Nitrous Oxide Production and Methane Oxidation by Different Ammonia-Oxidizing Bacteria

Abstract

ABSTRACT Ammonia-oxidizing bacteria (AOB) are thought to contribute significantly to N 2 O production and methane oxidation in soils. Most of our knowledge derives from experiments with Nitrosomonas europaea , which appears to be of minor importance in most soils compared to Nitrosospira spp. We have conducted a comparative study of levels of aerobic N 2 O production in six phylogenetically different Nitrosospira strains newly isolated from soils and in two N. europaea and Nitrosospira multiformis type strains. The fraction of oxidized ammonium released as N 2 O during aerobic growth was remarkably constant (0.07 to 0.1%) for all the Nitrosospira strains, irrespective of the substrate supply (urea versus ammonium), the pH, or substrate limitation. N. europaea and Nitrosospira multiformis released similar fractions of N 2 O when they were supplied with ample amounts of substrates, but the fractions rose sharply (to 1 to 5%) when they were restricted by a low pH or substrate limitation. Phosphate buffer (versus HEPES) doubled the N 2 O release for all types of AOB. No detectable oxidation of atmospheric methane was detected. Calculations based on detection limits as well as data in the literature on CH 4 oxidation by AOB bacteria prove that none of the tested strains contribute significantly to the oxidation of atmospheric CH 4 in soils.