Abstract: Over the last century, the Mediterranean basin has been widely affected by the abandonment of farming activities, leading to a natural succession towards forested ecosystems. This process is resulting in a carbon (C) stock increase at an ecosystem level, often assessed through the measurement of aboveground biomass, while the contribution of soil organic carbon (SOC) remains unclear. We investigated C changes caused by secondary succession on previously grazed areas in central Italy, specifically focusing on the SOC pool. The natural succession is described through a chronosequence approach over four successional stages: pastures, shrublands, young and mature forests. Eight replicates per stage were studied, and C stock was estimated in the mineral soil down to a 30-cm depth, and in all other ecosystem C pools: aboveground and belowground biomass, deadwood and litter. In the mature forests, SOC stock was significantly higher (p < 0.05) than in pastures by 40 ± 8 Mg ha^–1, corresponding to 28% of the total ecosystem C stock gain. The same trend was observed for aboveground biomass, the pool that increased the most (62 ± 23 Mg ha^–1), with a 43% contribution to total ecosystem gain. Our results point to a substantial contribution of SOC to overall C stock during secondary succession in Mediterranean ecosystems.

Key words: Climate change mitigation, Pastures · Forests, Secondary succession, Soil organic carbon