The objective of this study was to gain insight into the functional diversity of ammonia-oxidizing bacteria (AOB) in agricultural soils treated with dairy-wastes. We wanted to make connections between the diversity of these nitrifying bacteria and the kinetics of the nitrification process. The kinetic characteristics were determined for agricultural soils treated with (NH₄)₂SO₄, dairy compost or no N amendment repeatedly over a five year period. The relationship between NH₄⁺ substrate concentrations (ranging from 0 mM-20 mM NH₄⁺) and nitrification rate was examined in soil-slurry laboratory experiments. The rate data were fit to both the Michaelis-Menten and Haldane kinetic models. All the soils exhibited decreased rates or NH₄⁺inhibition at concentrations higher than 2 mM NH₄⁺. Therefore, the Haldane model fit the data better than the standard Michaelis –Menten model. V_max was higher in the N treated soils versus the no N control and was the only parameter that was significantly different between soil treated differentially. While kinetic parameters such as K_m may reflect differences in AOB community composition; in this study we did not find significant differences in K_m. Therefore, we would not expect to find significant AOB community differences between these soils. This was confirmed by extraction of DNA from the soils and analyses of clone libraries of partial amoA genes. All soil AOB communities were found to be dominated by strains related to Nitrosospira multiformis. The kinetics of soil AOB cultures enriched under low NH₄⁺ concentrations from these same soils are under further examination.