The effects of nitrogen availability on the response of terrestrial ecosystems to elevated carbon dioxide

Abstract

Human activities are increasing the concentration of carbon dioxide (CO2) in the atmosphere, warming the planet. Terrestrial ecosystems currently sequester about a quarter of human CO2 emissions, slowing climate change. The principal mechanism believed to be responsible for this is an increasing rate of plant growth (that is, “CO2 fertilization”). However, the fate of this ecosystem service is uncertain, as it has been proposed that soil nitrogen (N) availability will limit plants’ capacity to continue absorbing increasing quantities of CO2. Whether N will limit the CO2 fertilization effect in the future will determine the rate at which human CO2 emissions will accumulate in the atmosphere, thereby influencing the climate. In this thesis, I have collected and synthesized the large body of information about the N limitation of CO2 fertilization, using data from experiments in which atmospheric CO2 concentration is manipulated. I have found that the hypothesis that the increase in the strength of the CO2 fertilization effect will be eliminated by restricted N availability is simplistic. Based on the experimental data available, I have found evidence supporting a mechanism by which plants under elevated CO2 can acquire additional N in exchange for carbohydrates via symbiotic fungi. Using this framework, I have quantified the magnitude of the terrestrial CO2 fertilization effect on plant biomass worldwide, and identified the areas of the global land mass that could potentially experience a greater enhancement in biomass under elevated CO2. I propose a framework and areas of further research that may help models better simulate the interactions between the carbon and nitrogen cycles under elevated CO2 using a plant- economics approach, in which nitrogen is a resource that can be acquired by plants in exchange for energy.