30 Minute Presentation
Spatial and Temporal Dimensions of Nitrogen Cycling in Urban Stormwater Bioretention Facilities
Urban development is transforming landscapes at unprecedented rates. Human activities and landscape modifications associated with urbanization extensively increase hydrologic demands and modify natural hydrologic systems; consequently, population growth occurring in urban areas increases pressure on water resources. Urban aquatic ecosystems are vulnerable to impacts associated with increased connectivity with urban surfaces and hydrologic changes that initiate long-term changes in receiving waterbodies. Nitrogen (N) loading from urban and suburban catchments to receiving surface waters can lead to impairment of aquatic ecosystems and is a concern in many cities with water quality issues. To improve urban water quality, cities are increasingly adopting the use of bioretention facilities (BRFs), systems that are designed to imitate natural hydrological and ecological processes, in an attempt to mitigate adverse impacts of urban hydrology on developed sites and provide additional ecosystem services. Among the desired functions of BRFs, nutrient cycling and pollutant removal are important services for water quality.
While many ecological functions of BRFs remain poorly understood, there is growing interest among researchers in examining the capacity of BRFs to provide N removal processes. Denitrification is of particular interest as it is the only permanent pathway for ecosystem N removal. High potential for N removal via denitrification and other N cycling processes has been observed, however, there have been limited on-the-ground assessments of how N cycling processes in BRFs vary across different seasonal conditions and regional climates. The objective of this study was to provide a detailed assessment of soil N process rates and variability in BRFs across seasons and geographic contexts in the United States. We present a 3-part survey of BRFs across the country in Portland, OR, Baltimore, MD, Charlotte, NC, New York City, NY, Phoenix, AZ, and Syracuse, NY in which we examine influences of soil drainage processes, seasonal conditions, and regional context on soil N cycling.
The results from this study showed that seasonal variability was not a primary influence on N cycling rates. N cycling rates showed some variability across regions, but signatures of the soil ecology in individual BRFs were similar across regions and distinct from natural riparian areas. These results also suggest that there may be important tradeoffs in the ecosystem services provided by BRFs, and that designers should consider the priorities of the stormwater management programs in order to achieve a balance between these tradeoffs. This study is one of several that examines potential N cycling in BRFs, but it extends the temporal and spatial dimensions of this body of research, showing that the potential of N cycling processes in BRFs does not change significantly across seasonal conditions but may be impacted by design and maintenance decisions across regions. Ultimately, providing hydrologic ecosystem services, such as high infiltration rates, and important nutrient cycling services like denitrification may require a balance between soil drainage rates that support stormwater volume mitigation while providing substantial enough retention times to support pollutant removal processes.