Soil Organic Matter (OM) promotes modification in chemical, physical and biological properties of soil, and its presence is of great value to soil quality. The magnitude of OM influences is dependent on its quantity (total C, C_T) and quality (labile C, C_L) factors. The labile fraction is more easily affected by changes in soil management and, for this reason, it can be used as an indicator of management quality and agriculture sustainability. The Carbon Management Index (CMI) proposed by Blair et al. (1995) has the advantage of involving both quantity and quality factors of OM in its estimation. However, the method to obtain the C_L suggested by the authors, based on C oxidation with 333 mM KMnO₄, is not adequate for some types of soil and climate, requiring different KMnO₄ concentration for each situation. On the other hand, physical fractionation seems to be accurate and suggest that it could be used to estimate the C_L and the CMI. Therefore, the purposes of this work were: a) to compare the C_L obtained from chemical (333 and 60 mM KMnO₄) with physical (density and granulometry) methods of OM fractionation; and b) to analyze the quality of different soil management systems through estimating CMIs. In order to achieve these goals, studies were conducted in three long-term experiments located in the South of Brazil, with the following treatments: 1) seven cropping systems in no tillage (NT) and two rates of N fertilizers (1983-2002); 2) conventional tillage (CT) and NT, two cropping systems, and two rates of N fertilizers (1985-2003); and 3) CT and NT systems with and without irrigation (1995-2003). The results indicate that the labile fractions of C obtained by the two methods (physical fractionation and oxidation with 60 mM KMnO₄) had a significant correlation with annual addition of organic C to the soil and similar quantity of C_L. However, the C_L obtained from oxidation with 333 mM KMnO₄ did not have a significant correlation with annual input of C and the mean quantity of C_L were about three times higher than the C_L obtained from the other methods. Among the physical methods of fractionation, the granulometric fraction was best correlated with the annual input of organic C to the soil. The high correlations among C_L obtained from physical fractionations and 60 mM KMnO₄ highlight the possibility of using physical fractionation to estimate OM lability and CMI. In general, management systems affected both C Pool Index (CPI) and Lability Index (LI). The presence of leguminous crops, especially tropical leguminous, resulted in higher CMI values than cropping systems with only gramineous crops. It is due to the higher addition of organic C to the soil by these species, thus increasing total organic C stocks as well as high labile C stocks. No tillage system and application of 180 kg N ha^-1 yr^-1 increased values of CMI by two times, as compared with the conventional tillage and no N application. The presence of irrigation decreased the CMI under no tillage system, but had a small effect under conventional tillage. Based on these results, it can be inferred that: (1) physical methods of OM fractionation are sensitive in detecting small changes in C_L and are reliable for CMI estimation; (2) the use of tillage systems with minimum plowing, together with leguminous crops and N-fertilizer application enhances management quality of the soil, as evidenced by the highest CMI values; (3) irrigation decreases IMC values under NT, but this effect was not observed in soil under CT. Reference: BLAIR, G.J., LEFROY, R.D.B. & LISLE, L. Soil carbon fractions based on their degree of oxidation, and the development of a Carbon Management Index, for agricultural systems. Aust. J. Agric. Res., v.46, p. 1459-1466. 1995.