Organic matter properties of soils were studied in a territory covered by Stagnosols after afforestation. We quantified the impact of afforestation on the amount and distribution of free organic matter, microaggregates (unstable and stable under low-intensity sonification) and their components in the upper horizons of former arable soils overgrown by different species of forest vegetation. The duration of 45 years after afforestation was revealed to be not sufficient for a complete renewal of soil humus state, which appears only after till 115 years of the duration of forest cenosis. The carbon storage in light fractions remains lower by 21–40 % relative to that of forest soil, including significant losses of free organic matter (42–58 %) and occluded organic matter (12–33 %), which present the most active part of soil organic matter. The positive impact of the deposit regime, expressed by a sharp improvement of the carbon cycle balance, is reflected by the Cunstable/Cstable ratio, which decreases from 6.2 in arable soil to 2.4 on average in forest stands.