Nutrient recovery strategies for sustainable urban agriculture in soilless and open-field systems

Global food systems face increasing pressure from population growth, land scarcity, soil degradation, and the need to reduce greenhouse gas emissions. Urban agriculture can contribute to more sustainable food production by shortening supply chains, yet it remains highly dependent on synthetic fertilizers with substantial environmental impacts. Nutrient recovery from organic waste streams offers a circular alternative, but its agronomic and environmental performance in urban and peri-urban systems remains insufficiently quantified. This dissertation evaluates the feasibility of recovered nutrients—struvite, compost, and ramial chipped wood (RCW)—as substitutes for mineral fertilizers in soilless and soil-based horticultural systems. Two contrasting experimental contexts were investigated. First, long-term hydroponic tomato production was analysed in the ICTA-UAB integrated rooftop greenhouse (i-RTG), assessing yield, resource use, and environmental impacts using Life Cycle Assessment. Second, field experiments in the peri-urban agricultural area of El Baix Llobregat evaluated the effects of RCW, compost, and struvite on spinach and sweet potato yield, soil properties, and gaseous emissions (CO₂, CH₄, N₂O, and NH₃). Results show that granulated struvite can fully replace mineral phosphorus and magnesium and partially substitute nitrogen in hydroponic tomato systems without compromising yield, fruit quality, or consumer acceptance, while reducing environmental impacts when applied at optimal rates. In soil-based systems, compost and struvite maintained high productivity, while RCW enhanced soil organic carbon stocks and significantly reduced N₂O emissions, performing best when combined with nitrogen-fixing crops. Overall, recovered nutrients can effectively replace mineral fertilizers in urban and peri-urban horticulture, supporting productive, resilient, and environmentally sustainable food systems.