Nitrification and dissimilatory ammonium production and their effects on nitrogen flux over the sediment-water interface in bioturbated coastal sediments

Nitrification and dissimilatory reduction of nitrate to ammonium were measured concomitantly with nitrogen release from marine coastal sediment samples from 2 fjord sill stations. Dissimilatory ammonium production (DAP) and nitrification were measured using core injections of "NO; and H1"CO;, respectively. DAP was detected in all segments of the cores by tracing "NH: evolved from the added "NO;. 15NH: recovery increased with increasing core depth, ranging from 1.6 to 10 % for Stn H and from 0.3 to 2.9 % for Stn L. Nitrification activity at Stn L was in the order of 10 nmol cm-3 h-' in the upper 2 cm but was not demonstrated in deeper strata. Consequently, DAP was most pronounced in the upper few centimeters although it was anticipated that more than 97 % of the nitrate produced was denitnfied. Mean fluxes of ammonlum out of the sediment were 24 and 12 pm01 m-2 h-' for Stns H and L, respectively, and corresponding nitrate fluxes were 2 and -24 pm01 m-2 h-'.