Response of CH₄ oxidation and methanotrophic diversity to NH₄ ⁺ and CH₄ mixing ratios

Methane oxidising activity and community structure of 11, specifically targeted, methanotrophic species have been examined in an arable soil. Soils were sampled from three different field plots, receiving no fertilisation (C), compost (G) and mineral fertiliser (M), respectively. Incubation experiments were carried out with and without pre-incubation at elevated CH₄ mixing ratios (100 ml CH₄ l-¹) and with and without ammonium (100 mg N kg-¹) pre-incubation. Four months after fertilisation, plots C, G and M did not show significant differences in physicochemical properties and CH₄ oxidising activity. The total number of methanotrophs (determined as the sum the 11 specifically targeted methanotrophs) in the fresh soils was 17.0x10⁶, 13.7x10⁶ and 15.5x10⁶ cells g-¹ for treatment C, G and M, respectively. This corresponded to 0.11 to 0.32% of the total bacterial number. The CH₄ oxidising activity increased 10⁵-fold (20-26 mg CH₄ g-¹ h-¹), the total number of methanotrophs doubled (28-76x10⁶ cells g-¹) and the methanotrophic diversity markedly increased in treatments with a pre-incubation at elevated CH₄ concentrations. In all soils and treatments, type II methanotrophs (62-91%) outnumbered type I methanotrophs (9-38%). Methylocystis and Methylosinus species were always most abundant. After pre-incubation with ammonium, CH₄ oxidation was completely inhibited; however, no change in the methanotrophic community structure could be detected.