This study investigates the soil organic carbon (SOC) and whole tree biomass carbon (C), soil bulk density (BD) as well as changes in these parameters in afforested areas in Latvia. The study covered 24 research sites in afforested areas—juvenile forest stands dominated by Scots pine, Norway spruce and Silver birch. The initial measurements were conducted in 2012 and repeated in 2021. The results show that afforestation mostly leads to a general decrease in soil BD and SOC stock in 0–40 cm soil layer and an increase in C stock in tree biomass across afforested areas with various tree species, soil types, and former land uses. The physical and chemical properties of the soil could explain the differences in changes in soil BD and SOC caused by afforestation, as well as the impact of past land use may have persisted. When comparing the changes in SOC stock with the increase in C stock in tree biomass due to afforestation, taking into account the decrease in soil BD and the resulting elevation of soil surface level, the afforested areas at juvenile development stage can be considered a net C sink.