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Abstract
Enumeration of populations of nitrate respiring bacteria in estuarine sediments of the River Tay, Scotland, showed that bacteria capable of dissimilating NO− 3 to NH+ 4 predominated over those denitrifying NO− 3 to N2. On the other hand, seasonal data and depth profile studies, using 15NO− 3, showed that denitrification was the principal route of dissimilatory NO− 3 reduction (78–90% of NO− 3 respired), with maximum rates of both processes occurring in the summer. Population densities of both populations of NO− 3 respiring bacteria were highest in the 0-2 cm horizon in Tay estuary mud-flats where maximum rates of NO− 3 respiration were also recorded [28-56 μg N d−1 (g dry wt sediment)−1]. Autotrophic nitrification rates in Tay estuary sediments showed a distinct seasonality, highest rates [0·93 μg N d−1 (g dry wt sediment)−1] occurring during the summer. Nitrification rates declined rapidly with sediment depth and were not detectable below the oxidized zone (3 cm). Population densities of autotrophic NH+ 4 and NO− 2 oxidizing bacteria followed a similar pattern of distribution. Heterotrophic nitrification appears to play an insignificant role in Tay estuary sediments.
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