Assessment of two techniques for measuring nitrification rates in aerated slurries of seagrass-bed sediments

Two methods were assessed to measure rates of nitrification in aerated slurries of seagras-bed sediments; (1) monitoring ammonium concentrations in samples with and without nitrapyrin ((2-chloro-6-(trichloromethyl) pyridine) treatment and (2) monitoring the accumulation of nitrate plus nitrite. Nitrifying bacteria were inhibited completely after 14 days exposure to nitrapyrin at 5 mg⁻¹, but the effect was poor over shorter periods, probably owing to limited diffusion of the inhibitor. Hence, nitrapyrin was not a satisfactory inhibitor to use in these sediments. Nitrification rates could not be measured by monitoring the concentrations of nitrate plus nitrite, because nitrate reduction occurred simultaneously with nitrification, even though the slurries contained from 3.5 to 8.9 mg O₂ l⁻¹.