Nitrous oxide production and methane oxidation by different ammonia-oxidizing bacteria

Ammonia-oxidizing bacteria (AOB) are thought to contribute significantly to N2O production and methane oxidation in soils. Most of our knowledge deriv es from experiments,vith Nitrosamonas europaea, which appears to be of mino r importance in most soils compared to Nitrosospira spp. We have conducted a comparative study of levels of aerobic N2O production in six phylogenetic ally different Nitrosospira strains newly isolated from soils and in two N. enropaea and Nitrosospira multiformis type strains. The fraction of oxidiz ed ammonium released as N2O during aerobic growth was remarkably constant ( 0.07 to 0.1%) for all the Nitrosospira strains, irrespective of the substra te supply (urea versus ammonium), the pH, or substrate limitation. N. europ aea and Nitrosospira multiformis released similar fractions of N2O when the y were supplied with ample amounts of substrates, but the fractions rose sh arply (to 1 to 5%) when they were restricted by a low pH or substrate limit ation. Phosphate buffer (versus HEPES) doubled the N2O release for all type s of AOB. No detectable oxidation of atmospheric methane,vas detected. Calc ulations based on detection limits as well as data in the literature on CH4 oxidation by AOB bacteria prove that none of the tested strains contribute significantly to the oxidation of atmospheric CH4 in soils.