Nitrification and Denitrification in Lake and Estuarine Sediments Measured by the 15 N Dilution Technique and Isotope Pairing

Abstract

The transformation of nitrogen compounds in lake and estuarine sediments incubated in the dark was analyzed in a continuous-flowthrough system. The inflowing water contained 15 NO 3 - , and by determination of the isotopic composition of the N 2 , NO 3 - , and NH 4 + pools in the outflowing water, it was possible to quantify the following reactions: total NO 3 - uptake, denitrification based on NO 3 - from the overlying water, nitrification, coupled nitrification-denitrification, and N mineralization. In sediment cores from both lake and estuarine environments, benthic microphytes assimilated NO 3 - and NH 4 + for a period of 25 to 60 h after darkening. Under steady-state conditions in the dark, denitrification of NO 3 - originating from the overlying water accounted for 91 to 171 μmol m -2 h -1 in the lake sediments and for 131 to 182 μmol m -2 h -1 in the estuarine sediments, corresponding to approximately 100% of the total NO 3 - uptake for both sediments. It seems that high NO 3 - uptake by benthic microphytes in the initial dark period may have been misinterpreted in earlier investigations as dissimilatory reduction to ammonium. The rates of coupled nitrification-denitrification within the sediments contributed to 10% of the total denitrification at steady state in the dark, and total nitrification was only twice as high as the coupled process.