Net Sediment N₂ Fluxes in a Coastal Marine System--Experimental Manipulations and a Conceptual Model

Environmental factors, including the supply of organic matter and inorganic nutrient concentrations, are thought to influence rates of two opposing processes in the nitrogen (N) cycle, denitrification and N fixation. Using sediment cores from a temperate estuary we examined the effects of nitrate, phosphate, and ammonium enrichment of the overlying water on the net N₂ flux across the sediment-water interface. In addition, we used sediment cores and large marine mesocosms to determine the effect of organic matter amendment. The addition of nitrate (50 μM), ammonium (50 μM), and phosphate (10 μM) to the overlying water had no effect on the net N₂ flux. However, high ammonium fluxes (145 μmol m⁻² h⁻¹) were observed in the nitrate-amended cores, which is suggestive of dissimilatory nitrate reduction to ammonium. Organic matter enrichment first increased rates of N fixation, but ultimately switched the sediments from being a net source (N fixation dominated) to a net sink (denitrification dominated) of N. We observed a threshold where N fixation took place when organic matter deposition fell below about 0.3 g C m⁻² day⁻¹. On the basis of these results and the findings of others, we developed a conceptual model that links net sediment N₂ flux with the organic matter production of a system.