Urbanization effects on nitrogen dynamics in the critical zone
Urbanization alters the structure and function of the critical zone by changing the way that water moves across the landscape and affecting connections between different components of the landscape. Addition of impervious surfaces to urban watersheds decreases groundwater recharge and increases the energy and amount of stormflow to streams. These changes alter the coupling between uplands, riparian zones and streams and the capacity of these areas to cycle and retain nitrogen moving across the landscape. Urbanization effects on critical zone coupling between landscape components and nitrogen cycling has been a major theme of the National Science Foundation funded urban Long Term Ecological Research (LTER) project in Baltimore, the Baltimore Ecosystem Study. Long-term data, experiments and models have been used to analyze how changes in critical zone structure alter ecosystem functions related to nitrogen cycling and ecosystem services important to water quality in receiving waters. These analyses are of great interest in the nitrogen-sensitive waters of Chesapeake Bay which receives drainage from the Baltimore metropolitan area.
Long-term monitoring has shown reductions in water table levels in riparian zones in urban compared with forested reference watersheds. Urban riparian zones are drier and support lower rates of denitrification, an anaerobic microbial process that consumes reactive nitrogen and higher rates of nitrification, an aerobic process that produces reactive nitrogen. These changes can convert riparian zones from sinks to sources of reactive nitrogen in the urban landscape. In streams, urban storm runoff degrades stream banks and reduces the presence of geomorphic features such as organic debris dams and pools that have significant potential for denitrification. Efforts to restore urban watersheds, riparian zones and streams have the capacity to reverse these effects by restoring connections between uplands, riparian zones and streams. Upland interventions can reduce the amount and energy of runoff and stream/riparian restorations can recreate interactions between streams and riparian zones that facilitate denitrification. The success of these efforts depends on multi-disciplinary analysis crossing biophysical and social sciences.