Simulation Model of the Coupling between Nitrification and Denitrification in a Freshwater Sediment

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A model was constructed to simulate the results of experiments which investigated nitrification and denitrification in the freshwater sediment of Lake Vilhelmsborg, Denmark (K. Jensen, N. P. Sloth, N. Risgaard-Petersen, S. Rysgaard, and N. P. Revsbech, Appl. Environ. Microbiol. 60:2094-2100, 1994). The model output faithfully represented the profiles of O2 and NO3- and rates of nitrification, denitrification, and O2 consumption as the O2 concentration in the overlying water was increased from 10 to 600 μM. The model also accurately predicted the response, to increasing O2 concentrations, of the integrated (micromoles per square meter per hour) rates of nitrification and denitrification. The simulated rates of denitrification of NO3- diffusing from the overlying water (Dw) and of NO3- generated by nitrification within the sediment (Dn) corresponded to the experimental rates as the O2 concentration in the overlying water was altered. The predicted Dw and Dn rates, as NO3- concentration in the overlying water was changed, closely resembled those determined experimentally. The model was composed of 41 layers 0.1 mm thick, of which 3 represented the diffusive boundary layer in the water. Large first-order rate constants for nitrification and denitrification were required to completely oxidize all NH4+ diffusing from the lower sediment layers and to remove much of the NO3- produced. In addition to the flux of NH4+ from below, the model required a flux of an electron donor, possibly methane. Close coupling between nitrification and denitrification, achieved by allowing denitrification to tolerate some O2 (∼10 μM), was necessary to reproduce the real data. Spatial separation of the two processes (no toleration by denitrification of O2) resulted in too high NO3- concentrations and too low rates of denitrification.

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