Denitrification rates as measured by the isotope pairing method and by the acetylene inhibition technique in continental shelf sediments of the North Sea

Denitrification rates in continental shelf sediments of the North Sea were measured by the simultaneous application of the acetylene inhibition technique (AIT) and the isotope pairing method (IPM). Nitrification was the predominant source of nitrate in these sediments. Both techniques were able to detect coupled nitrification-denitrification activity. Denitrification rates measured by the AIT were only measurable during the first 60 min of the incubation. Rates obtained by the AIT in this period ranged between 5.4 and 7.3 mu mol N m(-2)h(-1). Continued incubation of acetylene amended sediment cores resulted in decreasing denitrification rates, suggesting that accumulated nitrous oxide was consumed despite the presence of acetylene. Denitrification rates obtained by the application of the IPM ranged between 9.8 and 13.2 mu mol N m(-2)h(-1). Constant production of single-labelled ((NN)-N-14-N-15) relative to double-labelled ((NN)-N-15-N-15) dinitrogen indicated a homogeneous mixing of both isotopic nitrate species through the denitrification zone. The present results suggest that previously published data severely underestimated denitrification rates in North Sea continental shelf sediments. Concomitant measured fluxes of nitrate and ammonium across the sediment-water interface reveal that in these sediments most of the deposited nitrogen is returned to the water column as ammonium and nitrate.