The purpose of this article was to assess the participation of young soils of ash dump forest communities in carbon sequestration by soils of southern taiga forests, considering the physico-chemical properties of the ash substrate and forest litter (pH, TOC, TN, content of P and K mobile compounds, and exchangeable Ca and Mg ions). It was revealed that on three Middle Ural ash dumps (composed of fly ash from various brown coals) over 50–60 years, forest communities spontaneously formed according to the zonal type (with the dominance of Betula pendula Roth and Populus tremula L.) with poorly differentiated young soils—technosols. For the first time, as a result of using an integrated approach to assess the direction of forest ecosystem formation on fly ash dumps, a tendency to increase carbon stocks in technogenic soils that have not reached the level of zonal soils was revealed, as well as the dependence of C accumulation on some physico-chemical characteristics of ash was established. Carbon stocks in Technosols are on average equal to 44 t/ha but vary significantly. It was shown that there is a medium negative relationship between the content and stocks of organic carbon in soils formed on a technogenic substrate and the content of mobile phosphorus compounds in them (the correlation coefficient is −0.58 and −0.53, respectively). The average carbon stocks in the litter of technosols, which is the main source of organic carbon in forest soils, are 3.2 t/ha. It was revealed that the carbon stocks in the litter are most influenced by the content of exchangeable calcium cations and magnesium in it (the correlation coefficients are −0.68 and −0.69, respectively). Any correlation between the studied litter parameters and carbon accumulation in the soils of ash dumps was found. The study revealed that the carbon stocks in the technosols of ash dump forest communities are two times less than the carbon stocks in the zonal forest soils of the Middle Ural southern taiga. The stocks of this element in the litter of young soils are equal to 1/3 of the litter of zonal soils. The composition of the humus substance system formed in the soils of forest areas of ash dumps and zonal soils is similar. The results of this study can serve to fill gaps in the knowledge about carbon sequestration by soils and aim to draw attention to forest communities of technogenic ecosystems to consider the contribution of their components to carbon sequestration.