Isotopes And Pollen: What Can They Tell Us About Land Use History And Nitrogen Source To Wetland Biota?

Emily M. Elliott, Department of Geography & Environmental Engineering, Johns Hopkins University
Grace S. Brush, Department of Geography & Environmental Engineering, Johns Hopkins University

Ecosystem health is routinely monitored using nutrient concentrations in streamwater. In particular, the concentration of nitrate and ammonium in streamwater can be used to estimate nutrient fluxes, construct nitrogen budgets, and assess watershed nitrogen retention capacity. However, consistent monitoring records rarely span more than 30 years. For that reason, most monitoring records do not cover the peak period of nitrogen oxide emissions (~1970), extensive fertilizer application (post-1950s), or the height of land clearance (~1860-1920). Moreover, monitoring nitrate or ammonium concentration does not provide information regarding the actual source of the nitrogen to surface waters.

This research investigates the use of nitrogen isotopes as indicators of nitrogen source to terrestrial wetlands, over extended temporal and spatial scales. Sediment cores extracted from wetlands throughout the Chesapeake Bay watershed are used to reconstruct changes in sedimentation rates, land cover, and nitrogen source to wetland biota since European settlement (~300 years). The 15N of bulk sedimentary organic matter is used to indicate the relative importance of various nitrogen sources (e.g., human or animal waste, atmospheric deposition, biological nitrogen fixation). Changes in land use and sedimentation rates are reconstructed using alynology and historical records. Preliminary results indicate that this method can be used to elucidate the relative influence of multiple nitrogen sources, including animal and human waste effluent, atmospheric deposition, and biological nitrogen fixation.

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