Nitrogen (N) is an essential nutrient for plant growth that constrains the fixation and storage of carbon (C) in many ecosystems. Understanding how environmental change, especially increasing N deposition, carbon dioxide concentrations, and soil temperature, alters the N limitation of forest growth is critical for accurately predicting future C storage and climate change. Accurate predictions depend on developing a historical and present day evaluation of N controls on C storage and using this knowledge to assess and improve global models. In this dissertation, I first demonstrate that N deposition has increased C storage in trees during the 1980s and 1990s across the northeastern U.S. Second, I show how integrating four different observational and experimental datasets (N fertilization experiments, N deposition gradients, 15N tracer studies, and small catchment N budgets) provide unique insights for testing and improving Earth System models. By comparing model output to globally-distributed N fertilization experiments, I demonstrate that two prominent Earth System models (the CLM-CN and O-CN) differ widely in their sensitivity to step increases in N fertilization. Third, a separate analysis focused on the CLM-CN found that the model was not sensitive enough to N deposition in comparison to historical N deposition data. By comparing CLM-CN output to both 15N tracer studies and small catchment N budgets, I show that the low response to N deposition is partially due to low ecosystem retention of N. Model improvements to the CLM-CN that decreased photosynthesis and introduced a more closed N cycle (i.e., lower N inputs relative to internal cycling) increased ecosystem retention of N, decreased the productivity response to N fertilization, and increased the productivity response to N deposition, thereby yielding much more similar model predictions to observations. Overall, this dissertation increases our knowledge of how N deposition influences C storage and is the first to explicitly benchmark C and N interactions in Earth System models using a range of observations. In addition, my work sets a foundation for estimating the impact of N cycling on climate and creates a framework for future evaluations of Earth System models.