Abstract
Urban agriculture systems can significantly contribute towards mitigating the impacts of inefficient and complex food supply chains and increase urban food sovereignty. Moreover, improving these urban agriculture systems in terms of nutrient management can lead to a better environmental performance. Based on a rooftop greenhouse in the Barcelona region, we propose a cascade system where the leachates of a tomato cycle from January to July (donor crop) are used as the main irrigation source for five successive lettuce cycles (receiving crop). By determining the agronomic performance and the nutrient metabolism of the system, we aimed to define the potential of these systems to avoid nutrient depletion and mitigate eutrophication, while scaling the system in terms of nutrient supply between the donor and the receiving crops. The results showed that low yields (below 130 g per lettuce plant) are obtained if a cascade system is used during the early stage of the donor crop, as the amount of nutrients in donor’s leachates, specially N (62.4 mg irrigated per plant in the first cycle), was not enough to feed the lettuce receiving crop. This effect was also observed in the nutrient content of the lettuce, which increased with every test until equaling the control (4.4% of N content) as the leachates got richer, although too high electrical conductivity values (near 3 dS/m) were reached at the end of the donor crop cycle. Findings on the uptake of the residual nutrient flows showed how the cascade system was able to take advantage of the nutrients to produce local lettuce while mitigating the effect of N and P in the freshwater and marine environments. Considering our case study, we finally quantified the scale between the donor and receiving crops and proposed three major ideas to optimize the nutrient flows while maintaining the yield and quality of the vegetables produced in the receiving crop.
Highlights
Cities cover 3% of the Earth’s surface area, but host 55% of the world’s population (United Nations, 2014; SEDAC, 2016)
The low yields obtained in the receiving crop in the cascade system coincide with the findings by Muñoz et al (2012), who used tomato as both donor and receiving crop and Muñoz et al (2017), who used tomato as donor crop and lettuce as receiving crop
The present paper has presented an evaluation of a cascade system with a long-cycle tomato donor crop and five successive cycles of lettuce
Summary
Cities cover 3% of the Earth’s surface area, but host 55% of the world’s population (United Nations, 2014; SEDAC, 2016). The global nutritional demand concentrates in urban areas, resulting in long and complex supply chains. Similar to conventional agricultural systems, nutrient discharge resulting from intensive fertilizer use is problematic due its impact on water eutrophication (Muñoz et al, 2008; Torrellas et al, 2012; Romero-Gámez et al, 2014, 2012; Boneta et al, 2019). Closing the nutrient cycles in soilless systems maintains the utility and value of scarce resources (Bocken et al, 2017), produces a regenerative effect on the environment (Ellen MacArthur Foundation and McKinsey Center for Business and Environment, 2015), and contributes to a reduction in water and fertilizer consumption (Carmassi et al, 2005; Rufí-Salís et al, 2020)
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