A multi-scale assessment of the coupling between the nitrogen cycle and the terrestrial carbon sink under global change

Abstract

It is now unequivocal that the main driver of greenhouse gas (GHG) accumulation in Earth's atmosphere over the industrial era has been due to anthropogenic global change. Despite the recent and rapid rise in GHGs, global terrestrial ecosystems have slowed the rate of carbon dioxide accumulation in the atmosphere, and thus the rate of climate change, through the uptake and storage of carbon (C) in plant biomass and soil organic matter. However, the global cycles of C and nitrogen (N) are tightly coupled through primary production and the turnover of soil organic matter, highlighting a stoichiometric relationship that is critical to understanding the future stability and strength of the terrestrial C sink. Thus, our ability to understand the response of the terrestrial C sink to global change hinges on critical -- but often overlooked -- feedback with N cycling. In this dissertation, I examine how different aspects of global change and land management practices are impacting the terrestrial C sink by evaluating interactions with the N cycle in both managed and natural ecosystems. I use observational and experimental methods to quantitatively assess C and N dynamics at plot, landscape, and global scales. I focus this work on crop- and range-lands of the Upper Missouri River Basin in the United States, and in equatorial mountain forests across the globe to address three overarching research questions: 1. What are the controls over the soil-atmosphere gas exchange of greenhouse gases, and how do they change in response to management and vegetation cover? 2. How do changes in plant community composition influence ecosystem C and N dynamics? 3. How is global change altering the stability and rate of C and N accumulation and storage?