CQESTR simulates the effect of several management practices on soil organic carbon stocks. The model had been calibrated and validated in temperate regions. Our objective was to simulate carbon sequestration in Oxisols under plowed and no-tillage systems in southeastern and northeastern Brazil using the CQESTR model. In the southeast (20.75 S 42.81 W) tillage systems consisted of no-tillage (NT), reduced tillage (RT) (one disc plough and one harrow leveling (RT1), or one heavy disc harrow and one harrow leveling (RT2)) and conventional tillage (CT) (two heavy disc harrow followed by one disc plough and two harrow leveling). In the northeast (7.55 S 45.23 W), tillage systems consisted of NT, RT (one chisel plow and one harrow leveling) and CT (one disk plow, two heavy disk-harrowing and two harrow leveling). For the two experiments, CQESTR estimated a decrease in total organic carbon (TOC) stocks in plowed and no-tillage systems with respect to initial conditions (native forest). The model estimated carbon emissions varying from 1.28 (NT) to 3.67 Mg ha^-1 year^-1 (CT) in the southeast and from 1.13 (NT) to 3.04 (CT) Mg ha^-1 year^-1 in the northeast. CQESTR underestimated TOC in both experiments, especially under no-tillage systems, meaning that adjustments like the inclusion of clay mineralogy are necessary to simulation of TOC in the tropics. In spite of this, measured and simulated values of TOC were well correlated in the 0-20 cm layer (southeast, R² =0.94, p<0.01; northeast, R²=0.88, p<0.05) showing acceptable performance in the prediction of TOC in a tropical soil.