Methane oxidation activity in various soils and freshwater sediments: Occurrence, characteristics, vertical profiles, and distribution on grain size fractions

Abstract

CH 4 oxidation activities from various soils and freshwater sediments were measured at low (≤2 parts per million by volume (ppmv)) and high (≥1000 ppmv) CH 4 mixing ratios. Most of the tested soils acted as sinks for atmospheric CH 4 . A correlation between the CH 4 oxidation activity and the numbers of methanotrophs was only observed at high (1000 ppmv) CH 4 mixing ratios. This indicates that the counted methanotrophs were not the bacteria which are oxidizing atmospheric CH 4 (≤1.7 ppmv). The CH 4 oxidation was due to prokaryotic microorganisms active only under oxic conditions. The CH 4 oxidation activity decreased at O 2 mixing ratios below 1–3% and was rather insensitive for the variation of O 2 at mixing ratios >3%. Undisturbed, stratified soils, and freshwater sediments showed vertical profiles of CH 4 oxidation activities with a distinct maximum. Sediments showed an exact correspondence between the number of methanotrophs and the maximum of CH 4 oxidation both being localized at the surface sediment layer. The oxic soils showed maxima of CH 4 oxidation activities generally located in subsurface layers. The maxima of CH 4 oxidation activities were slightly shifted below the maxima of the numbers of methanotrophs indicating that the counted bacteria (incubation under 20% CH 4 ) might not represent the active population which oxidizes atmospheric CH 4 . Plowed, agricultural soils showed no distinct maxima, neither of the CH 4 oxidation activities nor of the numbers of methanotrophs. The grain size fractionation by centrifugation or wet sieving of slurries of two forest soils showed that the bulk (80–96%) of the CH 4 oxidizing activity was attached to the smaller mineral fractions (clay, silt, fine sand) of these soils. Within the mineral fractions, greater particles had higher specific activities of CH 4 oxidation than smaller particles.