Promoting biological weed regulation via competition for resources requires better understanding the func- tioning of heterogeneous canopies in nitrogen-deficient situations. Mechanistic simulation models are powerful tools to reach this goal. Our objective was to integrate plant-plant competition for nitrogen into the preexisting FLORSYS model simulating the effects of cropping systems on weed dynamics and crop production. The formalisms were either created or inspired from other models and adapted to make them compatible with the individual- based representation of FLORSYS. Plant nitrogen uptake was simulated by confronting plant nitrogen demand (driven by shoot growth) to plant nitrogen supply (depending on root characteristics, soil-nitrogen availability and the presence of neighboring plants with roots in the same soil zone). A nitrogen stress index allowed ac- counting for the impact of plant nitrogen nutrition on plant photosynthesis, biomass allocation and morphology. The new formalisms consisted of only seven species-specific parameters. Despite simplifying hypotheses in formalisms, predictions were in good agreement with knowledge on canopy functioning and crop-weed in- teractions. We provide the first mechanistic cropping system model focusing on weeds that simulates plant-plant competition for nitrogen (in addition to competition for light). It will be useful to understand the role of nitrogen in crop-weed interactions and identify agroecological management strategies promoting weed regulation by competition.