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Riverine nitrogen (N) export has decreased in forested and mixed land-use watersheds of the Chesapeake Bay (CB) in recent decades, but the factors driving these water-quality improvements are uncertain. This knowledge gap impedes the development of science-based strategies to project future changes in water quality. One factor that may explain these trends is reduced atmospheric N deposition, but existing data cannot address this hypothesis. Recent advances in the analysis of stable oxygen isotopes in streamwater nitrate provide an unparalleled opportunity to trace atmospheric nitrate and help account for its contributions to surface waters.
The goal of the proposed research is to assess how land use affects the amount of atmospheric nitrate that contributes directly to stream nitrate loads, which is highly responsive to Maryland Sea Grant's call for research in the area of "Resilient Ecosystem Processes and Responses". We hypothesize that retention of atmospheric nitrate varies based on the proportion of forest, agricultural, and urban lands in a watershed. We predict that atmospheric nitrate will be retained to a greater extent in predominantly forested watersheds where N is more limiting and therefore more likely to be rapidly processed and immobilized. In contrast, urban and agricultural settings provide opportunities for bypassing and incomplete nitrate removal because of greater N availability and alterations of hydrologic flow paths. Thus, reductions in atmospheric N deposition may lead to greater water-quality improvements in urban and agricultural, than forested, systems.
We propose to partner with the Maryland Department of Natural Resource's (DNR) Water Quality Monitoring Program to collect and analyze concentrations and delta 17 O values of nitrate in ~768 water samples. These samples will be collected across a spectrum of hydrological conditions from 12 gaged streams spanning gradients of forest, agricultural, and urban land-use. These data will be used in conjunction with atmospheric deposition and hydrological data to (1) quantify the proportion, load, and retention of atmospheric nitrate in streams and (2) assess the effects of land use on variations in export and retention of atmospheric nitrate.
Our outreach efforts are aimed at helping policy makers, land-use and natural resource managers, and high school teachers and students understand how land use and nutrient loading affect the export of N from watersheds. There is strong interest in quantifying the amount of atmospheric nitrate exported by different types of lands at national (e.g., US Environmental Protection Agency), regional (e.g., CB Program), and state (e.g., Maryland DNR) levels. We will share project results with members of each of these groups via workshops, seminars, and/or webinars. Additionally, our outreach efforts will give western Maryland teachers and their students the uncommon opportunity to apply science practices to investigate a real-world and locally relevant environmental issue. Building on data from this study, we will develop and provide additional curricular materials and training to expand an existing NOAA-funded student-teacher-scientist partnership, which will raise awareness and knowledge about the influence of land-use on retention and export of atmospheric nitrate.