KINETICS OF THE DENITRIFICATION PROCESS IN A SOIL UNDER PERMANENT PASTURE

The dynamics of mineral N species involved in the processes of denitrification were studied in laboratory systems using a soil from permanent pasture. Experiments were carried out to test a mechanistic model linking C mineralization and denitrification. This involved the use of competitive Michaelis-Menten type enzyme kinetics to simulate the reduction of NO3- and the formation of NO2-, N2O and N2. The rate of reduction of each N oxide was assumed to be dependent upon its concentration and on a weighting factor for competition between electron acceptors. Concentrations of 20, 200 and 2000 mg NO3--N kg-1 were applied to a soil slurry and anaerobically incubated for 5 days at 25-degrees-C. The production of CO2, N2O and N2, and concentrations of NO3- and NO2- were monitored daily. The ratio between CO2 produced and nitrate reduced was 0.7; much lower than the value expected if glucose was the only C substrate and all nitrate was reduced to N2. The nitrite concentration remained below 0.5 mg NO2--N kg-1 in each treatment indicating that all NO2- formed, was very rapidly reduced to N2O. The reduction of N2O was much slower: no N2 was formed within the first day when 200 mg NO3--N kg-1 was applied, and the rate of N2O production remained higher than the rate of N2O reduction for the first 4 days of the incubation. Only 75% of the N2O formed was reduced within 1 day when 20 mg NO3--N kg-1 was added. The dynamics of the different forms of nitrogen generated in the denitrification process at different concentrations of available nitrate could be simulated using a weighting factor or an affinity of 1 for NO3-, around 1000 for NO2- and 0.75 for N2O. It was not necessary to include inhibitory effects of nitrate or nitrite in the model to simulate the observed patterns of mineral N dynamics. It was concluded from this study that low affinity for N2O as compared to the high affinity for NO2-could lead to high losses of nitrous oxide in the field even if only low quantities of NO3- were available.