• Hydrogen-oxidizing bacteria;
  • Soil enzymes;
  • Atmospheric H2;
  • H2 oxidation kinetics;
  • H2 threshold

Abstract Hydrogen oxidation rates were measured in a neutral compost soil and an acidic sandy loam at H2 mixing ratios of 0.01 to 5000 ppmv. The kinetics were biphasic showing two different Km values for H2, one at about 10–40 nM dissolved H2, the other at about 1.2–1.4 μM H2. The low-Km activity was less sensitive to chloroform fumigation than the high-Km activity. If sterile soil was amended with Paracoccus denitrificans or a H2-oxidizing strain isolated from compost soil, it exhibited only a high-Km (0.7–0.9 μM) activity. It also failed to utilize H2 mixing ratios below a threshold of 1.6–3.0 ppmv H2 (160–300 mPa). A similar result was obtained when fresh soil samples were suspended in water, and H2 oxidation was determined from the decrease of dissolved H2. However, H2 was again utilized to mixing ratios lower than 0.05 ppmv, if the supernatant of the soil suspension or the settled soil particles were dried onto sterile soil or purified quarz sand. Obviously, soils contain two different activities for oxidation of H2: (1) a high-Km, high-threshold activity which apparently is due to aerobic H2-oxidizing bacteria, and (2) a low-Km, low-threshold activity whose origin is unknown but presumably is due to soil enzymes.