The nitrogen budget of an urban watershed: Zeekoevlei, Cape Town

Abstract

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 ecosystems, 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.