Methane oxidation potential of boreal landfill cover materials: The governing factors and enhancement by nutrient manipulation
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Methanotrophs inhabiting landfill covers are in a crucial role in mitigating CH4 emissions, but the characteristics of the cover material or ambient temperature do not always enable the maximal CH4 oxidation potential (MOP). This study aimed at identifying the factors governing MOPs of different materials used for constructing biocovers and other cover structures. We also tested whether the activity of methanotrophs could be enhanced at cold temperature (4 and 12 °C) by improving the nutrient content (NO3-, PO43-, trace elements) of the cover material. Compost samples from biocovers designed to support CH4 oxidation were exhibiting the highest MOPs (4.16 µmol CH4 gdw-1 h-1), but also the soil samples collected from other cover structures were oxidising CH4 (0.41 µmol CH4 gdw-1 h-1). The best predictors for the MOPs were the NO3- content and activity of heterotrophic bacteria at 72.8 %, which were higher in the compost samples than in the soil samples. The depletion of NO3- from the landfill cover material limiting the activity of methanotrophs could not be confirmed by the nutrient manipulation assay at 4 °C as the addition of nitrogen decreased the MOPs from 0.090 µmol CH4 gdw-1 h-1 to < 0.085 µmol CH4 gdw-1 h-1. At 12 °C, all nutrient additions reduced the MOPs. The inhibition was believed to result from high ionic concentration caused by nutrient addition. At 4 °C, the addition of trace elements increased the MOPs (> 0.096 µmol CH4 gdw-1 h-1) suggesting that this was attributable to stimulation of the enzymatic activity of the psychrotolerant methanotrophs.