Methane Fluxes In Human-Dominated Ecosystems: From Pastures To Cities

The biosphere regulates Earth's climate through the exchange of greenhouse gases between ecosystems and the atmosphere. Human activities have altered terrestrial ecosystems to a substantial degree, with consequence to greenhouse gas exchange rates and climate. Anthropogenic impacts to ecosystem methane (CH4) fluxes are poorly understood and require further study, as CH4 emissions are growing and the causes are uncertain. This dissertation explores CH4 fluxes from subtropical cattle pastures and cities to better understand greenhouse gas exchange in human-dominated ecosystems. These disparate systems are globally relevant because pastures are the most common, and cities are the fastest growing, land use. Chapter One describes CH4 fluxes from subtropical cattle pastures in south Florida using a variety of observational approaches. We found that highly variable emissions from the flooded landscape were the dominant CH4 source and, unexpectedly, cattle-emitted CH4 was a minor component of ecosystem emissions. Chapter Two further investigates the hydrologic controls of pasture CH4 fluxes using eddy covariance measurements and laboratory manipulations. We found that flooding of a thin horizon of surface organic soil drove pasture CH4 fluxes, suggesting that small changes in pasture water table dynamics could induce large changes in emissions. Chapter Three assesses the impact of water management, and consequent CH4 emissions, on pasture greenhouse gas budgets by combining eddy covariance observations with analysis of water retention data from south Florida pastures. Subtropical pastures were net CO2 sinks, but strong greenhouse gas sources when accounting for CH4 emissions, and water retention practices are likely responsible for only a minor component of pasture and regional greenhouse gas emissions. Chapter Four assesses the impact of natural gas use on greenhouse gas emissions from Ithaca, New York, using mobile surveys and atmospheric monitoring of CO2 and CH4 concentrations and isotopes. Pipeline CH4 leakage rates in Ithaca were low, likely due to a well-maintained pipeline system, but we observed clear signs of natural gas leakage and combustion from the atmospheric monitoring site. This result, combined with spatial analysis, demonstrates a natural gas heat and electricity cogeneration plant is a significant emissions source in Ithaca. In all, this dissertation demonstrates that human-dominated ecosystems play an important role in terrestrial CH4 exchange and that management choices influence ecosystem greenhouse gas emissions