Soil organic carbon sequestration over 50 years in resampled afforestation chronosequences on former cropland

Afforestation of agricultural land is widely promoted as a nature-based solution to enhance carbon (C) sequestration and mitigate atmospheric CO₂ levels. However, the temporal dynamics of soil organic carbon (SOC) after afforestation, particularly in mineral soils, remain uncertain due to the complex interaction of biogeochemical processes and their spatial variability. We investigated changes in SOC sequestration over five decades of afforestation on former cropland by extending the chronosequence approach with three repeated soil inventories in oak (Quercus robur L.) and Norway spruce (Picea abies (L.) Karst.) stands. Aboveground biomass C stocks were also quantified to evaluate the contribution of SOC to post-agricultural ecosystem C stocks. Forest floor C stocks increased rapidly in the early years and stabilized after approximately three decades, with consistently higher accumulation under Norway spruce than oak. In contrast, mineral SOC stocks in 0-25 cm depth increased with forest age by 0.18 ± 0.06 Mg ha^-1 yr^-1 under oak and 0.44 ± 0.07 Mg ha^-1 yr^-1 under Norway spruce. These contrasting trends in forest floor and mineral soil indicated a shift in C source-sink strength over time and between species. After 50 years of afforestation, total ecosystem C stocks in afforested stands reached up to 75% of those in a 200-year-old forest, with most new C stored in biomass (84-86%), followed by mineral soil (10-11%) and forest floor (4-5%). Despite higher sequestration of new C in Norway spruce stands, the relative distribution across ecosystem compartments was similar between tree species.