Abstract: The Century model was used to simulate soil carbon (C) and nitrogen (N) cycling, and crop production dynamics in an ongoing field experiment in Uruguay (started in 1963). The model was calibrated using observed data from three treatments (crop or crop?pasture rotations) and validated with a fourth treatment. The model correctly predicted the impact of different treatments on microbial biomass, N mineralization, soil respiration, and crop yields. The model and observed data show that soil respiration, N mineralization, soil C, and crop yields increase with increasing plant derived C inputs caused by increasing the frequency of pastures in the rotations. This is one of the first papers which show the strong positive correlation of observed soil C with plant C soil inputs to field?observed microbial biomass, soil respiration, and N mineralization. The results also showed that reducing tillage and transitioning to a no?till system increased soil C and reduced soil erosion. The main path of soil C losses was heterotrophic microbial respiration which accounted for 66% of the total C lost in a continuous crop rotation and no fertilizers, 71% in a continuous crop rotation with fertilizers, and 86% in a crop?pasture rotation with fertilizers. Model results from a degraded cropping system showed that adding grass/clover pastures greatly increased plant production and soil C, while reducing the frequency of grass/clover pastures in high?fertility cropping systems from 50% of the time to 25% reduces crop yields and soil C. Including cover crops substantially increases crop production and maintains soil C in high fertility and degraded cropping systems