Urbanization impacts nitrogen and organic matter quality along different US streams
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Along urban streams and rivers, various processes, including road salt application, sewage leaks, and weathering of the built environment, contribute to novel chemical cocktails made up of metals, salts, nutrients, and organic matter. Common sources and in situ processes can lead to co-mobilization of nitrogen and changes in organic matter with elevated salinity, which can have broader implications for ecosystem health. To improve our understanding of these interactions, we investigated how salt, nitrogen, and organic matter varied: 1) along nine rural-to-urban flowpaths across five major U.S. cities, 2) seasonally over time at four intensive sites in Washington, D.C. and Baltimore sites, and 3) during a road salting event at 3 intensive Washington, D.C. sites. In the smaller watersheds studied (area < 900 km2), changes in organic matter indices (such as BIX, HIX, and the Coble’s peak T to C ratio) indicated that organic matter tends to become more microbially-derived downstream except at Scotts Level Branch, where nitrogen concentrations are elevated at the start of the flowpath. Across all sites, principal component analysis identified two principal components that explain 68.7% of the variance and revealed that total dissolved nitrogen, BIX, and FI along with salt-derived ions were positively correlated with one another, while HIX and SUVA254 were negatively correlated. These results suggested that labile organic matter and nitrogen increase with elevated salinity along stream flowpaths. Seasonally, sodium concentrations peak during road salting events, while fluxes relate to seasonal discharge. During a road salting event, changes in total dissolved nitrogen, dissolved organic carbon, and organic matter properties appear to be controlled by changes both in salinity and discharge. Overall, these results provide new insights into the interactions among in-stream salts, nitrogen, and organic matter.