Nitrification and Denitrification in Lake and Estuarine Sediments Measured by the 15N Dilution Technique and Isotope Pairing |
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| Authors: | Sren Rysgaard Nils Risgaard-Petersen Lars Peter Nielsen and Niels Peter Revsbech |
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| Affiliation: | Søren Rysgaard, Nils Risgaard-Petersen, Lars Peter Nielsen, and Niels Peter Revsbech |
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| 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 15NO3-, and by determination of the isotopic composition of the N2, NO3-, and NH4+ pools in the outflowing water, it was possible to quantify the following reactions: total NO3- uptake, denitrification based on NO3- from the overlying water, nitrification, coupled nitrification-denitrification, and N mineralization. In sediment cores from both lake and estuarine environments, benthic microphytes assimilated NO3- and NH4+ for a period of 25 to 60 h after darkening. Under steady-state conditions in the dark, denitrification of NO3- 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 NO3- uptake for both sediments. It seems that high NO3- 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. |
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