Channel complexity and nitrate concentrations drive denitrification rates in urban restored and unrestored streams

Stream restoration projects constructed in urbanizing watersheds provided an opportunistic setting to determine how channel bedform influenced nitrate removal by denitrification. We measured denitrification rates in streambed sediments seasonally at eight streams in North Carolina, USA to characterize the physicochemical drivers of nitrogen transformations in restored urban streams. Mean denitrification rates were highly variable (97 ± 58 to 585 ± 214 μmol m−2 h−1) and most significantly influenced by nitrate concentrations; baseflow nitrate concentrations ranged from 0.06 to 1.07 mg/L. A multiple linear regression approach also revealed the importance of channel complexity, water depth and temperature, which together with nitrate concentrations explained 60% of the variability among sites and seasons. Controlling for nutrient limitation, we observed significantly greater denitrification rates in more geomorphically complex streams, particularly near grade control structures and in deep pools. However, these trends were not consistently observed across all sites, which suggest the added influence of watershed scale drivers (e.g., urban hydrology) as well as reach scale drivers including the design, construction and age of the restoration project.