Conversion of agricultural land to forests can change soil properties. Information on these changes is important mainly for greenhouse gases inventory in the LULUCF sector, as an increase in soil carbon stocks is one of the most pronounced and relevant consequences. In the 1950s and 1960s, the main reason for land-use change in Slovakia was the afforestation of eroded soils on slopes, gullies, and other degraded areas. Since the 1990s, the most extensive process has been natural forest expansion on abandoned agricultural soils. Several methodological approaches have been used in research projects to study possible changes in soil following land use change from arable land or grassland to forest land. The most reliable results were obtained from soil carbon measurements along transects at the “forest edge”. This type of forest expansion is usually neglected. Due to grassland mowing practices, the grassland directly adjacent to forest stands is often not mown, allowing tree seedlings to survive and grow. In this way, the forest edge can shift 10-20 m over several decades. In the research site, soil organic carbon (SOC) density in topsoil was significantly higher in forests (both old forests and young forest stands in the transition zone) than in adjacent grasslands.