Agronomic and Environmental Assessment of a Polyculture Rooftop Soilless Urban Home Garden in a Mediterranean City

Urban planning has been focusing its attention on urban rooftop agriculture as an innovative way to produce local and reliable food in unused spaces in cities. However, there is a lack of quantitative data on soilless urban home gardens and their contribution to self-sufficiency. The aim of the present study is to provide quantitative agronomic and environmental data on an actual soilless urban garden to estimate its degree of self-sufficiency and sustainability. For this purpose, an 18 m2 soilless polyculture rooftop urban home garden in the city center of Barcelona was analyzed. From 2015 to 2017, 22 different crops were grown to feed 2 people in an open-air soilless system, and a life cycle assessment was performed. A total productivity of 10.6 kg/m2/year was achieved, meaning that 5.3 m2 would be needed to fulfill the yearly vegetable requirements of an average citizen (in terms of weight). Considering the vegetable market basket of Catalonia, an 8.2 m2 soilless garden would be needed to cover 62% of the market basket for one person. The top 5 most productive crops were tomato, chard, lettuce, pepper and eggplant, accounting for 85.5% of the total production. The water consumption was 3.7 L/m2/day, and 3.3 kg/year/m2 of waste was generated. A high degree of self-sufficiency was achieved, although adjustments could be made to adapt the production to the market basket. The environmental assessment showed that the fertilizers and their associated leachates accounted for the highest environmental impacts in all the studied impact categories. Overall, 0.6 kg CO2 eq. was generated per kg of vegetables produced. The quantitative data provided by the present study offer a reference from which urban planners and researchers can project future implementations of rooftop urban agriculture (UA) on a large scale

Assessing the environmental behavior of alternative fertigation methods in soilless systems : the case of Phaseolus vulgaris with Struvite and Rhizobia inoculation

Unidad de excelencia María de Maeztu CEX2019-000940-MAltres 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 Universitat Autònoma de Barcelona for awarding a research scholarship to M. Rufí-Salís (PIF-UAB 2017)Urban agriculture, while being a promising solution to increase food sovereignty in cities, can lead to an unprecedented discharge of nutrient and fertilizer-related emissions into the urban environment. Especially relevant are nitrogen (N) and phosphorus (P), due to their contribution to marine and freshwater eutrophication. Therefore, alternative methods of fertilization need to be put into practice to avoid such impacts to the surrounding environment. Struvite, has been studied as a potential slow releasing fertilizer due to its high P content, while the bacteria rhizobium has been used to fix N directly from the atmosphere. Legumes, like the common bean are N-demanding crops capable of symbiosis with the bacteria rhizobium and have previously shown positive responses to fertilization with struvite. This study aims to analyze the performance of plant production in hydroponic systems combining rhizobium inoculation and struvite (2g, 5g, 10g, 20g) irrigated with a N and P deficient nutrient solution, as well as a life cycle analysis (LCA) to determine the possible impacts. The nutrient content of in- and out-going irrigation was analyzed as well as in plants and beans. The functional unit for the LCA was 1kg of fresh beans. The results obtained indicate a yield reduction of 60% to 50% in comparison to the control which was irrigated with a full nutrient solution. The impacts from operational stage are less in all impact categories, where most significant reductions up to 69% and 59% are seen in marine-eutrophication and global warming respectively. Although the infrastructure does not change between treatments, its impacts increase due to lower yields. We determine that below a 10% of conventional yield, the alternative systems have more impact than the use of conventional mineral fertilizers in almost all impact categories, thus pointing to the importance of infrastructure to truly reduce environmental impacts for urban agriculture

Mapping direct N2O emissions from peri-urban agriculture : the case of the Metropolitan Area of Barcelona

Unidad de excelencia María de Maeztu CEX2019-000940-MAltres ajuts: acords transformatius de la UABGeographically explicit datasets reflecting local management of crops are needed to help improve direct nitrous oxide (N2O) emission inventories. Yet, the lack of geographically explicit datasets of relevant factors influencing the emissions make it difficult to estimate them in such way. Particularly, for local peri-urban agriculture, spatially explicit datasets of crop type, fertilizer use, irrigation, and emission factors (EFs) are hard to find, yet necessary for evaluating and promoting urban self-sufficiency, resilience, and circularity. We spatially distribute these factors for the peri-urban agriculture in the Metropolitan Area of Barcelona (AMB) and create N2O emissions maps using crop-specific EFs as well as Tier 1 IPCC EFs for comparison. Further, the role of the soil types is qualitatively assessed. When compared to Tier 1 IPCC EFs, we find 15% more emissions (i.e. 7718 kg N2O-N year−1) than those estimated with the crop-specific EFs (i.e. 6533 kg N2O-N year−1) for the entire AMB. Emissions for most rainfed crop areas like cereals (e.g. oat and barley) and non-citric fruits (e.g. cherries and peaches), which cover 24% and 13% of AMB's peri-urban agricultural area respectively, are higher with Tier 1 EF. Conversely, crop-specific EFs estimate higher emissions for irrigated horticultural crops (e.g. tomato, artichoke) which cover 33% of AMB's peri-urban agricultural area and make up 70% of the total N2O emissions (4588 kg N2O-N year−1 using crop-specific EFs). Mapping the emissions helps evaluate spatial variability of key factors such as fertilizer use and irrigation of crops but carry uncertainties due to downscaling regional data to represent urban level data gaps. It also highlighted core emitting areas. Further the usefulness of the outputs on mitigation, sustainability and circularity studies are briefly discussed

Recovered phosphorus for a more resilient urban agriculture : assessment of the fertilizer potential of struvite in hydroponics

Altres ajuts: Acord transformatiu CRUE-CSICAltres ajuts: the authors are grateful to Universitat Autònoma de Barcelona for awarding a research scholarship to M. Rufí-Salís (PIF-UAB 2017), to the Spanish Ministry of Economy, Industry and Competitiveness (Spain) for the grant awarded to V. Arcas-Pilz (FPI-MINECO 2018), and to the National Commission for Scientific and Technological Research (Chile) for the grant awarded to F. Parada (PFCHA-CONICYT 2018 - Folio 72180248). A. Petit-Boix thanks the German Federal Ministry of Educa-tion and Research for thefinancial support of the research group"Circulus - Opportunities and challenges of transition to a sustainablecircular bio-economy", grant number 031B0018.Unidad de excelencia María de Maeztu CEX2019-000940-MUrban agriculture (UA) is a means for cities to become more resilient in terms of food sovereignty while shortening the distance between production and consumption. However, intensive soilless UA still depends on the use of fertilizers, which relies on depleting non-renewable resources such as phosphorous (P) and causes both local and global impact for its production and application. With the aim to reduce such impacts and encourage a more efficient use of nutrients, this study assesses the feasibility of using struvite precipitated from an urban wastewater treatment plant as the unique source of P fertilizer. To do so, we apply various quantities of struvite (ranging from 1 to 20 g/plant) to the substrate of a hydroponic Phaseolus vulgaris crop and determine the yield, water flows and P balances. The results show that treatments with more than 5g of struvite per plant produced a higher yield (maximum of 181.41 g/plant) than the control (134.6 g/plant) with mineral fertilizer (KPO4H2). On the other hand, P concentration in all plant organs was always lower when using struvite than when using chemical fertilizer. Finally, the fact that different amounts of struvite remained undissolved in all treatments denotes the importance to balance between a correct P supply to the plant and a decrease of P lost through the leachates, based on the amount of struvite and the irrigated water. The findings of this study show that it is feasible for UA to efficiently use locally recovered nutrients such as P to produce local food

Closed-loop crop cascade to optimize nutrient flows and grow low-impact vegetables in cities

Unidad de excelencia María de Maeztu CEX2019-000940-MAltres 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

Optimizing irrigation in urban agriculture for tomato crops in rooftop greenhouses

Unidad de excelencia María de Maeztu CEX2019-000940-MAltres ajuts: Acord transformatiu CRUE-CSICThe rise of population in urban areas makes it ever more important to promote urban agriculture (UA) that is efficient in terms of water and nutrients. How to meet the irrigation demand of UA is of particular concern in urban areas where water sources are often limited. With the aim of determining how to reduce water use for irrigation while maintaining productivity and reducing environmental impacts in UA, this study explores the agronomic performance and environmental life cycle impacts and benefits of three different fertigation management practices used in a rooftop greenhouse for tomato crop in Barcelona: 1) open management (OP); 2) recirculation (RC), in which 30% of the drained, unused water is used to irrigate the crop; and 3) the same recirculated management of RC with a further reduction in fresh water input of 15%(RR). Despite the recirculation and reduction of water and nutrients, all three irrigation management practices resulted in similar yields: 16.2, 17.9, and 16.8·kg·m−2 for OP, RC, and RR, respectively. In terms of water-use efficiency, RR management was the most efficient, requiring 48.7·liters·kg−1 of tomato, followed by RC (52.4·L·kg−1) and OP (75.2·L·kg−1). RR presented an improvement of 7% in water-use efficiency. In terms of environmental performance, RC had the best performance in almost all impact categories during the operational phase, especially in regard to marine and freshwater eutrophication, with 44% and 93% fewer impacts than OP due to the recirculation of nutrients and reduced nutrient loss through leachates. In terms of infrastructure, even though recirculation management requires additional equipment, the materials present better performance in the range from 0.2 to 14% depending on the impact category. This study can support evaluation of agricultural projects in the city, through yields and water consumption presented, incentivizing good practices aligned with the sustainability of UA

Improving the fertigation of soilless urban vertical agriculture through the combination of struvite and rhizobia inoculation in Phaseolus vulgaris

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-2015-0552Soilless 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

Extended use and optimization of struvite in hydroponic cultivation systems

Altres ajuts: Acord transformatiu CRUE-CSICUnidad de excelencia María de Maeztu CEX2019-000940-MHydroponic systems are an attractive form of urban agriculture due to their low weight load, inert substrate conditions, and overall better control of plant nutrition and growth. However, gaining urban food sovereignty cannot be at the cost of increasing environmental impacts, such as eutrophication and nonrenewable resource depletion, associated with phosphorus fertilizer use. Struvite, a wastewater byproduct, is a potential slow-releasing P source that can serve as a substitute for mineral P fertilizer. In this study, we explored the adequacy struvite in hydroponic systems, testing different quantities (5 g, 10 g and 20 g per plant) compared with monopotassium phosphate for pepper and lettuce hydroponic production. The results show competitive productions for both crops with the use of struvite, especially during the first lettuce harvest (225.5 g, 249.9 g, 272.6 g, and 250 g for 5 g, 10 g, 20 g and control, respectively) where a greater struvite dissolution was seen. Although all struvite treatments in pepper show low phosphorous content in the biomass, yields do not deviate greatly from the control (3.6 kg, 4.3 kg, 7.5 kg and 5.3 kg for 5 g, 10 g, 20 g and control, respectively). The environmental performance of all lettuce treatments showed a reduction in all impact categories, especially freshwater eutrophication and mineral resource scarcity, except for marine eutrophication. All impact categories were reduced for all pepper treatments with 10 g and 20 g of struvite. When the results are extrapolated to a full year of production, we find that the slow dissolution of struvite can sustain competitive production with an initial 20 g, with less impact in all categories except marine eutrophicatio

Agronomic and Environmental Assessment of a Polyculture Rooftop Soilless Urban Home Garden in a Mediterranean City

Unidad de excelencia María de Maeztu MdM-2015-0552Altres ajuts: Generalitat de Catalunya grant (FI-DGR 2016) and the Universitat Autònoma de Barcelona scholarship (PIF-UAB 2017)Urban planning has been focusing its attention on urban rooftop agriculture as an innovative way to produce local and reliable food in unused spaces in cities. However, there is a lack of quantitative data on soilless urban home gardens and their contribution to self-sufficiency. The aim of the present study is to provide quantitative agronomic and environmental data on an actual soilless urban garden to estimate its degree of self-sufficiency and sustainability. For this purpose, an 18 m2 soilless polyculture rooftop urban home garden in the city center of Barcelona was analyzed. From 2015 to 2017, 22 different crops were grown to feed 2 people in an open-air soilless system, and a life cycle assessment was performed. A total productivity of 10.6 kg/m2/year was achieved, meaning that 5.3 m2 would be needed to fulfill the yearly vegetable requirements of an average citizen (in terms of weight). Considering the vegetable market basket of Catalonia, an 8.2 m2 soilless garden would be needed to cover 62% of the market basket for one person. The top 5 most productive crops were tomato, chard, lettuce, pepper and eggplant, accounting for 85.5% of the total production. The water consumption was 3.7 L/m2/day, and 3.3 kg/year/m2 of waste was generated. A high degree of self-sufficiency was achieved, although adjustments could be made to adapt the production to the market basket. The environmental assessment showed that the fertilizers and their associated leachates accounted for the highest environmental impacts in all the studied impact categories. Overall, 0.6 kg CO2 eq. was generated per kg of vegetables produced. The quantitative data provided by the present study offer a reference from which urban planners and researchers can project future implementations of rooftop urban agriculture (UA) on a large scale

Identifying eco-efficient year-round crop combinations for rooftop greenhouse agriculture

Unidad de excelencia María de Maeztu CEX2019-000940-MPurpose: Rooftop greenhouses (RTGs) are agricultural systems that can improve the food supply chain by producing vegetables in unused urban spaces. However, to date, environmental assessments of RTGs have only focused on specific crops, without considering the impacts resulting from seasonality, combinations of crops and nonoperational time. We analyze vegetable production in an RTG over 4 years to determine the crop combinations that minimize yearly environmental impacts while diversifying food supply. Methods: The system under study consists of an integrated RTG (i-RTG) with a hydroponic system in Barcelona, in the Mediterranean region. By using life cycle assessment (LCA), we evaluate the environmental performance of 25 different crop cycles and 7 species cultivated during the period 2015-2018. Three functional units are used: 1 kg of edible fresh production, 1 unit of economic value (€) in the wholesale market and 1 kcal of nutritional value. The system boundaries consider two subsystems: infrastructure (greenhouse structure, rainwater harvesting system and auxiliary equipment) and operation (fertilizers and their emissions into water and substrate). In addition, we perform an eco-efficiency analysis, considering the carbon footprint of the crop cycles and their value at the wholesale market during their harvesting periods. Results and discussion: Spring tomato cycles exert the lowest impacts in all categories, considering all three functional units, due to the high yields obtained. In contrast, spinach and arugula have the highest impacts. Regarding relative impact, the greenhouse structure presented a large impact, while fertilizer production had notable relative contributions in tomato cycles. Moreover, nitrogen and phosphorus emissions from fertigation are the main causes of freshwater and marine eutrophication. By combining the most eco-efficient cycles, we can see that growing two consecutive tomato cycles is the best alternative with the functional unit of yield (0.49 kg CO2 eq./kg), whereas a long spring tomato cycle combined with bean and lettuce cycles in the autumn/winter is the best scenario when using market (0.70 kg CO2 eq./€) and nutritional value (3.18·10−3 kg CO2/ kcal). Conclusions: This study shows that increasing the diversity of the system leads to better environmental performance of greenhouse urban agriculture if suitable crops are selected for the autumn/winter season. The functional unit involving the economic value and the eco-efficiency analysis are useful to demonstrate the capability of the growing system to produce added-value vegetables under harsher conditions while categorizing and classifying the crops to select the most suitable combinations based on economic and environmental parameters