An agricultural soil was treated with dairy waste compost or ammonium sulfate fertilizers as N-sources and silage corn was planted for 5 consecutive years. At the end of this time the kinetics of nitrification were determined in laboratory experiments for soil samples from the field plots. A range of NH4+ concentrations from 0 mM to 20 mM NH4+ was added to soil from the three soil treatments (control no N addition, ammonium sulfate (200 kg N/ha) and dairy waste compost (high rate equivalent to approximately 200 kg N/ha available N). A shaken slurry assay was sampled over a 24h period and concentrations of substrate (ammonium) and product (nitrite+nitrate) were measured. Haldane kinetics incorporates a term for substrate inhibition of the transformation at high substrate concentrations (V = Vmax S / (Km + S + S2 / Ki). Overall, the Haldane model was a better fit to the experimental data indicating that substrate inhibition may be significant under high ammonium concentrations that may be attained immediately after fertilization or animal waste application. The maximum rate of nitrification (Vmax) was significantly different for the soil treatments with the ammonium sulfate and compost treated soils exhibiting higher Vmax values of 1.74 and 1.50 mm kg-1 soil day–1 versus the control at 0.98 mM kg-1 soil day–1. The Km and Ki values were not significantly different for the three soils and had overall average values of 0.02 mM and 27 mM respectively. Our results suggest that the different N-sources resulted in changes in the nitrifier community related to population size as reflected in Vmax while the communities did not have significantly different enzyme affinities or sensitivity to ammonium. The Km and Ki values are comparable to those of pure cultures of Nitrosomonas oligotropha but much lower than those found for other cultured ammonium-oxidizing bacteria.
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