Soil organic matter contains various organic compounds and plays an important role in the circulation of organic carbon (SOC) and affects the quality of soils. Soil carbon loss via soil erosion and slope processes results in SOC variations along the slope. Deposition of silt and clay on peat-muck soils protects them from mineralization of organic carbon compounds and release of CO₂ to ambient air. Slope processes occurring in northeastern Poland contribute to the reduction of SOC in eroded soils and sequestration of carbon in deluvial soils, mucky soils, and peat-muck soils. The pool of SOC and the lability of carbon can be used to assess the quality of soil in various conditions on the basis of the carbon management index (CMI). In the studied landscapes, the values increased down the slope and reached higher values in the soils covered with grasslands.