Title: Linkages among Land Use, Riparian Zones, and Uptake and Transformation of Nitrate in Stream Ecosystems

Land use alters the physical and biological structure of stream ecosystems and potentially alters their capacity to process nitrogen (N), an essential nutrient that has nearly doubled in abundance on the biosphere during the past century from human activities. In this dissertation, I quantified uptake and transformation of nitrate (NO3-) in small ( ≤ third-order) streams and related these dynamics to aquatic ecosystem processes, including primary production and organic matter decomposition, and attributes of riparian zone structure and vegetation composition. I also analyze patterns of stream NO3-processing among three classes of adjacent land use practices (forest, agriculture, and urban). In Chapter 2, ambient rates of NO3- uptake and transformation were measured with 24-hr releases of 15N-labeled NO3- in nine stream reaches in the Willamette River Basin of western Oregon during summer low flow (July – August). Three reaches each were surrounded by forested, agricultural or urban land use. After standardizing reaches to a 500-m length,