Unidad de excelencia María de Maeztu CEX2019-000940-M

Altres ajuts: the authors are grateful to Universitat Autònoma de Barcelona for awarding a research scholarship to MR-S (PIF-UAB 2017), the Spanish Ministry ofEconomy, Industry and Competitiveness (Spain) for the grant awarded to VA-P (FPI-MINECO 2018), and the National Commission for Scientific and Technological Research (Chile) for the grant awarded to FP (PFCHA- CONICYT 2018 - Folio 72180248). This work was supported by the "María de Maeztu" program for Units of Excellence in R&D [CEX2019- 000940-M]. AP-B thanks the German Federal Ministry of Education and Research for the financial support of the research group "Circulus - Opportunities and challenges oftransition to a sustainable circular bio-economy," grant number 031B0018. This work was supported by and ERC Consolidator grant awarded to Gara Villalba (818002-URBAG).

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.