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. Vmax 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
Km may reflect differences in AOB community composition; in this
study we did not find significant differences in Km. 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.