Increased CH₄ Oxidation in Arctic Tundra Ecosystems Caused by Vegetation-Mediated Soil Drying

Arctic tundra soils can act as an important sink for atmospheric methane (CH₄). However, the role and magnitude of this process, and how it will change during future climate scenarios, are poorly understood. The vegetation is changing with a warmer Arctic climate, with taller plants, more shrubs, and altered vegetation patterns. These changes are predicted to be strongest in moist to wet regions, areas usually associated with CH₄ production. Additionally, these changes in growth patterns can increase evapotranspiration rates, leading to enhanced soil aeration, favouring CH₄ oxidation. Here, we investigate CH₄ dynamics within long-term (> 25 years) passive air warming treatments, using five plant communities with contrasting soil moisture and nutrient regimes. These treatments reveal a strong increase in atmospheric CH₄ oxidation in two dry ecosystems (140.4% ± 8.1% and 204.2% ± 19.3% for a Dry Heath and Dry Meadow, respectively), and a strong reduction of CH₄ emissions (91.2% ± 18.6%) in a Tussock Tundra community. In contrast, our investigation of Mesic and Wet Meadows showed no significant treatment effects, with only limited CH₄ exchange in the Wet Meadow. Furthermore, when inhibiting CH₄ oxidation in the surface soil, we found evidence of CH₄ production even at the driest site (Dry Heath), indicating a potential for CH₄ production throughout the landscape. Although soil temperature and moisture have been put forward as strong regulators of CH₄ fluxes, they did not consistently explain our observed changes. Instead, we argue for interactions between vegetation change and near-surface soil characteristics. The observed shift in plant composition and increased vegetation height, along with warmer air temperatures, enhanced evapotranspiration and surface soil aeration, thereby stimulating methanotrophy and leading to increased CH₄ oxidation. This vegetation-induced climate feedback would aid the predicted temperature-dependent increase of CH₄ oxidation in the Arctic, potentially mediating CH₄ emissions from the region.