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 i nvestigated nitrification and denitrification in the freshwater sedime nt of Lake Vilhelmsborg, Denmark (K. Jensen, N. P. Sloth, N. Risgaard- Petersen, S. Rysgaard, and N. P. Revsbech, Appl. Environ. Microbiol. 6 0:2094-2100, 1994). The model output faithfully represented the profil es of O-2 and NO3- and rates of nitrification, denitrification, and O- 2 consumption as the O-2 concentration in the overlying water was incr eased from 10 to 600 mu M. The model also accurately predicted the res ponse, 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 ov erlying water (D-w) and of NO3- generated by nitrification within the sediment (D-n) corresponded to the experimental rates as the O-2 conce ntration in the overlying water was altered. The predicted D-w and D-n rates, as NO3- concentration in the overlying water was changed, clos ely resembled those determined experimentally. The model was composed of 41 layers 0.1 mm thick, of which 3 represented the diffusive bounda ry layer in the water. Large first order rate constants for nitrificat ion and denitrification were required to completely oxidize all NH4+ d iffusing from the lower sediment layers and to remove much of the NO3- produced. In addition to the flux of NH4+ from below, the model requi red a flux of an electron donor, possibly methane. Close coupling betw een nitrification and denitrification, achieved by allowing denitrific ation to tolerate some O-2 (similar to 10 mu M), was necessary to repr oduce the real data. Spatial separation of the two processes (no toler ation by denitrification of O-2) resulted in too high NO3- concentrati ons and too low rates of denitrification.