Urban ecosystems present special considerations and challenges in researching and evaluating ecosystem functions and services. A case study of nitrate retention and loss in forested, urban wetlands illustrates these challenges. Water table dynamics, in situ nitrogen cycling rates, and nitrate concentrations and isotope values in throughfall and soil leachate were measured for 1 to 5 years in 9 to 14 palustrine, forested wetlands in northern
Ongoing research at the U.S. Environmental Protection Agency’s Urban Watershed Management Branch (UWMB) investigates the performance of engineered, green infrastructure/low impact development structures, including wetland mesocosms, swales, porous pavement, and rain gardens, in mitigating peak flows and pollutant levels in urban stormwater systems. Studies have shown heavy metal and phosphorus removal, but commonly used sandy media with high infiltration rates and low organic matter content could be a challenge for nitrate retention. Current research efforts focus on promoting organic matter hotspots and saturation at depth in rain garden media to facilitate nitrate removal via denitrification.
Results/Conclusions
Variability in hydrology, nitrogen cycling, and atmospheric deposition can, in some cases, be explained in the context of known ecological effects of urban land use and stormwater management practices. However, these stressors did not always exhibit the predicted effects, as demonstrated by dry and/or flashy water tables in less developed watersheds and denitrification rates that did not always reflect hydrological conditions. In this study, visual assessment of “alteration” did not successfully identify usable reference sites, a finding with implications for comparing urban data to baseline conditions and conducting rapid assessments of ecosystem functions in urban areas.
Studying the ecological functions of urban engineered structures can be an opportunity for promoting ecological literacy. If rain garden design specifications can be modified to improve nitrate removal capacity, these findings will be disseminated to managers and extension specialists who develop rain garden design manuals for governmental and residential bioretention systems. The green infrastructure systems at UWMB’s Urban Watershed Research Facility serve as demonstration projects visited by research scientists, graduate students, stormwater professionals, and legislative aides. This research provides an opportunity to introduce an ecological perspective to the design of engineered systems, thus promoting cross-disciplinary experiences, approaches, and understanding in the green infrastructure field.