Fine root turnover is a critical component of below-ground forest ecology, which regulates nutrient dynamics, forest net primary productivity, carbon input to soils, and soil respiration. Understanding fine root responses to changing environmental conditions is critical for predicting the productivity and carbon sequestration potential of forest ecosystems during the 21st century. The first objective of this study is to demonstrate that a mechanistic model can realistically simulate spatial and temporal fine root demography in temperate forests on the basis of two hypotheses: (1) absorption of mineral N (N) stimulates the production of new roots, and (2) fine root longevity decreases with increasing N availability. Based on this model, my second objective is to predict fine root responses to changing atmospheric CO2 levels and N deposition rates. To meet these objectives, an extensive description of the N cycle and the new fine root module were implemented in the ASPECTS model. In agreement with a wide body of literature information, the new model predicted: (1) a preferential colonisation by fine roots of the uppermost soil layer, and (2) a flush of fine root growth in the spring. The simulations indicate that fine root biomass will increase in response to elevated CO2 under the double effect of (1) an increase in root longevity due to increased N stress, and (2) larger amounts of assimilates available to the growth of plant tissue due to increased photosynthesis. Although the simulated total fine root biomass increased under both increasing N deposition rates and atmospheric CO2 concentrations, the model predicts that the distribution of fine roots among soil layers will be altered. This suggests that experimental studies must consider the full depth of the root system in order to accurately assess effects of environmental changes on fine root dynamics. The model also suggests that fine root longevity is a plastic parameter, which varied from less than 1 year to more than 3 years depending on forcing values of N deposition rates and atmospheric CO2 concentrations. Finally, the model indicates that the increase in net ecosystem exchange (NEE) and soil respiration in temperate forests under elevated CO2 will be proportional to the amount of available N, with little to no response in low N conditions and up to +28% for both NEE and soil respiration under the highest deposition rate (7.0 g N m−2 y−1).