The research compares the C sequestration of on the one hand an old pasture and on the other hand two young forest stands, planted on former pasture lands. The stands are respectively ash and pin oak stands, planted 27 and 26 years ago, with a basal area of 21.3 m2 and 13.10 m2. C-content was assessed, based upon Hohenadl's model trees, for all compartments. The total C-content of the pasture land presently amounts to 128 t/ha, whereas it reaches respectively 173 t/ha and 118 t/ha for the ash and pin oak stands. The soils of the forest stands contain respectively 74 and 50 t carbon/ha. So, remarkable changes occurred in the C quantity and allocation by the afforestation. Afforestation of a rich pasture leads in the first phase to a noticeable decrease of the C content in the soil. The C content of the A1 horizon is significantly greater in the pasture than in the young forest stands. Nevertheless afforestation of a pasture considerably increases, already after 25 years, the C sequestration of the ecosystem, provided the used tree species are suitable, i.e. they should be fast growing and produce a good quality litter. This way, not only the aboveground C accumulation is useful, but also the quantity and the quality of the soil humus. Research on the evolution of the C sequestration after afforestation of agricultural lands deserves priority.