Nitrate enhancement of nitrification depth in sediment/water microcosms |
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| Authors: | James G Cooke Robert E White |
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| Institution: | (1) Soil Science Laboratory, University of Oxford, Oxford, United Kingdom;(2) Present address: Water Quality Centre, Ministry of Works & Development, Hamilton, New Zealand;(3) Present address: Dept. of Soil Science, Massey University, Palmerston North, New Zealand |
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| Abstract: | Streams draining agricultural catchments in loward England commonly have high nitrate concentrations. Laboratory microcosms
were used to study nitrogen transformations in the sediment from such a stream. Solutions of either 0.5 mmol dm−3 calcium nitrate or calcium chloride (control) were passed once only over the sediment surface. After 5 days incubation, Eh
in the nitrate treatment stabilized at approximately 500 mV at 5 mm below the water/sediment interface, and 285 mV at 10 mmn,
whereas in the chloride treatment Eh was −200 mV to −250 mV at all measured depths (5 mm and below). Inorganic N mesurements
of the seidment profiles showed that much more ammonium-N was being lost from the nitrate-treated sediment that the control.
It was hypothesized that, when nitrate was present in the streamwater, the combined processed of nitrate diffusion and denitrification
progressively consumed organic matter, thereby allowing dissolved oxygen to diffuse further into the sediment. A consequence
of these processes would be a greater depth of nitrification and hence more ammounium loss.
The plausibility of this hypothesis was evaluated by constructing a deterministic model which simulated the nitrate and ammonium
profiles from the processes of diffusion, denitrification, nitrification, and ammonification. Kinetic parameters and diffusion
impedance factor were determined from independent experiments. This calibrated model provided reasonable simulations of experimental
data obtained from an additional experiment, which also showed that peaks in denitrification and nitrification activity were
offset from each other by 2–3 mm, and that they migrated down the profile with increasing incubation time. |
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