Kinetics of CH 4 oxidation in oxic soils exposed to ambient air or high CH 4 mixing ratios

Abstract

The kinetic parameters of CH 4 oxidation ( K m, V max, apparent threshold = Th a) 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 CH 4 mixing ratios (20%). The preincubation resulted in an increase of the most probable number of methanotrophic bacteria. In fresh soils, CH 4 oxidation followed Michaelis-Menten kinetics with a low K m (30–51 nM CH 4), low V max (0.7–3.6 nmol CH 4 h −1g −1dw soil), and low Th a (0.2–2.7 ppmv CH 4). In preincubated soils, CH 4 oxidation exhibited biphasic kinetics in which two different CH 4 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 K m (13–470 nM CH 4), low V max (2.1–150.0 nmol CH 4 h −1g −1dw) and low Th a (0.3–4.1 ppmv CH 4) being similar to the kinetic parameters in fresh soils; and a low-affinity activity with high K m (1740–27 900 nM CH 4), high V max (270–3 690 nmol CH 4 h −1g −1dw) and high Th a (11–45 ppmv CH 4) 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 CH 4 mizing ratios (>5% CH 4). Bacteria culturable as methanotrophs are probably responsible for the low-affinity activity which is typical for the soils exposed to high CH 4 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 CH 4 mixing ratios. Both high- and low-affinity activities were inhibited by autoclaving and by acetylene.