Ecosystem restoration and denitrification in an urban stream.
Kaushal, Sujay*,1, Groffman, Peter, Findlay, Stuart, 1 Institute of Ecosystem Studies, Millbrook, New York, USA
ABSTRACT- Substantial removal of nitrate can occur in the riparian and hyporheic zones of small streams. Urbanization may lead to impairment of processes leading to this nitrate removal, however, by altering aboveground sources of carbon, and routing water to deeper flow paths away from denitrification hot spots in surface soil horizons. We have been quantifying denitrification in the riparian and hyporheic zones of restored and unrestored reaches of an urban stream using an in situ 15N tracer technique. Recovery of conservative SF6 tracer showed that this push-pull technique could be applied to a wide range of locations using an incubation time of 4 hours. Rates of denitrification in hyporheic wells were higher in the restored reach of the stream as compared to the unrestored reach across different seasons. In the restored reach, sandy riparian areas showed significantly higher rates of denitrification as compared to grassy riparian areas. Maximum rates of denitrification in the restored reach ranged from 100-112 micrograms N/kg soil/day. These high rates of denitrification were consistent with significantly lower concentrations of nitrate in hyporheic water in the restored reach relative to the unrestored reach throughout different seasons. Mean concentrations of dissolved organic carbon also differed between the two reaches and were higher in hyporheic and stream water from the restored reach. Activities of 8 enzymes in hyporheic wells suggested that there were seasonal differences in the functional diversity of microbial communities in both reaches. Considerable denitrification may occur in urban riparian and hyporheic zones after ecosystem restoration.
Key words: streams, urbanization, denitrification, hyporheic zones
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