Glacier melting promotes methane emission via increased methanogenic activity in the foreland alpine meadow

Annual glacier melting alters hydrothermal conditions of the foreland alpine meadows, and causes significant fluctuations in methane (CH₄) flux. Previously we found that Tibetan glacier foreland alpine meadow shifts to CH₄ source from sink during the melting season, but the potential mechanisms remain unclear. This study, via combination of in-situ measurement of seasonal CH₄ flux and survey of microbial species that may involve in CH₄ metabolism, explores the causes of glacier melting on CH₄ flux in a glacier foreland alpine meadow on Tibetan Plateau. We determined a pronounced CH₄ emission (13.95 μg·m^-2·h^-1) in August (melting season) but CH₄ uptake in June (-3.76 μg·m^-2·h^-1) and October (-17.77 μg·m^-2·h^-1), and 1.4-fold higher soil moisture in August than the other two months. This showed a direct correlation of CH₄ flux with glacier melting increased soil water. Additionally, glacier melting caused more CH₄ fluxes increase in hollows than in hummocks. Amplicon sequencing determined 126-fold higher abundance of mcrA, the methanogenic marker gene, in August than in June and October, and a higher relative abundance of a fungal phylum Mortierellomycota and syntrophic bacteria that convert the fatty acids, the degradation intermediates of organic complexes to CO₂ and acetate, the methanogenic substrates like in August. However, no seasonal variation of pmoA, the marker gene of aerobic methanotrophs, was observed. It appears that glacier melting promotes the CH₄ producing but not the consuming microorganisms, thus leading to increased CH₄ emission. The findings of this work indicate that global warming resulted glacier melting would increase global CH₄ emissions, and in turn worsens global warming, so an alarming positive feedback loop.