Altres ajuts: the authors are grateful to the Spanish Ministry of Economy, Industry and Competitiveness (Spain) for the grant awarded to V. Arcas-Pilz (FPI-MINECO 2018) to the National Commission for Scientific and Technological Research (Chile) for the grant awarded to F. Parada (PFCHA-CONICYT 2018 - Folio 72180248); and to Universitat Autònoma de Barcelona for awarding a research scholarship to M. Rufí-Salís (PIF-UAB 2017).

Unidad de excelencia María de Maeztu MdM-2015-0552

Soilless crop production is a viable way to promote vertical agriculture in urban areas, but relies intensively on the use of mineral fertilizer. Thus, the benefits of fresher, local food and of avoided transportation and packaging associated to reduced food imports could be counteracted by an increase in nutrient-rich wastewater, contributing to freshwater and marine eutrophication. The present study aimed to explore the use of mineral fertilizer substitutes in soilless agriculture. Phaseolus vulgaris (common bean) was fertilized with the combination of slow-releasing fertilizer struvite (a source of N, P, and Mg) that is a by-product of wastewater treatment plants and inoculated with Rhizobium (a N2-fixing soil bacteria). The experiment included three bean production lines: A) 2g/plant of struvite and rhizobium inoculation, B) 5g/plant of struvite and rhizobium inoculation, both irrigated with a Mg, P and N- free nutrient solution, and C) control treatment irrigated with a full nutrient solution and no inoculation. Plant growth, development, yield and nutrient content were determined at 35, 62 and 84 days after transplanting, as well as the biological N2 fixation using the 15N natural abundance method. Treatments A and B resulted in lower total yields per plant than the control C (59.35± 26.4ga plant-1 for A, 74.2±23.0ga plant-1 for B and 147.71± 45.3gb plant-1 for C). For A and B, nodulation and N2 fixation capacity seamed to increase with the initially available struvite, but overtime reached deficient levels of Mg and close to deficient levels of P which could explain the lower yields. Nevertheless, we conclude that the combination of struvite and the N2-fixing bacteria covered N needs of the plant throughout the growth cycle. However, further studies are needed to determine optimal struvite quantities for vertical agriculture systems that can meet P and Mg requirements throughout the lifetime of the plant.