Abstract Soil organic carbon (SOC) sequestration in agricultural soils requires appropriate cropping systems and conservation practices that increase crop biomass production. No tillage agriculture provides several advantages to establish “carbon friendly” production systems. Conservation agriculture has greatly expanded over the past three decades in the North Central region of Rio Grande de Sul, the southernmost state of Brazil, with about 6 million ha of land under dryland grain crop agriculture. A shift from intensive conventional tillage to mulch tillage and then no tillage has resulted in increased SOC levels across much of the region as verified in field measurements. In this study a modeling system was constructed by coupling the interactive Century (i_Century) software running Century 4.0 and ArcGIS data layers to simulate the long-term SOC trends in the Ibiruba region, which occupies a land area of 83,500 ha. This system was capable of accounting for cropping system and management practices trends from 1900 to the present times, as well as simulating projections out to the year 2050, capturing periods of decreasing or increasing SOC trends. Simulations performed with the modeling system show that SOC stocks in Ibiruba, estimated at 5.05 Tg C before human activity, decreased to 3.71 Tg C (31%) by 1985. An estimated recovery to 3.97 Tg C by 2005 in response to adoption of SOC-enhancing practices such as conservation tillage and no tillage was observed. Simulations to 2050 with maintenance of these cropping systems and soil management showed that total C stocks could reach 4.21 Tg C, which would still be 21% less than originally present in those soils. The integration of Century with GIS was greatly expedited with the use of the i_Century interface and its readily-accessible database format. Data transfer between the modeling and geoprocessing environment was facilitated for quick visualization and further processing. This approach can be applied for carbon monitoring and accounting schemes at several scales, from the field to the national level.