Chloride and Nitrogen Dynamics in Urban Streams
Rebecca Hale and Peter Groffman

Chloride pollution from road salt and fertilizer use can have a major impact on the water quality of urban streams. We looked at debris dams in three streams within the Gwynn Falls watershed in Baltimore County to determine the effects of elevated chloride concentrations on nitrogen cycling in organic debris dams. Samples were collected from debris dams in two rural streams and one urban stream and incubated in polluted stream water and ambient stream water with chloride (80 mgL-1 and 2500 mgL-1) and nitrate (2 mgL- 1) additions. Samples were taken at 0, 5, 10, 20, and 30 days and analyzed for denitrification potential, potential net nitrification and N- mineralization, and microbial respiration. The effects of Cl- additions were immediately visible. Denitrification was highest in streams with high stream NO3- levels and was increased by NO3- additions in streams with low NO3-. This effect of NO3- on denitrification rates was moderated by the addition of Cl- in the rural stream. Denitrification in streams with a history of Cl- pollution was not affected by Cl- additions. In all streams Cl- additions stimulated nitrification through the lysing of cells and subsequent influx of organic N. Results suggest that Cl- pollution could have a net negative effect on stream N levels by increasing input of NO3- through nitrification and decreasing rates of N removal through denitrification.

Keywords: denitrification, nitrification, chloride, urban streams

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