KINETICS OF METHANE OXIDATION IN OXIC SOILS

We investigated the kinetics of CH4 oxidation in three different soils (cultivated cambisol, forest cambisol, meadow cambisol) under oxic conditions. CH4 oxidation was apparently due to aerobic microorganisms which were inhibited by autoclaving, and followed a Michaelis-Menten kinetic with a half saturation constant (Km) of about 20-45 ppmv and a maximum oxidation rate (Vmax) of about 0.6-3.6 nmol CH4 h-1 g-1 dry weight soil. The determined Km values were much smaller than those known from previous experiments with soil or methanotrophic bacteria. Extrapolation of the linear part of the Michaelis-Menten kinetic (range of first- order kinetics) to zero CH4 oxidation usually resulted in threshold mixing ratios (Tha) of about 0. 2-0.3 ppmv CH4, i. e. lower than the atmospheric mixing ratio. CH4 was oxidized even below these threshold mixing ratios, but at a rate that was much lower than expected from the first-order rate constant of CH4 oxidation. These results show that oxidation of atmospheric CH4 in soil is not a simple first-order reaction emphasizing careful interpretation of kinetic data with respect to field conditions.