NITRIFICATION AND DENITRIFICATION IN LAKE AND ESTUARINE SEDIMENTS MEASURED BY THE N-15 DILUTION TECHNIQUE AND ISOTOPE PAIRING

The transformation of nitrogen compounds in lake and estuarine sedimen ts incubated in the dark was analyzed in a continuous-flowthrough syst em. The inflowing water contained (NO3-)N-15, and by determination of the isotopic composition of the N2, NO3-, and NH4+ pools in the outflo wing 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 mineraliza tion. In sediment cores from both lake and estuarine environments, ben thic microphytes assimilated NO3- and NH4+ for a period of 25 to 60 h after darkening. Under steady-state conditions in the dark, denitrific ation of NO3- originating from the overlying water accounted for 91 to 171 mumol m-2 h-1 in the lake sediments and for 131 to 182 mumol 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- upt ake by benthic microphytes in the initial dark period may have been mi sinterpreted in earlier investigations as dissimilatory reduction to a mmonium. The rates of coupled nitrification-denitrification within the sediments contributed to 10% of the total denitrification at steady s tate in the dark, and total nitrification was only twice as high as th e coupled process.