.|  Baltimore Ecosystem Study
Processing of nitrogen in urban streams
An important change resulting from urbanization is an increase in impervious surface cover, which affects the hydrology of urban areas. Impervious surfaces increase surface runoff and flood peak discharges and decrease infiltration and groundwater recharge. The engineering of stream channels with channels and storm drainage systems compound the effects of impervious surface cover by directing surface runoff directly into streams. These increased flows have dramatic effects on stream channel geomorphology widening and deepening channels and eroding stream banks (Figure 1). High flows and peaks can also lead to stream scouring, reducing the presence of large woody debris in urban streams. Large woody debris are important to the formation and maintenance of organic debris dams which obstruct stream flow and function as "hot spots" of nutrient cycling in streams. The systematic degradation of urban streams has come to be referred to as "urban stream syndrome" (Walsh et al. 2005).

Figure 1. Conceptual diagram showing streams and associated riparian zones in forested and urban watersheds. High runoff during storm events leads to incised stream channels in urban watersheds, which in combination with reduced infiltration in impervious urban uplands, can lead to reduced riparian groundwater levels.

The ability of streams to convert inorganic nitrogen (N) into organic forms or N gas is an important function of these ecosystems that is strongly affected by urban stream syndrome. Inorganic N, especially nitrate is of concern as a drinking water pollutant and as a cause of eutrophication in coastal waters. Watershed-scale strategies to control N pollution problems focus on reducing sources (e.g., fertilizer, sewage) and/or increasing "sinks" of inorganic N. Sinks are defined as areas and/or processes that prevent the movement of inorganic N to receiving waters and include uptake by autotrophs, immobilization by heterotrophs, and denitrification, the anaerobic conversion of nitrate into N gas. Because different components of stream ecosystems can function as either sources or sinks for inorganic N, BES research has focused on understanding N processing in different stream features and how these features are affected by environmental factors, natural and human disturbance, and ecosystem management.
One approach to analysis of stream N sink and source dynamics is to consider different stream features, characterize their sink or source potential and then quantify their importance in the overall flow of water. While most water in a stream flows in its main, open channel, stream water often diverges from the main channel into sediments below the stream, gravel bars next to the stream or organic debris dams in the middle of the stream. Analysis of C and N dynamics in these features is useful for characterizing their potential to produce and consume inorganic N. An ecosystem-scale assessment requires coupling this analysis with hydrologic information on the amount of water that passes through the different features. Consideration of the natural and anthropogenic factors that influence the genesis and maintenance of different features, e.g., flow regime, riparian vegetation, geologic substrate) is an additional necessary component of these studies.

Figure 2. Denitrification potential (top) and organic content (bottom) of stream features sampled in four streams in the Baltimore metropolitan area in summer 2002. Values are mean (standard error) of 2 - 8 replicates of each feature. Bars with different superscripts are significantly different at p < 0.05 in a one-way analysis of variance with a Duncan's Multiple Range test. From Groffman et al. (2005)

In several studies, we have measured denitrification potential, potential net mineralization and nitrification, respiration and organic matter content in sediments from features in streams in and around Baltimore (Groffman et al. 2005, Hale and Groffman 2006, Harrison et al. 2012). Denitrification potential is consistently highest in organic debris dams and organic-rich gravel bars - features with high organic matter content (Figure 2). Organic debris dams in suburban streams have higher denitrification than debris dams in the forested reference stream, likely due to the higher nitrate content of the suburban streams. This result suggests that debris dam denitrification increases in response to high nitrate levels and that this process may be an important "sink" for nitrate in urban or suburban streams. It is important to note however that high denitrifying features (e.g., organic debris dams) are difficult to maintain in urban streams with high storm flows. Features in N rich streams also supported higher rates of nitrification than features in the forested reference stream suggesting that source/sink nitrate dynamics in urban stream features will be a complex function of organic matter, water level and N loading factors.
We have also found that the high salt levels in urban streams may have a negative effect on denitrification (Hale and Groffman 2006). In laboratory microcosms, DEA in debris dam material from a forested reference stream was increased by nitrate additions. However, chloride additions constrained the response of DEA to nitrate additions in material from the forested stream, but had no effect on DEA in material from streams with a history of high chloride levels. Chloride additions changed the sign of net N mineralization from negative (consumption of inorganic N) to positive in debris dam material from the forested reference stream, but had no effect on net mineralization in material from streams with a history of exposure to chloride. Understanding the factors regulating the maintenance and N cycling activity of organic debris, and incorporating them into urban stream management plans, could have important effects on N dynamics in suburban watersheds.
Literature Cited
Groffman, P. M., A. M. Dorsey, and P. M. Mayer. 2005. N processing within geomorphic structures in urban streams. Pages 613-625.
Hale, R. L. and P. M. Groffman. 2006. Chloride effects on nitrogen dynamics in forested and suburban stream debris dams. Journal of Environmental Quality 35:2425-2432.
Harrison, M. D., P. M. Groffman, P. M. Mayer, and S. S. Kaushal. 2012. Microbial biomass and activity in geomorphic features in forested and urban restored and degraded streams. Ecological Engineering 38:1-10.
Walsh, C. J., A. H. Roy, J. W. Feminella, P. D. Cottingham, P. M. Groffman, and R. P. Morgan. 2005. The urban stream syndrome: current knowledge and the search for a cure. Journal of the North American Benthological Society 24:706-723.
This research was supported by funding from the NSF Long-term Ecological Research (LTER) Program. This material is based upon work supported by the National Science Foundation under Grant No. 1027188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.