Changes in the carbon stocks of stem biomass, organic layers and the upper 50 cm of the mineral soil during succession and afforestation of spruce (Picea abies) on former grassland were examined along six chronosequences in Thuringia and the Alps. Three chronosequences were established on calcareous and three on acidic bedrocks. Stand elevation and mean annual precipitation of the chronosequences were different. Maximum stand age was 93 years on acid and 112 years on calcareous bedrocks. Stem biomass increased with stand age and reached values of 250–400 t C ha−1 in the oldest successional stands. On acidic bedrocks, the organic layers accumulated linearly during forest succession at a rate of 0.34 t C ha−1 yr−1. On calcareous bedrocks, a maximum carbon stock in the humus layers was reached at an age of 60 years.
Total carbon stocks in stem biomass, organic layers and the mineral soil increased during forest development from 75 t C ha−1 in the meadows to 350 t C ha−1 in the oldest successional forest stands (2.75 t C ha−1 yr−1). Carbon sequestration occurred in stem biomass and in the organic layers (0.34 t C ha−1 yr−1on acid bedrock), while mineral soil carbon stocks declined.
Mineral soil carbon stocks were larger in areas with higher precipitation. During forest succession, mineral soil carbon stocks of the upper 50 cm decreased until they reached approximately 80% of the meadow level and increased slightly thereafter. Carbon dynamics in soil layers were examined by a process model. Results showed that sustained input of meadow fine roots is the factor, which most likely reduces carbon losses in the upper 10 cm. Carbon losses in 10–20 cm depth were lower on acidic than on calcareous bedrocks. In this depth, continuous dissolved organic carbon inputs and low soil respiration rates could promote carbon sequestration following initial carbon loss.
At least 80 years are necessary to regain former stock levels in the mineral soil. Despite the comparatively larger amount of carbon stored in the regrowing vegetation, afforestation projects under the Kyoto protocol should also aim at the preservation or increase of carbon in the mineral soil regarding its greater stability of compared with stocks in biomass and humus layers. If grassland afforestation is planned, suitable management options and a sufficient rotation length should be chosen to achieve these objectives. Maintenance of grass cover reduces the initial loss.