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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]

Abstract: In this study, the effects of soil bulk density, total porosity, organic matter content, aluminium saturation and cation exchange capacity (CEC) on water and ethanol sorptivity and hydro-repellency index of various sized aggregates of horizons A and B were investigated. Soil samples were taken from horizons A and B in 17 different locations of Rio Grande do Sul, Brazil. Three samples were collected from each of the horizons for the determination of soil sorptivity in water and ethanol. Soil sorptivity was determined by a steady-state flow using a mini-infiltrometer. Hydro-repellency of samples was estimated from the ethanol/water sorptivity values. Aggregates from most soil samples exhibited low hydro-repellency index, R (1.24–3.12), except for luvisol and vertisol, which had R ≥5.79. The very low coefficients of determination, r 2 =0.18 and 0.22 at p =0.01 between hydro-repellency index and organic matter content, and between angle of contact and organic matter content, respectively is an indication that the quantity of organic matter have little influence on hydro-repellency, but the composition of organic matter and other soil properties. The mean pH of sampled soils ranged from 3.90 and 5.50 in both horizons A and B but presents no significant effect on the hydro-repellency of soil at p =0.01. The results show that soil hydraulic and sorptivity properties are dependent on interaction between aggregate sizes, which is mainly a function of the soil clay minerals. [Copyright &y& Elsevier]