DENITRIFICATION RATES AS MEASURED BY THE ISOTOPE PAIRING METHOD AND BY THE ACETYLENE INHIBITION TECHNIQUE IN CONTINENTAL-SHELF SEDIMENTS OFTHE NORTH-SEA

Denitrification rates in continental shelf sediments of the North Sea were measured by the simultaneous application of the acetylene inhibit ion technique (AIT) and the isotope pairing method (IPM). Nitrificatio n was the predominant source of nitrate in these sediments. Both techn iques were able to detect coupled nitrification-denitrification activi ty. Denitrification rates measured by the AIT were only measurable dur ing the first 60 min of the incubation. Rates obtained by the AIT in t his period ranged between 5.4 and 7.3 mu mol N m(-2)h(-1). Continued i ncubation of acetylene amended sediment cores resulted in decreasing d enitrification rates, suggesting that accumulated nitrous oxide was co nsumed despite the presence of acetylene. Denitrification rates obtain ed 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) re lative to double-labelled ((NN)-N-15-N-15) dinitrogen indicated a homo geneous mixing of both isotopic nitrate species through the denitrific ation zone. The present results suggest that previously published data severely underestimated denitrification rates in North Sea continenta l shelf sediments. Concomitant measured fluxes of nitrate and ammonium across the sediment-water interface reveal that in these sediments mo st of the deposited nitrogen is returned to the water column as ammoni um and nitrate.