Nitrogen fluxes and dynamics in an urban watershe

Nitrogen fluxes have been reported in urban ecosystems in recent years. Little is known about N fluxes from areas with intense fertilization/irrigation, reclaimed water irrigation, or no irrigation, which are also common to an urban city. The main purpose of this dissertation was to identify major N sources in an urban watershed, the Lake Alice watershed on the campus of the University of Florida, and how N fluxes changed before they reached the receiving water, as well as the flow path in which the source N reached to the headwater. It can help land managers to prioritize the controlling N sources and make decisions for N management. This dissertation was comprised of five parts. First, a literature review provided background information about N cycling in global and regional (urban) scale, urban N management practices and introduction to in situ nitrate monitoring devices. Then N fluxes from three small urban catchments of different land uses were determined and compared regarding different management practices from each catchment. Sports Field Catchment (SFC) with intense fertilization/irrigation was found to produce the greatest N load (37 kg yr-1) compared to Reclaimed Water Irrigated Catchment and Control Catchment (with no irrigation). In addition, baseflow N was dominating in the N fluxes from SFC, indicating regular fertilization/irrigation practices were the major drivers for the great N load. Thirdly, spatial changes in N fluxes from catchments to basins in urban streams were investigated to determine the major N contributor to Lake Alice watershed, and how hydrology drove the spatial changes of NO3-N concentrations was discussed. The results suggested that sports fields could be the largest N contributor to Lake Alice watershed and the streams delivering flow from sports fields could produce the greatest N load to Lake Alice. A dilution effect happened along the flow paths of urban streams to Lake Alice, which was a sink for N although the amount of N load may be insufficient to cause eutrophication or limit the reproduction of biomass. Fourthly, a comparison between high resolution in situ Submersible Ultraviolet Nitrate Analyzers (SUNAs) and autosamplers was made to determine the more effective approach in characterization storm events. It showed that SUNAs exhibited a better performance than autosamplers as they could capture more information during the storms than autosamplers, providing immediate signals for NO3-N response to hydrological changes. Finally the relationship between NO 3-N concentrations and discharge was examined by SUNAs. There was a significant relationship between NO3-N concentrations and discharge. NO3-N concentration changes during storms were clockwise loops, suggesting immediate dilution effect. It can also be inferred from SUNA's curves that the increasing NO3-N concentrations after storms were attributed to NO3-N-rich groundwater rather than surface runoff.