Effects of stream restoration on denitrification in an urbanizing watershed

Increased delivery of nitrogen due to urbanization and stream ecosystem degradation is contributing to eutrophication in coastal regions of the eastern United States. We tested whether geomorphic restoration involving hydrologic "reconnection" of a stream to its floodplain could increase rates of denitrification at the riparian-zone-stream interface of an urban stream in Baltimore, Maryland. Rates of denitrification measured using in situ ¹⁵N tracer additions were spatially variable across sites and years and ranged from undetectable to >200 micrograms N·(kg sediment)⁻¹·d⁻¹. Mean rates of denitrification were significantly greater in the restored reach of the stream at 77.4 ± 12.6 micrograms N·kg⁻¹·d⁻¹(mean ± SE) as compared to the unrestored reach at 34.8 ± 8.0 micrograms N·kg⁻¹·d⁻¹. Concentrations of nitrate-N in groundwater and stream water in the restored reach were also significantly lower than in the unrestored reach, but this may have also been associated with differences in sources and hydrologic flow paths. Riparian areas with low, hydrologically "connected" streambanks designed to promote flooding and dissipation of erosive force for storm water management had substantially higher rates of denitrification than restored high "nonconnected" banks and both unrestored low and high banks. Coupled measurements of hyporheic groundwater flow and in situ denitrification rates indicated that up to 1.16 mg ${\rm{NO}}_{\rm{3}} ^{\rm{ - }} {\rm{ - N}}$ could be removed per liter of groundwater flow through one cubic meter of sediment at the riparian-zone-stream interface over a mean residence time of 4.97 d in the unrestored reach, and estimates of mass removal of nitrate-N in the restored reach were also considerable. Mass removal of nitrate-N appeared to be strongly influenced by hydrologic residence time in unrestored and restored reaches. Our results suggest that stream restoration designed to "reconnect" stream channels with floodplains can increase denitrification rates, that there can be substantial variability in the efficacy of stream restoration designs, and that more work is necessary to elucidate which designs can be effective in conjunction with watershed strategies to reduce nitrate-N sources to streams.