Are natural biogeochemical processes important in urban ecosystems?
Peter M. Groffman, Richard V. Pouyat, Lawrence E. Band, Kenneth T. Belt, Neely L. Law, Gary T. Fisher
In this presentation we use data on natural and anthropogenic pools and fluxes of nitrogen (N) in watersheds and streams in the Baltimore metropolitan area to test the hypothesis that natural biogeochemical processes such as mineralization, nitrification and denitrification are insignificant compared to anthropogenic processes such as atmospheric deposition, food consumption, sewage and fertilization in urban watersheds. Data were taken from long-term monitoring of stream exports and terrestrial processes, studies of instream retention processes and nitrogen input/output budget analyses conducted as part of BES. In contrast to our expectations, natural processes appear to be quite important in the urban and suburban watersheds of Baltimore. For example, the magnitude and variability of mineralization and nitrification are much greater than atmospheric deposition (50 versus 8 – 20 kg N ha-1 y-1), more than 70% of atmospheric and fertilizer inputs are “retained” in watershed soils and vegetation, and organic debris dams in urban streams function as a negative feedback on nitrate levels, with denitrification increasing in response to stream nitrate concentrations. These results suggest that natural processes are important, and manageable, controllers of stream ecology and water quality in urban and suburban watersheds.
Keywords: biogeochemistry, watersheds, nitrogen

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