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

Abstract

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 NO 3 - and rates of nitrification, denitrification, and O 2 consumption as the O 2 concentration in the overlying water was increased from 10 to 600 μM. 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 NO 3 - diffusing from the overlying water ( D w ) and of NO 3 - 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 NO 3 - 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 NH 4 + diffusing from the lower sediment layers and to remove much of the NO 3 - produced. In addition to the flux of NH 4 + 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 (∼10 μM), was necessary to reproduce the real data. Spatial separation of the two processes (no toleration by denitrification of O 2 ) resulted in too high NO 3 - concentrations and too low rates of denitrification.