Abstract
Soilless crop production is a viable way to promote vertical agriculture in urban areas, but it relies extensively on the use of mineral fertilizer. Thus, the benefits of fresher, local food and avoiding the transportation and packaging associated with food import could be counteracted by an increase in nutrient-rich wastewater, which could contribute 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 a combination of slow-releasing fertilizer struvite (a source of N, P, and Mg), which is a byproduct of wastewater treatment plants, and inoculation with Rhizobium (a N2-fixing soil bacteria). The experiment included three bean-production lines: (A) 2 g/plant of struvite and rhizobial inoculation; (B) 5 g/plant of struvite and rhizobial inoculation, both irrigated with a Mg-, P-, and N-free nutrient solution; and (C) a control treatment that consisted of irrigation with a full nutrient solution and no inoculation. Plant growth, development, yields, and nutrient contents were determined at 35, 62, and 84 days after transplanting as well as biological N2 fixation, which was determined using the 15N natural abundance method. Treatments A and B resulted in lower total yields per plant than the control C treatment (e.g., 59.35 ± 26.4 g plant–1 for A, 74.2 ± 23.0 g plant–1 for B, and 147.71 ± 45.3 g plant–1 for C). For A and B, the nodulation and N2 fixation capacities appeared to increase with the amount of initially available struvite, but, over time, deficient levels of Mg were reached as well as nearly deficient levels of P, which could explain the lower yields. Nevertheless, we conclude that the combination of struvite and N2-fixing bacteria covered the N needs of plants throughout the growth cycle. However, further studies are needed to determine the optimal struvite quantities for vertical agriculture systems that can meet the P and Mg requirements throughout the lifetime of the plants.
Highlights
From 1950 to 2018, the population living in urban areas grew more than fourfold to an estimated 4.2 billion people
Experimental Site, Materials, and Growth Conditions. This experiment was conducted in the Rooftop Greenhouse Laboratory (RTG-Lab) of the Environmental Science and Technology Building (ICTA-UAB), which is located in the Universitat Autònoma de Barcelona Campus (42◦29 24 E, 45◦94 36 N) (Sanjuan-Delmás et al, 2018)
Bean seeds were treated with a commercial product (e.g., Nadicom GmbH©) which contained a mixture of Rhizobium phaseoli and Rhizobium giardinii strains for inoculation before planting
Summary
From 1950 to 2018, the population living in urban areas grew more than fourfold to an estimated 4.2 billion people. Vertical agriculture relies extensively on the use of mineral fertilizer, which results in nitrates and phosphate being discharged into wastewater, which can contribute to freshwater and marine eutrophication (Anton et al, 2005; Gopalakrishnan et al, 2015; Sanjuan-Delmás et al, 2018) This extensive use of mineral fertilizers affects the environment but can be related to a high cost of production and extraction, as is the case for nitrogen fertilizers due to the Haber-Bosch process (Cherkasov et al, 2015) and for phosphorous due to phosphate rock extraction (Cordell and White, 2013). The high energy cost of synthetic nitrogen production and the ever-depleting sources of phosphate rock, when added to the environmental cost of their disposal and emissions to water and air (Rufí-Salís et al, 2020a,b), necessitates the search for alternatives to further implement these technologies in a sustainable way
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