Small-scale distribution of interstitial nitrite in freshwater sediment microcosms: The role of nitrate and oxygen availability, and sediment permeability

The spatial distribution of interstitial NO(2)(-) concentrations was studied in NO(3)(-)-exposed freshwater sediment microcosms, using pore water extractions as well as ion-selective microsensors. Porewater extractions revealed ecotoxicologically critical NO(2)(-) concentrations in hypoxic and anoxic sediment layers in which significant NO(3)(-) consumption took place. In contrast, the use of ion-selective microsensors demonstrated the high capacity of the thin oxic surface layer of the sediments to consume NO(2)(-) and to produce NO(3)(-). Two modes of NO(3)(-). Supply to the sediments were compared: In treatments with NO(3)(-) supply to the overlying water, a subsurface maximum of NO(2)(-) concentration was observed, coinciding with the site of maximum NO(3)(-) consumption. When NO(3)(-) was perfused up through the sediment cores, however, NO(2)(-) accumulated throughout the entire sediment column. Such spatially extensive NO(2)(-) accumulations were only observed in sediments poor in organic matter with a relatively high permeability. By manipulating the O(2) content of the overlying water, the release of NO(2)(-) from the sediments could be influenced: In treatments with air-saturated overlying water, the sediments did not release detectable amounts of NO(2)(-) into the water phase. When kept hypoxic (25% air saturation) instead, significant NO(2)(-) accumulations were recorded in the overlying water. These findings suggest that in treatments with air-saturated overlying water, NO(2)(-) that was produced in deeper sediment layers (denitrifying conditions) was completely consumed at the oxic sediment surface (nitrifying conditions) before it could reach the overlying water.