Nitrogen Constraints On Terrestrial Carbon Sequestration, From Trees To The Globe

Abstract

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.