Effects of ammonium on the activity and community of methanotrophs in landfill biocover soils

The influence of NH₄⁺ on microbial CH₄ oxidation is still poorly understood in landfill cover soils. In this study, effects of NH₄⁺ addition on the activity and community structure of methanotrophs were investigated in waste biocover soil (WBS) treated by a series of NH₄⁺-N contents (0, 100, 300, 600 and 1200 mg kg⁻¹). The results showed that the addition of NH₄⁺-N ranging from 100 to 300 mg kg⁻¹ could stimulate CH₄ oxidation in the WBS samples at the first stage of activity, while the addition of an NH₄⁺-N content of 600 mg kg⁻¹ had an inhibitory effect on CH₄ oxidation in the first 4 days. The decrease of CH₄ oxidation rate observed in the last stage of activity could be caused by nitrogen limitation and/or exopolymeric substance accumulation. Type I methanotrophs Methylocaldum and Methylobacter, and type II methanotrophs (Methylocystis and Methylosinus) were abundant in the WBS samples. Of these, Methylocaldum was the main methanotroph in the original WBS. With incubation, a higher abundance of Methylobacter was observed in the treatments with NH₄⁺-N contents greater than 300 mg kg⁻¹, which suggested that NH₄⁺-N addition might lead to the dominance of Methylobacter in the WBS samples. Compared to type I methanotrophs, the abundance of type II methanotrophs Methylocystis and/or Methylosinus was lower in the original WBS sample. An increase in the abundance of Methylocystis and/or Methylosinus occurred in the last stage of activity, and was likely due to a nitrogen limitation condition. Redundancy analysis showed that NH₄⁺-N and the C/N ratio had a significant influence on the methanotrophic community in the WBS sample.