The effect of nitrate in stream water on the relationship between gentrification and nitrification in a stream‐sediment microcosm

Abstract

SUMMARY 1. Microcosm experiments were carried out to simulate, in the laboratory, the conditions occurring at the water‐sediment interface of a stream draining agricultural land. Constant boundary conditions were attained by passing synthetic 'stream water', saturated with dissolved oxygen and containing 1 mmol NO3−N dm−3 (or 1 mmol Cl− dm−3, control), once only over the sediment surface.2. Measurements were made of inorganic‐N (nitrate, nitrite, ammonium), redox potential, potential denitrification and nitrification activities, and readily mineralizable carbon sediment profiles at three incubation times up to 24 days. The peaks in denitrification and nitrification activity moved down the profile with time in the nitrate‐treated sediment, but stayed relatively stationary in the control treatment. Although the zone of nitrification was restricted to the top 2–3 mm of sediment in the control treatment, high fluxes of both dissolved oxygen and NH4−N maintained a high nitrifier activity within this zone for the duration of the experiment.3. Increases in denitrifier activity immediately below the nitrifier activity peak indicated that a coupled nitrification‐denitrification sequence was operating in both the control and nitrate‐treated sediment. The greater depth of nitrification when nitrate was present in the ‘stream water’ was attributed to a feedback mechanism in which enhanced denitrification in the sediment reduced the local demand for oxygen and permitted dissolved oxygen to diffuse further into the sediment. The progressively greater depth to which oxygen penetrated caused the contiguous peaks of potential nitrifier and denitrifier enzyme activity to migrate farther from the interface. However, diffusion rates of the reactants limited the depth to which these coupled reactions could extend.4. The possible effect of this feedback mechanism on the nitrate status of natural sediment‐stream water systems is briefly discussed.