Soils contain two different activities for oxidation of hydrogen

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