Poster: Geomorphic Controls on Carbon and Nitrogen Processing in a Degraded Urban Stream
Paul Mayer, Elise Striz, Edward Doheny, Robert Shedlock, and Peter Groffman

Elevated nitrate levels in streams and groundwater pose human and ecological threats. Microbial denitrification removes nitrate from groundwater, but requires anaerobic (saturated) conditions and adequate supply of dissolved organic carbon (DOC) from detritus and organic soils. Conditions favorable for denitrification may be under geomorphic control. For example, stream incision due to urbanization may cause a disconnection between the stream channel and adjacent floodplain, thereby inhibiting denitrification and/or promoting nitrification. We examined the effects of stream geomorphology and stream restoration on stream geochemistry. Bioreactive N (nitrate+nitrite) concentration in the hyporheic zone was inversely related to DOC, suggesting that processes that remove nitrogen from groundwater (e.g. denitrification) were limited by carbon availability. DOC decreased with depth in the hyporheic zone, suggesting that the source of DOC is allochthonous. Examination of soil cores in the hyporheic zone indicates that some stream features possess and/or retain more total carbon and thus, may serve as hot spots for denitrification. Bioreactive N concentration in the groundwater was inversely related to C:N ratios in the soil, further suggesting that nitrogen in the groundwater is influenced by C availability. Bioreactive N concentrations in the stream were lowest in a drought year, suggesting that lower base flow, which increases residence times of water in the system, promoted conditions (e.g. lower dissolved oxygen, prolonged mixing with dissolved organic carbon) that removed N from the stream. Preliminary analyses of post-restoration conditions in this stream indicated that certain restoration techniques (e.g. bank re-shaping) may promote nitrate removal by re-establishing the connection between the stream and the floodplain. N concentrations are lower in the restored stream reach than in the degraded reach. Additionally, the relationship between between N and DOC, or N and dissolved oxygen are highly variable in the degraded reach, suggesting that incised streams cannot efficiently process N or respire C.


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