The soil environment is modified by fertilizer use, tillage and crop rotations. Three treatments were selected from a long-term experiment established in 1963 at INIA La Estanzuela Experimental Station located in Uruguay (34oS, 57oW), to examine several aspects of soil carbon dynamics in different moments. The experiment is located in an Argiudoll soil with moderate slope (3-4 %). Total organic carbon (TOC), organic C in particulate organic mater (C-POM), and C mineralization rates under field and laboratory conditions were evaluated. Selected treatments were: CS-1 (continuous cropping without fertilizer); CS-2 (continuous cropping with N and P fertilizer); CS-5, a six-year crop – pasture rotation with 50 % of the time under crops and 50 % under grass-legume pastures. Conventional tillage was used throughout the experiment. Crop rotations including pastures (CS-5) showed the highest TOC levels due to a higher residue return in crops and pastures and a reduction in soil erosion. Thus, this treatment maintained the TOC levels of 1963. Continuous cropping (CS-1 and CS-2) showed declines in TOC levels due to a lower residue return and increases in soil erosion. After forty years TOC levels were 80 and 50 % lower for CS-1 and CS-2 compared to CS-5. Treatments also significantly affected the more active fractions of soil organic matter (C-POM). Inclusion of pastures (CS-5) resulted in significantly higher values of C-POM than continuous cropping systems and the relative magnitude of these differences were more important than those observed with TOC. Carbon mineralization rates measured under field conditions were highly correlated with TOC levels and soil temperature for the selected treatments. Under laboratory conditions, minimum and maximum values of percent of organic C mineralized were 1.32 and 1.48 for CS-1 and CS-5 respectively.