Denitrification in marine sediment: measurement of capacity and estimate of in situ rate.

The acetylene block technique was used to assay denitrification in undisturbed sediment cores of an intertidal mud flat. Nitrogen loss due to this process was estimated at 1 to 41 kg of N/hectare (ha) per year. Anaerobic nitrate-saturated slurry of the same sediment had a denitrification capacity of 1,026 kg of N/ha per year. The acetylene block technique failed at low nitrate concentrations, so that denitrification at average in situ nitrate concentrations could not be determined. Denitrification followed zero-order kinetics at nitrate concentrations high enough to allow successful blockage of N(2)O reduction. Thus, an estimate of in situ rates based on kinetic parameters and in situ nitrate concentrations was impossible. No denitrification was observed in a slurry of the top 1.5 cm under aerated conditions and nitrate saturation. In undisturbed sediment, significant denitrification occurred in few discrete sites within a matrix of undetectable or low activity. Despite numerous errors contributing to the uncertainty of the estimate of in situ rates, the result obtained by this method was considered more valuable than the determination of denitrification capacities. Methods which include severe changes of physical and chemical parameters may frequently lead to overestimates of denitrification rates.