Methane emission from feather moss stands.

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH . However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH cycling. Therefore, we investigated methanogenic rates and pathways together with the methanogenic microbial community composition in feather moss stands from temperate and boreal forests. Potential rates of CH emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH h gdw . Temperature and water content positively influenced CH emission. Methanogenic potentials determined under N atmosphere in darkness ranged between 22 and 157 pmol CH h gdw . Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 10 and 10 copies per gram dry weight moss of the mcrA gene coding for a subunit of the methyl CoM reductase. Archaeal 16S rRNA and mcrA gene sequences in the moss stands were characteristic for the archaeal families Methanobacteriaceae and Methanosarcinaceae. The potential methanogenic rates were similar in incubations with and without methyl fluoride, indicating that the CH was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH produced was depleted in C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3-62 mM). The δ C of acetate was similar to that of the moss biomass, indicating acetate production by fermentation. Our study showed that the feather moss stands contained active methanogenic microbial communities producing CH by hydrogenotrophic methanogenesis and causing net emission of CH under ambient conditions, albeit at low rates.