Soil organic carbon dynamics along alpine greening gradients on different bedrock

Alpine greening due to climate warming represents a major environmental change in mountain ecosystems, generally assumed to increase soil organic carbon (SOC) storage due to higher C inputs. However, data on SOC dynamics in high elevation soils remain scarce. In this study, we investigated SOC stocks, radiocarbon (¹⁴C)-based turnover rates, and mineral C saturation (reflected by the ratio of SOC to pedogenic oxides, PO) along three alpine elevation gradients from (sub)alpine forests to the periglacial zone on different bedrock types. SOC stocks showed pronounced declines from ~10 to 1 kg C m^-2 across the transition from alpine grasslands (2000 – 2750 m a.s.l.) to the nival zone (2850 – 3100 m a.s.l.), accompanied by a decrease of ¹⁴C-based turnover rates from decades to millennia. Underlying bedrock significantly influenced SOC stocks, with dolomitic soils storing 50% less C than siliceous soils probably due to slower weathering and reduced SOC stabilisation. Soils in the sparsely and non-vegetated periglacial zone showed low SOC:PO ratios, indicating a high capacity to stabilize new incoming C while alpine grassland and forest soils at lower elevation exhibited high SOC:PO ratios and limited additional storage capacity. Below the vegetation line, SOC stocks in alpine grasslands exhibited only minor variation with decreasing elevation, while ¹⁴C-derived turnover rates increased. This apparent decoupling suggests that greater plant-derived C inputs under warmer conditions are counterbalanced by enhanced microbial decomposition, thereby limiting long-term SOC accumulation. Overall, these results indicate that SOC sequestration under alpine greening will be limited to a small area around the current vegetation line, with parent material influencing the magnitude of C uptake. Our study provides critical baseline data for predicting carbon cycling and sequestration potential in alpine soils under ongoing environmental change.