topography and tidal hydrology of TFRs are as important as the denitrification rate in determining the nitrogen loss from the riparian zone relative to riverine through-flow.