| Cities have expanded in conjunction with rapid population growth, often at the expense of agricultural lands. Simultaneously, Earth's biogeochemical cycles have been altered by human activity. Concentrations of fixed nitrogen (e.g., nitrate, ammonium, and NOx) have been intentionally and unintentionally increased by human practices including synthetic fertilizer application and fossil-fuel combustion. Yet ecosystem services of nitrogen retention are threatened as ecosystem structure is altered by human activities. The research presented here addresses the question: how do the hydrologic and geomorphic structure of a stream affect patterns of nitrogen availability and retention? Research was conducted in Indian Bend Wash (IBW), a rapidly urbanizing watershed of Scottsdale, AZ.; Patterns of urbanization over the past half century were quantified using aerial photography. Changes in the mainstem of IBW were particularly marked. A series of artificial lakes was constructed in the low-flow channel, and adjacent floodplain sediments were stabilized with turf grass. Canals and groundwater pumping allowed the creation of a perennial reach in what was historically a dry wash. Fluctuations in proportion of groundwater and floodwater entering the lake series drove variations in the concentrations of nitrogen and phosphorus, causing phytoplankton growth to alternate between phosphorus and nitrogen limitation. A mass balance of the flux of nitrogen through the lake chain and its adjacent floodplain indicated that a significant proportion of the nitrogen was not exported. Experiments demonstrated that denitrification was the primary retention mechanism in floodplain soils while storage, presumably in sediments, was more important in lakes.; This study demonstrates that urbanization, by creating novel linkages between patches, can unintentionally increase nitrogen concentrations, disrupting fundamental processes like primary production. Although they are human-designed to serve other functions (e.g., flood management), artificial lakes and turf floodplains can act as important sites of denitrification, suggesting a possible means for managers to control nitrogen loading to recipient ecosystems. However, this potential is limited, and whereas the proportion of nitrogen retained by the stream-floodplain complex was large, the massive influx of nitrogen overwhelmed the ecosystem's nitrogen removal capacity and its ability to prevent export of significant quantities of nitrogen. |
