Localization of atmospheric H2-oxidizing soil hydrogenases in different particle fractions of soil

Low atmospheric H₂ concentrations (0.55 ppmv) are oxidized in soils by a high-affinity activity with typical characteristics of an abiontic soil enzyme. This activity was measured in a meadow cambisol and a forest cambisol. In both soils, the maximum activity was reached at a soil moisture of about 20% water-holding capacity, and was localized in the top A_h horizon. The soils were fractionated by dry sieving and wet filtration into nine different particle-size fractions, ranging from 3 to 2000 µm in size. H₂ oxidation was measured by three different assays and was compared to the ATP content and microscopic counts of bacteria in the same fractions. In the meadow soil, the specific activities of H₂ oxidation increased with the particle size (maximum at 200–500 µm), whereas ATP and bacterial counts showed no trend. In the forest soil, the specific activities of H₂ oxidation increased with the particle size up to 50–100 µm, and then decreased again. ATP and bacterial counts, however, showed the opposite trend, i.e., decreased with an increasing particle size. Thus the H₂-oxidizing activity was not correlated with characteristic microbial biomass parameters. Although significant percentage (29–64%) of randomly isolated heterotrophic bacteria was able to oxidize H₂, this activity was too small to account for the H₂ oxidation in the soil. In both soils, most of the activity present was found in particles of 100–500 µm in size. The recovery shifted to smaller size fractions when larger soil aggregates were broken up by wet instead of dry sieving. Attempts to extract the H₂-oxidizing activity from the soil particles were unsuccessful.