Inhibition of methane consumption in forest soils and pure cultures of methanotrophs by aqueous forest soil extracts

Aqueous extracts of forest soils inhibited atmospheric methane consumption by boreal and temperate forest soils by up to 100%. Extracts from the upper soil layers (0–5 cm) were generally inhibitory, while those from deeper in the soil column (5–12 cm) were not. The inhibitory effect was concentration-dependent and transient, becoming insignificant after as little as 3 d of exposure of soil to the aqueous extracts. Ammonium concentration alone could not explain the inhibitory effect and treatment with activated charcoal suggested involvement of an organic component. Methane oxidation by pure cultures of the methanotrophs Methylosinus trichosporium OB3b and Methylobacter luteus was significantly inhibited by the aqueous extracts of upper, but not lower soil layers. Removal of phenolic compounds by polyvinylpolypyrrolidone alleviated inhibition in the pure cultures but not in a forest soil. The results support the hypothesis that naturally occurring substances may prevent methane consumption in the upper layers of forest soils.