Riparian ecosystem function in urban watersheds (presentation)

Peter M. Groffman
Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545
Phone:(845) 677-5343
E-mail: groffmanp@ecostudies.org
FAX: (845) 677-5976

Kamau Crawford
Tuskegee University, Box 5472, Tuskegee AL 36088
Phone: 334-725-5263
E-mail: kamaucrawford@hotmail.com

Lawrence W. Band
Department of Geography, UNC-Chapel Hill, Chapel Hill, NC 27514
Phone: 919-962-3921
Fax: 919-962-1537
E-mail: lband@email.unc.edu

Riparian ecosystems have been shown to prevent the movement of pollutants from upland land uses to streams in many areas. This function is dependent on the capacity of riparian vegetation and microbial communities to intercept and process pollutants moving in surface runoff and/or groundwater flow.

We measured denitrification, an anaerobic microbial process that converts nitrate (a drinking water pollutant and cause of eutrophication in coastal waters) into nitrogen gas, and a series of soil variables (soil moisture, organic matter content, inorganic N) in four rural and four urban riparian zones in the Baltimore metropolitan area. Two of the riparian zones were forested and two had herbaceous vegetation in each land use context. There were no differences between urban and rural and grass and forest riparian zones, but variability was much higher in urban than rural sites (Figure 1). There were strong positive relationships between soil moisture (Figure 2) and organic matter content (Figure 3) and denitrification potential. The high variation in the urban context was caused by the presence of one very wet and three very dry sites.

The results suggest that urban riparian sites can have significant denitrification, if soils are wet. However, urban riparian zones are often characterized by dry soils and low water tables due to increases in upland surface runoff and downcutting of streams associated with increases in impervious surfaces in urban watersheds. These factors can cause urban riparian zones to be hydrologically isolated from adjacent uplands and streams and therefore to have lower levels of pollutant removal functions.

Figure 1. Denitrification potential in four urban and four rural riparian zones in the Baltimore metropolitan area, July 1998. There were two forested and two herbaceous sites in each land use context. There were no significant (p < 0.01) differences between urban and rural or forested versus herbaceous sites. Variability was much higher among the urban sites due to higher variability in water table depth and soil moisture content, i.e. one of the urban sites was very wet and three sites were very dry.

Figure 2. Denitrification potential versus soil moisture. in four urban and four rural riparian zones in the Baltimore metropolitan area, July 1998. There were two forested and two herbaceous sites in each land use context. Coefficient of determination (r2) = 0.55, p < 0.01.

Figure 3. Denitrification potential versus soil organic matter content in four urban and four rural riparian zones in the Baltimore metropolitan area, July 1998. There were two forested and two herbaceous sites in each land use context. Coefficient of determination (r2) = 0.68, p < 0.01.