One of the main objectives of this work was to study the effects of land use on soil carbon, total and extractable SOM and the properties of humic substances. Soil samples were collected in western and central Lithuania and represented Albeluvisol and Cambisol. Total C contents as well as C / N in the soil samples were determined by a dry combustion method using an automatic Vario EL III analyzer. Extractable SOM was measured using sodium pyrophosphate solution and hot water for the extraction procedure. Humic acids were investigated by the methods of gel permeation chromatography and absorption spectroscopy in UV / VIS and IR regions. Experimental evidence suggests that land use conversion had both positive and negative effects on soil total and extractable carbon and SOM. In Albeluvisols, grassland soil was found to contain by 0.2–0.6 g kg^–1 more C compared to fallow land (1–1.5 years old). In Cambisols, long-term pasture soil was found to contain by 2.4 g kg^–1 more C compared to arable land (16 years) converted from pasture. Results of humic acid analyses by gel chromatography showed a high molecular weight and polydispersity of humic acids. The highest content of humic acids in HWE as well as a high content (29%) of macromolecules with the molecular weight more than 100,000, and the lowest C / N ratio (10.3) were determined in long-term pasture soil.

Keywords: soil carbon, SOM, extractable SOM, Albeluvisol, Cambisol, land use, grassland, fallow