Kinetics of CH4 oxidation in oxic soils exposed to ambient air or high CH4 mixing ratios

Abstract

The kinetic parameters of CH4 oxidation (Km, Vmax, apparent threshold = Tha) were measured using different oxic soils (cultivated cambisol, forest luvisol, meadow cambisol, paddy soil) both in a fresh state and after 3 weeks preincubation under high CH4 mixing ratios (20%). The preincubation resulted in an increase of the most probable number of methanotrophic bacteria. In fresh soils, CH4 oxidation followed Michaelis-Menten kinetics with a low Km (30–51 nM CH4), low Vmax (0.7–3.6 nmol CH4 h−1g−1dw soil), and low Tha (0.2–2.7 ppmv CH4). In preincubated soils, CH4 oxidation exhibited biphasic kinetics in which two different CH4 saturation curves were apparently superimposed on each other. Eadie-Hofstee plots of the data showed two activities with different kinetic parameters: a high-affinity activity with low Km (13–470 nM CH4), low Vmax (2.1–150.0 nmol CH4 h−1g−1dw) and low Tha (0.3–4.1 ppmv CH4) being similar to the kinetic parameters in fresh soils; and a low-affinity activity with high Km (1740–27 900 nM CH4), high Vmax (270–3 690 nmol CH4 h−1g−1dw) and high Tha (11–45 ppmv CH4) being similar to the kinetic parameters known from methanotrophic bacteria. The low-affinity activity was also observed in a soil over a deep natural gas source which was permanently exposed to high CH4 mizing ratios (>5% CH4). Bacteria culturable as methanotrophs are probably responsible for the low-affinity activity which is typical for the soils exposed to high CH4 mixing ratios. However, the bacteria responsible for the high-affinity activity are still unknown. This activity is typical for the soils exposed to only ambient CH4 mixing ratios. Both high- and low-affinity activities were inhibited by autoclaving and by acetylene.