Autotrophic ammonium-oxidising bacteria in Swedish mor humus

Ammonium-oxidising bacteria were enumerated (MPN method) and characterised in soils from limed and unlimed plots of four separate coniferous forest experiments in Sweden. In the unlimed control sites (pH 4.2), the number of ammonium-oxidising bacteria were below the detection limit (5×102 g−1 soil dry weight). In the limed soils (6 y prior to our sampling) (pH 5–6), the numbers ranged from 103 to 105 g−1, and were found to be correlated with the measured soil pH. Urea appeared to have a positive effect on the number of ammonium-oxidising bacteria. Nitrosospira appeared to dominate in all limed soils. e oxidation rates were measured in one isolated Nitrosospira culture, and was found to agree reasonably well with the methane oxidation measured for other ammonium oxidisers. The theoretical methane oxidation rate by ammonium-oxidising bacteria in the soils was less than 0.1% of the actual methane oxidation rate measured. Thus, the study offers no support to the hypothesis that ammonium-oxidising bacteria contribute to the methane oxidation in soils.