Nitrate in Urban Riparian Zones

Evan Grant, Baltimore Ecosystem Study
Peter Groffman, Institute of Ecosystem Studies

Riparian zones are a critical interface between the terrestrial and aquatic components of a watershed which filter non-point source pollutants from entering streams. These zones may or may not be important sites of nitrate removal in urban systems given 1) the extensive disturbance of natural vegetation and 2) low water tables, due to stream channel incision from high runoff volume and velocity. Previous studies have focused on nitrate in groundwater in agricultural systems. A comparison was made between a forested watershed with a high water table (30 cm mean depth) and three sites with low water tables (70-80 cm mean depth) within urbanized watersheds. Soils were sampled at 5 depths. At each depth, we extracted inorganic N, and measured nitrification, mineralization, respiration and denitrification in a 10-day laboratory assay. Denitrification potential and soil organic matter content were also quantified. The urban sites had less mineralizable carbon, and lacked the organic layer that is present at the forested reference site. No detectable nitrate was found in the soil profile at the forested site, but all urbanized sites had high nitrate throughout the soil profile. Nitrification in the soil profile at the urban sites was high, but was non-existent at the forested site. Denitrification was low and did not differ among sites. In addition, the denitrification potential was lower in the forested reference site than in the urbanized sites, supporting the hypothesis that low denitrification is not responsible for high urban nitrate concentrations in the soil profile. Therefore, the urban sites most likely have high nitrate because of the low water tables, which creates an aerobic soil profile, and leads to higher nitrification rates.

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