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

Permafrost thaw in subarctic peatlands alters ecosystem methane (CH₄) fluxes. Collapsing permafrost peat plateaus (palsas) change soil hydrology, oxygen availability, and vegetation composition, and each of these factors contribute to net CH₄ flux by influencing CH₄ production, consumption and transport. However, changes in plant-mediated CH₄ fluxes have mostly been estimated with aboveground characteristics, such as biomass and leaf area, leaving belowground parts (roots and rhizomes) understudied despite their direct contact to depth-dependent CH₄ flux processes. Here, we explored the potential of using root and rhizome traits as proxies for plant-mediated CH₄ cycling along a peatland permafrost thaw gradient in subarctic Sweden. We investigated changes in plant belowground traits along the thaw gradient and the relationships between root and rhizome biomass, surface area (SA), diameter, tissue density (TD), and specific root length (SRL), and early, middle, peak and season median CH₄ fluxes by utilizing chamber CH₄ flux and pore water CH₄ concentration and isotopic measurements during the productive season. Shrub SRL, diameter and isotopic data suggested increased plant-mediated carbon substrates for acetoclastic methanogenesis along the thaw gradient. Root TD (root porosity proxy) decreased with thaw and had negative correlations with CH₄ fluxes throughout the season, and together with positive herbaceous rhizome SA-CH₄ flux associations and lower pore water CH₄ concentrations in the fully thawed stage. These results indicated increasing herbaceous plant-mediated transport of acetoclastically-produced CH₄ with thaw. Altogether, while confirming previous findings of increased plant-mediated acetoclastic methanogenesis with thaw, this study also demonstrated the benefit of belowground traits in revealing new aspects of plant-mediated CH₄ cycling in permafrost peatlands.