CAREER: Yin and yang - is there a balance between nitrogen fixation and denitrification in riverine ecosystems?

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Increased nitrogen loading from human activities degrades water quality and threatens aquatic biodiversity. It has long been assumed that as nitrogen loading to rivers increases, nitrogen production in the stream via biological nitrogen fixation (the conversion of atmospheric nitrogen gas to biologically available nitrogen) would cease. At the same time nitrogen loss via denitrification (the conversion of available nitrogen back to nitrogen gas) would increase, causing a net reduction in nitrogen in rivers and streams. Yet, recent work in estuaries and lakes suggests that high rates of both nitrogen fixation and denitrification can co-occur across a range of nitrogen loadings, challenging this widely-held assumption. This research will provide fundamental understanding about the response of rivers to anthropogenic nitrogen enrichment and will enable better management of nitrogen cycling at regional and global scales. This work also seeks to create a more ecologically-informed citizenry by integrating ecosystem ecology techniques into GK-12 and undergraduate education. The construction of a mobile analytical lab to facilitate sampling streams across the United States will be accomplished through a unique collaboration among engineering and biology-oriented undergraduates and faculty at Michigan Tech. The mobile lab will also be used to design new laboratory exercises for an undergraduate ecology course, support student research projects, and facilitate outreach to GK-12 students in the western Upper Peninsula of Michigan.This study will apply new analytical techniques and new stream models in the context of observational and experimental manipulations of nutrient loading to test hypotheses on the balance between nitrogen fixation and denitrification across gradients of nitrogen loads in streams and rivers at the continental scale. The study will address three fundamental questions: (1) Do N fixation and denitrification co-exist in streams across a natural range of nitrogen concentrations, as they do in other aquatic ecosystems?, (2) Do "hot spots" and "hot moments", i.e., spatial and temporal heterogeneity, facilitate this co-existence?, and (3) Do increasing nutrient loads shift the balance to favor losses via denitrification? The study will capitalize on the National Ecological Observatory Network (NEON) as a continental-scale experimental framework to test the hypotheses through three interacting research elements: a gradient study of N2 fixation and denitrification (years 1-4), a multi-scale spatial and temporal study to identify hot spots and hot moments of N2 fixation and denitrification (years 2-5), and an experimental study of nutrient enrichment effects on N2 fixation and denitrification in streams (years 4-5). The project will use MIMS as a new analytical technique along with acetylene reduction and acetylene block assays to measure N fixation and denitrification, and to couple these measurements with reach-scale models commonly used to estimate stream metabolism to estimate denitrification and N2 fixation.