Effect of CH4 concentrations and soil conditions on the induction of CH4 oxidation activity

Four different soils (meadow cambisol, forest luvisol, cultivated cambisol, paddy soil) were incubated under different CH4 mixing ratios (1.7 μl CH4 l−1 to 20% CH4) in air (20% O2, rest N2) and the CH4 oxidation activities as well as the numbers of methanotrophic bacteria were determined. All soils showed an increase of the microbial CH4 oxidation activity and the numbers of methanotrophic bacteria only if incubated at CH4 mixing ratios exceeding about 100–1000 μl CH4 l−1. The induction of the CH4 oxidation activity was strictly O2-dependent and was inhibited by acetylene or autoclaving, demonstrating that CH4 oxidation in the soil was due to methanotrophic bacteria. The induction process was influenced by physico-chemical soil variables such as soil moisture, pH, temperature, NH4+ concentration, Cu2+ concentration and aggregate size. Optimum ranges for the induction process were: soil water content of 20–35% H2O; pH 6.7–8.1; temperatures of 25–35°C; and NH4+ concentrations in the soil water phase of 12–61 mm. Copper concentrations >4.3 mm Cu2+ inhibited the induction of CH4 oxidation activity. Increasing soil aggregate sizes between 50 μm and >2 mm dia resulted in a slight but steady stimulation of the induction. Our results demonstrate that the development of an active methanotrophic population in oxic soils requires not only sufficient CH4 but also soil physico-chemical conditions that are suitable for growth or activation of methanotrophic bacteria.