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Abstract

CH4 oxidation activities from various soils and freshwater sediments were measured at low (less-than-or-equal-to 2 parts per million by volume (ppmv)) and high (greater-than-or-equal-to 1000 ppmv) CH4 mixing ratios. Most of the tested soils acted as sinks for atmospheric CH4. A correlation between the CH4 oxidation activity and the numbers of methanotrophs was only observed at high (1000 ppmv) CH4 mixing ratios. This indicates that the counted methanotrophs were not the bacteria which are oxidizing atmospheric CH4 (less-than-or-equal-to 1.7 ppmv). The CH4 oxidation was due to prokaryotic microorganisms active only under oxic conditions. The CH4 oxidation activity decreased at 02 mixing ratios below 1-3% and was rather insensitive for the variation of 02 at mixing ratios >3%. Undisturbed, stratified soils, and freshwater sediments showed vertical profiles of CH4 oxidation activities with a distinct maximum. Sediments showed an exact correspondence between the number of methanotrophs and the maximum of CH4 oxidation both being localized at the surface sediment layer. The oxic soils showed maxima of CH4 oxidation activities generally located in subsurface layers. The maxima of CH4 oxidation activities were slightly shifted below the maxima of the numbers of methanotrophs indicating that the counted bacteria (incubation under 20% CH4) might not represent the active population which oxidizes atmospheric CH4. Plowed agricultural soils showed no distinct maxima, neither of the CH4 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 CH4 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 CH4 oxidation than smaller particles.