Conservationist soil management systems such as no-tillage (NT) can be considered activities with potential to increase soil carbon sequestration. The objective of this study was to quantify the soil C stocks and, along estimations of the nitrous oxide (N2O) emissions, calculate the soil C sequestration under NT for different times of implantation in two crop successions. The experiment was installed on the farm Santa Branca in Tibagi (Paraná State, Brazil), on a clayey Oxisol (Typic Hapludox). The treatments were conducted in non-random strips with subdivided plots: no-tillage with corn/wheat and soybean/wheat crop successions (NT12 M/T and NT12 S/T, respectively) for 12 years and no-tillage (NT22 M/T and NT22 S/T, respectively) for 22 years. The soil C stock increased with the time of no-tillage adoption; the increase in soil C in 10 years was 35 % and the annual accumulation rate 1.94 t ha-1 yr-1. The simulation of the soil C stock with the use of the unicompartmental model showed that the high input of crop residues and cropping systems with legume species reduced the soil organic matter mineralization that favored soil C accumulation. The N-N2O emissions were 25 % higher in corn/wheat than in soybean/wheat succession, and the different period of NT adoption did not induce higher N-N2O emissions. The balance between the soil C accumulation rate and nitrous oxide emissions indicated a positive balance of the system in the soil C accumulation, representing a CO2 sequestration of 6 t ha-1 yr-1
soil carbon stocks; greenhouse gas emission; crop succession
