Graminoids Increase Greenhouse Gas Emissions From Thawed Permafrost at the End of the Growing Season

Amplified Arctic warming can induce strong ecosystem changes with adverse climate feedbacks through greenhouse gas (GHG) release. Shifting plant species and traits with permafrost thaw may contribute to the permafrost carbon feedback. How vegetation dynamics in thawing permafrost systems affect GHG release and how this varies with season, plant species, and soil conditions is poorly understood. Here, we assessed GHG emissions, redox potentials, and geochemical signatures as well as the carbon input in the form of root exudation along a vegetation density gradient and a permafrost thaw gradient over a growing season in Stordalen mire, Sweden. Ecosystem respiration and CH4 emissions increased along the thaw gradient from bog to fen, possibly due to high graminoid root carbon release rates into an anoxic soil, fuelling fast organic matter oxidation and lowering redox potentials to enhance methanogenesis. CH4 emissions increased seven-fold with increasing graminoid cover compared to non-vascular plant controls in the thawed soil. Plants may mediate CH4 transport, which was responsible for 80% of the graminoid-induced increase in CH4 emissions in the bog environment. In the fen environment, graminoid root carbon release stimulated CH4 formation, which dominated by contributing 70% of the graminoid-induced increase. Overall, photosynthesis-related CO2 fixation was substantial in the early and peak growing season, but when expressed as CO2 equivalents, CH4 release offset this uptake, resulting in net positive radiative forcings from graminoid-vegetated thawed soils throughout the growing season. Graminoids increased the net CO2-equivalent flux up to 8.9-fold compared to non-vascular plant locations with the strongest forcing toward late season in graminoid-vegetated fens. Our study showcases how fine-scaled, plant-mediated processes differently contribute to GHG emissions across a thawed bog and fen soil and how the time of growing season can overprint these effects to determine whether the system is a net GHG source or sink.