Simultaneous occurrence of denitrification and nitrate ammonification in sediments of the French Mediterranean Coast

Dissimilatory nitrate reductions in coastal marine sediment of Carteau Cove (French Mediterranean Coast) were studied between April 1993 and July 1994. Simultaneous determination of denitrification and dissimilatory nitrate reduction to ammonium was achieved by using a combination of acetylene blockage and N-15 techniques. After short incubations (maximum 5 h), a part of N-15 labelled nitrate added to the sediment was recovered as ammonium without incorporation in organic matter. The result indicate that a fraction of nitrate was reduced to ammonium by a dissimilatory mechanism instead of denitrifying. Denitrifying and nitrate ammonifying activities ranged from 0 to 19.8 mu mol l(-1) d(-1) and from 2.3 to 83.2 mu mol l(-1) d(-1), respectively. Denitrification rates were highest in early spring whereas nitrate ammonification were highest in fall. The recovery of nitrate reduced as N2O-N plus ammonium was between 40 and 100%, the highest nitrogen losses were recorded in July. Depending on the station and time of year denitrification accounted for between 0 and 43% of the total nitrate reduction whereas dissimilatory nitrate reduction to ammonium (DNRA) accounted for between 18 and 100%. The reduction rate data suggest that the pathway of nitrate reduction to ammonium may be important in coastal sediments.