Plant belowground traits indicate increased plant-mediated methane transport along a peatland permafrost thaw gradient

Permafrost thaw alters methane (CH 4 ) fluxes from subarctic peatland ecosystems. Collapsing permafrost palsas change hydrology, interstitial oxygen availability, and vegetation composition, and each of these factors contribute to net CH 4 flux by influencing CH 4 production, consumption and transport. Changes in plant-mediated CH 4 fluxes have mostly been estimated using aboveground characteristics, such as biomass and leaf area, leaving belowground parts (roots and rhizomes) understudied despite their direct contact to depth-dependent CH 4 flux processes. Here, we explored the potential of using root and rhizome traits as proxies for plant-mediated CH 4 cycling along a peatland permafrost thaw gradient in subarctic Sweden. We investigated changes in root and rhizome biomass, surface area (SA), diameter, tissue density (TD), and specific root length (SRL) along the permafrost thaw gradient, and how these traits relate to early-, middle-, peak- and season median CH 4 fluxes. We utilized chamber CH 4 flux and pore water CH 4 concentration and isotopic measurements during the productive season. Shrub SRL, diameter and isotopic data suggested increased plant-mediated carbon substrates available for acetoclastic methanogenesis across the thaw gradient. Root TD, proxy for root porosity, decreased with thaw and had negative correlations with CH 4 fluxes throughout the season. Simultaneously, herbaceous rhizome SA-CH 4 flux correlations were positive and pore water CH 4 concentrations were lowest in the fully thawed stage. These results indicated increasing herbaceous plant-mediated transport of acetoclastically-produced CH 4 with thaw. Our study demonstrates that integrating plant belowground traits with environmental and biogeochemical data can help improve CH 4 flux predictions in thawing landscapes and revealed key mechanistic insights regarding the interplay between substrate availability for methanogenesis and gas transport.