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The objective of this study was to evaluate the total organic carbon and the physical and chemical fractions of organic matter in an Alfisol under different use systems. The land use systems evaluated was homogeneous forestry of Eucalyptus grandis. agrosilvopastoral system and native grassland, in layers from 0.000-0.025 and 0.025-0.075m depth. The coarse fraction (CFG) and the carbon associated minerals (CAM) were obtained by physical fractionation. The free light fraction (FLL), light occluded (FLO) and heavy (FP) were obtained by densimetric physical fractionation. The non-humicfractions (NH), fulvic acid (AF), humic acid (HA) and liumin (HU) were obtained by chemical fractionation. The Eucalyptus grandis system promoted higher total organic carbon, CFG, FLL and FLO and AF. In the surface layer, the protective recalcitrance mechanisms by organic matter molecular and chemical stabilization are overlapping stability which result from aggregates occlusion. As is difficult to form aggregates due to sandy soil matrix, carbon youth which enter the system is decomposed by microorganisms going to a more advanced stage of decomposition, forming in this case associations with silt and clay particles. This occurs in soils where the clay percentage is low. Through the fractionation it was observed that most of the soil organic matter is stored in the form of HU. [ABSTRACT FROM AUTHOR]

Studies of the concentration of total organic carbon (TOC) in soil and organic matter fractions contribute significantly to the evaluation of soil quality, particularly in the study region, where there is no information on this topic. Moreover, there is also a worldwide concern about the carbon cycle, depending on the greenhouse effect that harms the world's climatic conditions. The aim of this study was to evaluate the effect of production systems in TOC concentration in a soil classified as dystrophic red clayey latosol, in the fraction of particulate organic carbon and in the mineral carbon fraction, i. e., the fraction of carbon associated with soil minerals and in various classes of aggregates of the soil planted with: 1) 13 and 20-year-old eucalyptus (Eucalyptus saligna), 2) pastures and 3) native forest where hygrophilous species and deciduous xerophytes dominate. Soil samples were collected in the district of Butiá, Rio Grande do Sul (Brazil) in 2006 in the depths 0.00 to 0.05, 0.05 to 0.10, 0.10 to 0.20 and from 0.20 to 0.40 m. Samples were air-dried at 20 °C and sieved according to the classes of granulometric fractionation, considering the aggregate classes of 4.76 to 8.00 mm, 2.00 to 4.76 mm, 1.00 to 2.00 mm, 0.50 to 1.00 mm, 0.25 to 0.50 mm and <0.25 mm. For each class of aggregate, particulate organic carbon and TOC was determined. The highest TOC concentrations, particulate organic carbon and mineral carbon occurred in the largest fraction (8.00 to 4.76 mm). In pasture soils greatest TOC accumulation was found at depth of 0.05 to 0.20 m, in classes of aggregates 8.00 to 0.50 mm. [ABSTRACT FROM AUTHOR]