Journal
LANDSCAPE AND URBAN PLANNING
Volume 227, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.landurbplan.2022.104540
Keywords
Nitrogen; Urban; Watershed; Wetlands; Denitrification
Categories
Funding
- Alliance for Collaboration on Climate and Earth System Science (ACCESS) SeasCape project
- NRF [114696]
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Urban watersheds receive excessive nitrogen loads and the capacity to remove this excess nitrogen depends on the functioning of riparian and wetland ecosystems. This study provides a preliminary quantification of nitrogen flux in a small urban watershed and highlights the important role of wetland systems in removing nitrogen.
Urban watersheds receive excessively high Nitrogen (N) loads predominantly via N deposition, fertilizer use, stormwater runoff, treated effluent discharge and sewage leaks. Beyond engineered solutions, the capacity of these systems to remove this excess N is largely dependent upon the functioning of riparian and wetland eco-systems, which in urban areas are highly modified, fragmented and increasingly under pressure. Appropriate management of urban watersheds needs to consider N flux and process rates that occur within the urban landscape. This research provides a preliminary quantification of the relative magnitudes of N flux in the context of a small urban watershed comprising key features of cities in the Global South: a mosaic of informal and formal residential, peri-urban agriculture and stressed service delivery. For a watershed system of 88 km2, total N inputs amount to 2.82 Gg N yr(-1), 78 % of which is in the form of household food. N outputs amount to 2.35 Gg N yr(-1) with 86 % of that coming out of wastewater streams either as treated effluent or during the denitrification process. An estimated 0.17 Gg N yr(-1) accumulates in the system as plant biomass, in groundwater, soils and landfill. The Zeekoevlei wetland system is hypothesised to remove 76 % of total N which drains off the urban landscape, indicating an important service provided to the City, in addition to other well documented ecosystem services. In this urban system, our estimates indicate N flux rates almost 3 times higher than what would be here naturally in a nutrient-deplete Fynbos system. This work daylights important monitoring and research needs in order to better understand the role of the urban landscape and wetland ecosystems in a highly human-mediated N cycle.
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