SIMULATION-MODEL OF THE COUPLING BETWEEN NITRIFICATION AND DENITRIFICATION IN A FRESH-WATER SEDIMENT

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 O-2 and NO3- and rates of nitrification, denitrification, and O-2 consumption as the O-2 concentration in the overlying water was increased from 10 to 600 mu M. The model also accurately predicted the response, to increasing O-2 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 (D-w) and of NO3- generated by nitrification within the sediment (D-n) corresponded to the experimental rates as the O-2 concentration in the overlying water was altered. The predicted D-w and D-n 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 O-2 (similar to 10 mu M), was necessary to reproduce the real data. Spatial separation of the two processes (no toleration by denitrification of O-2) resulted in too high NO3- concentrations and too low rates of denitrification.