The conversion of natural vegetation to cultivated land often results in changes in the quantity and quality of soil organic matter. The magnitude of these alterations varies according to the soil properties, climate, crops and soil management. Soil organic C is not always a sensitive indicator of soil changes. The distribution of C fractions is therefore being used to assess the soil organic matter quality more precisely. This study evaluated the changes in organic C fractions of a Humic Oxisol under different uses. Soil samples were collected from five different environments (native forest, scrub exposed to sporadic fires, a 30 year pasture, 25 year pasture and annual crops) in four layers (0.0-2.5; 2.5-5.0, 5.0-7.5, and 7.5-10.0 cm). In the 25 year-old pasture and annual crops the contents of soil organic C were reduced, especially in the surface layer 0.0-2.5 cm. In these areas, the biomass microbial C and water soluble C proved to be more sensitive to management changes. In the 30 year-old pasture, the contents of fulvic acids, humic acids and humina were higher than in the area under pasture for 25 years, in all layers.
soil organic matter; humic substances; microbial biomass
