Non-Linear Increases of Soil Carbon Stocks and Turnover Along Alpine Greening Gradients on Different Bedrocks

Alpine ecosystems are experiencing warming, leading to alpine greening-an upward migration and enhanced productivity of vegetation. Although soils represent the largest carbon (C) stocks in these ecosystems, impacts of greening on soil organic carbon (SOC) dynamics remain unknown, especially in the high alpine zone. We measured SOC stocks, quality and radiocarbon (¹⁴C) based turnover rates along elevation gradients from (sub)alpine forests to periglacial zones on three bedrock types, representing trajectories of changes in climate and vegetation and weathering regime. Data were linked to SOC stabilizing pedogenic oxides (PO) to assess the potential of soils to sequester C upon alpine greening. Results show pronounced decreases in SOC stocks (from 10 to 1 kg C m^-2) and increases in ¹⁴C-based turnover times (from decades to millennia) with increasing elevation. This pattern followed a non-linear sigmoidal function with a threshold at the vegetation line where grassland transitions to the nival zone. High alpine soils showed low SOC:PO ratios, indicating low C saturation and a high capacity to stabilize SOC on reactive minerals. However, negligible ¹⁴C-derived carbon inputs suggest that low vegetation productivity constrains SOC sequestration in these soils. In contrast, towards lower elevation with a warmer climate, grassland soils receive higher C inputs, but appear more C saturated, leading to greater SOC turnover but little SOC stock changes. Bedrock influenced SOC dynamics with approximately 50% smaller SOC stocks and faster turnover rates in dolomitic than in siliceous soils, due to slower weathering and lower SOC stabilization. Using elevation as a space-for-time substitution, we infer that SOC sequestration under alpine greening will be confined to areas around the vegetation line, with parent material influencing the magnitude of C uptake. Future projections of SOC dynamics should account for vegetation shifts and bedrock-driven stabilization to constrain the role of alpine soils in land carbon-climate feedbacks.