N processing within geomorphic structures in urban streams

Stream water often diverges from the main channel into sediments below the stream surface, gravel bars next to the stream, or organic debris dams in the middle of the stream. These geomorphic structures have the potential to support processes that produce or consume inorganic N (NH4+, NO3-) and thus affect streamwater quality. We measured production (potential net mineralization and nitrification) and consumption (denitrification potential, net immobilization) of inorganic N, respiration, and organic-matter content in sediments from geomorphic structures in 4 streams in and around Baltimore, Maryland, USA. We sampled sediments from stream pools, riffles, gravel bars (vegetated and nonvegetated), and organic debris dams in forested reference and suburban catchments, and also sampled degraded (incised channel) and restored reaches of one stream. Denitrification potential was highest in organic debris dams and organic-rich gravel bars-structures with high organic matter content. Organic debris dams in suburban streams had higher denitrification than debris dams in the forested reference stream, likely because of higher NO3- concentrations in suburban streams. These results suggest that denitrification in debris dams increases in response to high NO3- levels and that denitrification may be an important sink for NO3- in urban or suburban streams. However, such denitrifying structures as organic debris dams may be difficult to maintain in urban streams because of high storm flows and downstream displacement. Geomorphic structures in N-rich streams also supported higher rates of nitrification than structures in a forested reference stream, suggesting that these structures can become sources of NO3-. The ultimate effect of different structures on NO3- concentrations in urban streams will depend on the balance of these production and consumption processes, which is a complex function of a stream's ability to retain organic matter and resist hydrologic changes associated with urbanization and elevated NO3- levels.