Methane emission from feather moss stands

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH 4 . However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH 4 turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH 4 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 4 emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH 4 h −1 gdw −1 . Temperature and water content positively influenced CH 4 emission. Methanogenic potentials determined under N 2 atmosphere in darkness ranged between 22 and 157 pmol CH 4 h −1 gdw −1 . Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH 4 was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 10 4 and 10 5 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 4 was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH 4 produced was depleted in 13 C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3–62 mM ). The δ 13 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 4 by hydrogenotrophic methanogenesis and causing net emission of CH 4 under ambient conditions, albeit at low rates.