Nitrosospira, an important ammonium-oxidizing bacterium in fertilized coniferous forest soil

Among ammonium-oxidizing autotrophic nitrifiers only Nitrosospira was found in two pine forest soils fertilized with urea or wood ash in southern Finland. A strain isolated from an ash-treated soil was partially characterized. The cells were spirals, mostly of one to three turns; they were either flagellated or not and pili were found. The strain grew best at 20–27 °C at Po 2 of 0.21 (shortest doubling time, 29 h). K m (O 2 ) at 27 °C was 0.20 mg/L. Activity per cell during exponential growth ranged from 0.0060 to 0.0085 pmol [Formula: see text] and growth yield from 2.53 × 10 6 to 3.60 × 10 6 cells/μmol [Formula: see text]. Pure cultures could not be isolated from urea-fertilized soils. Hyphomicrobium- and seliberia-like bacteria were frequent contaminants of enrichment cultures in these soils. Reasons for the persistence of Nitrosospira in forest soil were discussed.