The aim of the study was to determine the effect of soil management systems of Brunic Arenosols on total content of organic carbon and its fraction susceptible to oxidation in comparison with soils under forests. Samples for study were taken from the humus horizon at sites located in forests and soils from little mid-forest cultivated fields (hunting plots). The agrotechnical treatments increased the content of plant-available forms of P, K and Mg in the soils of most hunting plots in comparison to forest soils. In the arable horizon of the hunting plots there was found a varied total content of organic carbon and its fraction susceptible to oxidation. Cultivation of soil in hunting plots caused a decrease in content of total organic carbon as well as its labile and non-labile fraction. To evaluate the carbon transformation in the soil of the cultivated plots against the forest (reference soil), the Carbon Management Index (CMI) was used. The decay rate of soil organic matter in a natural forest was lower than in the agricultural fields. A long-term tillage of Brunic Arenosols contributed to the degradation of the pool of organic carbon in sandy-textured soil.