A carbon-nitrogen cycling model (CNC) was developed and integrated into the CropSyst model. The soil carbon subroutine follows the model of Verberne et al. (1990) with several modifications. The CNC model runs on a daily or hourly time step. Inputs of carbon (C) are from crop residues, senescent leaves, and dead roots. Standing and flat residues are modeled separately. Each time there is a residue input, new pools are generated. Carbon inputs from residues are divided in three fractions: lignin, structural fraction, and non-structural fraction, the latter having the fastest turnover rate. Soil C is divided in four pools of decreasing turnover rates: microbial biomass, labile, metastable, and stable pools. The amount of biomass, except in the case of lignin, mediates C decomposition from residue and soil pools. Each microbial attack renders carbon dioxide and microbial biomass. The products of microbial decomposition are allocated to the different soil C pools depending on texture. No separation between active and dormant microbial biomass was attempted. All decomposition processes are simulated using first order kinetics. Rates of microbially-mediated processes are adjusted by temperature and moisture factors. Nitrogen (N) cycling is linked to C flow, except for the ammonium and nitrate pools that are independently modeled. Total denitrification and the fraction of N lost as nitrous oxide are modeled as a function of nitrate concentration, temperature, and soil moisture. Tillage events accelerate the decomposition rates in the layers affected by the operation. The residues incorporated in the soil by tillage events result in new residue C pools. Tillage also affects soil hydraulic properties. Parameterization of the CNC model was performed by interpretation of information in the literature and by interpretation of our own data on soil carbon dynamics. Simulations for different cropping systems and a sensitivity analysis of the model parameters are presented.