Future projections of land carbon uptake in Earth System Models are affected by land surface model responses to both CO2 and nitrogen fertilization. The Community Land Model, Version 5 (CLM5), contains a suite of modifications to carbon and nitrogen cycle representation. Globally, the CLM5 has a larger CO2 response and smaller nitrogen response than its predecessors. To improve our understanding of the controls over the fertilization responses of the new model, we assess sensitivity to eight parameters pertinent to the cycling of carbon and nitrogen by vegetation, both under present‐day conditions and with CO2 and nitrogen fertilization. The impact of fertilization varies with both model parameters and with the balance of limiting factors (water, temperature, nutrients, and light) in the pre‐fertilization model state. The model parameters that impact the pre‐fertilization state are in general not the same as those that control fertilization responses, meaning that goodness of fit to present‐day conditions does not necessarily imply a constraint on future transient projections. Where pre‐fertilization state has low leaf area, fertilization‐induced increases in leaf production amplify the model response to the initial fertilization via further increases in photosynthesis. Model responses to CO2 and N fertilization are strongly impacted by how much plant communities can increase their rates of nitrogen fixation and also directly affected by costs of N extraction from soil and stoichiometric flexibility. Illustration of how parametric flexibility impacts model outputs should help inform the interpretation of carbon‐climate feedbacks estimated by in fully coupled Earth system model simulations. Key Points The Community Land Model, Version 5, contains numerous modifications to representations of the vegetation carbon and nitrogen cycles The control state, and responses to CO2 and nitrogen, are sensitive to parameter choice, and to baseline water and nitrogen limitation Parameters controlling nitrogen fixation rates dominate fertilization responses, highlighting a need for greater scrutiny of this process