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

摘要

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.