Increased delivery of nitrogen due to land-use change has contributed to increased coastal eutrophication. Improved knowledge of nonpoint sources and transformations of nitrogen will be critical in managing the nitrogen cycle in response to interactive land-use and climate change. We investigated effects of land use, hydrologic conditions, and watershed restoration strategies on nitrogen sources and transformations in watersheds of the Baltimore Ecosystem Study Long-term Ecological Research (LTER) site in Baltimore, Maryland. Our major questions were related to how sources of nitrate and organic nitrogen in streams change with increasing land-use change and streamflow variability. In order to investigate this, we used a combination of long-term measurements of streamwater chemistry, watershed mass balance estimates, and analyses of N and O isotopes in nitrate and C and N isotopes in particulate organic matter.
Results/Conclusions
Results showed that discharge-weighted mean annual N concentrations in urbanizing watersheds decreased with increasing impervious surface coverage. Isotopic N and O signatures of nitrate in a forest stream were within the range typically reported for nitrification, and there were 2:1 linear increases in σ 15N and σ 18O in agricultural and low-residential catchments suggesting denitrification, primarily during baseflow conditions. Although mass balance estimates indicated that atmospheric deposition and lawn fertilizer were major inputs of N in urbanizing watersheds, elevated σ 15N and stable σ 18O indicated that leaks from septic systems and aging sanitary infrastructure were dominant outputs. Contributions of wastewater N vs. atmospheric sources to streams increased with increasing storm intensity due to “flushing” emphasizing the importance of hydrologic flowpaths. Our results suggest that managing nitrogen inputs near streams and below the rooting zone may play a disproportionately large role in regulating watershed exports. Examples of enhancing denitrification sinks at the groundwater-stream interface and at the larger stream network scale will also be discussed. Accurately identifying N sources will be critical in minimizing ecological and economic risks associated with watershed restoration and stormwater management strategies.