Nitrogen and carbon dynamics of two Norway spruce stands were compared using a dynamic soil-plant simulation model (SOILN). The objectives were to evaluate differences in properties between the two stands and to explore the consequences on C and N dynamics. The young stand (25 years old) was located in south-west Sweden (Skogaby) growing on a loamy sand soil. The old stand (70 years old) was growing on a sandy soil in western Denmark (Klosterhede). Differences in specific rates of processes between the two sites were estimated in terms of parameter values derived by calibration of the model. Differences in daily flow rates and pathways of N and C were examined and budget components were simulated over a six year period. Simulations showed that the different stands had different specific rates for growth, allocation, litter fall and N uptake resulting in higher N uptake in the younger trees whereas growth was more similar. The soil carbon balance was positive and soil C/N ratio increased for the young stand. For the old stand the soil carbon balance was negative and the C/N ratio decreased. The soil mineral N levels were much lower in the younger stand which was explained by a higher plant uptake of organic N. At both sites the average pools and fluxes over the years were fairly well simulated indicating that N leaching predictions on average might be reliable.