This article details a long-term experiment which attempted to determine stocks of soil organic matter (SOM) in southern Brazil under 3 different crop rotations in both conventional and zero tillage systems. Under no tillage the stocks of SOM were not affected by the different rotations. However, under conventional tillage two of the crop rotations with vetch and maize stimulated the decay of the original native SOM when compared to the wheat and soybean rotation. The authors hypothesize that the N2 fixation by the leguminous green manure (vetch) in the cropping system was the main cause of the C accumulation in the soil under zero tillage.