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.