Nitrogen cycling in urban wetlands

Background/Question/Methods

Urban land use change has been shown to have dramatic effects on watershed hydrology, with particularly dramatic effects on wetlands.  Human settlements require drainage which results in widespread deliberate wetland loss, while at the same time, there is deliberate wetland construction to treat stormwater in urban areas.  There is also widespread incidental loss of riparian wetlands due to degradation of urban stream channels, as well as incidental creation of wetlands associated with a variety of urban activities. Joan Ehrenfeld was one of the world’s leading researchers on urban wetlands, applying a powerful combination of approaches from plant, soil, microbial and hydrologic sciences to generate novel insights about these important ecosystems.  In the Baltimore Ecosystem Study (BES), one of two urban long-term ecological research (LTER) projects funded by the U.S. National Science Foundation, urban wetlands have been studied with several approaches, building on ideas developed by Professor Ehrenfeld.  A major focus has been on how geomorphic changes in stream channels have altered the ability of riparian wetlands to serve as “sinks” for reactive nitrogen (a prime cause of eutrophication in coastal waters) and on how stream restoration and stormwater infrastructure can reverse these effects. 

Results/Conclusions

Long-term monitoring of riparian water tables in Baltimore has shown that geomorphic changes in stream channels, especially incision, have led to lower water tables, drier soils and altered nitrogen cycling in urban and suburban riparian wetlands.  These drier soils support higher rates of nitrification (a source of reactive nitrogen) and lower rates of denitrification (a sink for reactive nitrogen). Stream restorations that increase surface or subsurface connectivity between the stream and hyporheic and riparian zones have the potential to restore riparian denitrification in urban watersheds.  Creation (deliberate or incidental) of floodplain wetlands is a particularly promising approach as these ecosystems contain sediments with high denitrification potential and can process and hold significant amounts of water. Stormwater detention basins also have potential to replace riparian denitrification functions lost to urbanization.  Sediments in these basins can have higher denitrification potential than natural riparian zones and they are engineered to have direct connections to upland reactive nitrogen sources, enhancing their capacity to serve as nitrogen sinks.