A Model-Based Evaluation of the Lateral Transport and Fate of Carbon Across the Terrestrial-Aquatic Continuum

The global carbon cycle regulates Earth’s climate and provides environmental conditions necessary for humans and other organisms to persist. With ever increasing anthropogenic emissions, land and ocean systems must take in more and more C to mitigate the impact of emissions on climate and of climate change on society. Within the land, C cycling occurs through heterogeneous terrestrial and aquatic ecosystems that are each vertically connected with the atmosphere and laterally connected to each other. The lateral C transport (LCT) from terrestrial to aquatic ecosystems is understudied and poorly constrained within the land C budget relative to vertical fluxes, yet determines when, where, and for how long C is stored. Because terrestrial and aquatic ecosystems respond differently to environmental conditions, quantifying the potential for land C uptake and storage to continue mitigating emissions requires constrained LCT estimates that can better resolve how stored C is partitioned among ecosystems.

In this dissertation, I take a coupled process model approach that leverages connectivity between terrestrial and aquatic ecosystems to constrain LCT, improve understanding of its drivers, and partition terrestrial LCT loss and aquatic fate. In Chapter 2, I develop a coupled terrestrial-aquatic C and hydrology process model for LCT. Based on a series of model experiments and sensitivity analyses, I create a conceptual model of the cross-scale interactions among climate, vegetation, and local responses that drive LCT at large continental scales. In Chapter 3, I apply the coupled terrestrial-aquatic model across a subset of watersheds with the Contiguous U.S. After validating the coupled terrestrial-aquatic model using terrestrial and aquatic data sources, I compare terrestrial vertical net C exchange, LCT losses, and aquatic emission of LCT across watersheds. In Chapter 4, I present a coupled model framework that leverages connectivity within the coupled terrestrial-aquatic process model to constrain C and water fluxes across terrestrial and aquatic ecosystems. I demonstrate that data collected in aquatic ecosystems can constrain vertical gross primary production fluxes in terrestrial ecosystems using a data assimilation approach. In combination, these chapters address critical questions about the drivers, quantity, and impacts of LCT within terrestrial, aquatic, and land C budgets, while providing a conceptual and process model framework that can be implemented to answer future questions about LCT.