Kinetics of nitrification of various amounts of ammonium added to soils

Nitrification rates in soils of Hokkaido upland fields were investigated in the laboratory. The soils, with various concentrations of ammonium sulfate added, were incubated at 20°C. The temporal changes in the concentrations of nitrate and ammonium in the soils were analyzed by mathematical models. Where the amount of ammonium added was <10μg Ng^-1, changes with time in the concentrations of ammonium and nitrate in the Brown Andosols could be described by the model that includes Michaelis-Menten kinetics on the assumption that the growth of the nitrifiers was negligible. The half-saturation constant and maximum velocity of nitrification were calculated to be 0.86 μg Ng^-1 and 3.3 μg N g^-1 day^-1, respectively. Where the amount of ammonium added was 200 μg N g^-1 and 400 μg N g^-1, changes with time in the concentration of the nitrate in the Brown Andosols and Ordinary Andosols were described well by the model that includes the growth of the nitrifiers. The initial velocities of the nitrification rate were calculated when the initial concentration of ammonium in soils was ≫300μgNg^-1. The higher the initial concentration of ammonium, the lower were the initial velocities of nitrification.