Nature-Based Solutions for Agriculture in Circular Cities: Challenges, Gaps, and Opportunities
1.1. Advantages and Challenges in the Contribution of Urban Agriculture towards Circularity in Cities
1.2. What Does Circularity Imply for Urban Agriculture?
1.3. The Objectives of This Study
- Define the input and output (I/O) streams, analyzing the inputs (I) necessary for the operation and the outputs (O) generated by UA related NBS (hereinafter UA-NBS);
- Summarize the main circularity aspects that are relevant for UA; and
2. Materials and Methods
- How do the NBS_u, NBS_i, and S_u contribute to food and/or biomass production?
- Which NBS_u, NBS_i, and S_u are relevant to UA?
- How is food and biomass production (UCC5) related to the other UCC?
- What are the main I/O streams of UA-NBS?
- What are the key opportunities and challenges for achieving circularity in UA?
2.1. Identification of Nature-Based Solutions Relevant for Urban Agriculture
- Evaluation of food and biomass production (UCC5): From a list of fifty-one NBS_u and NBS_i and ten S_u proposed by Langergraber et al.  and based on their contribution to the UCC5 , a separate evaluation regarding food and/or biomass production was made for each NBS_u/i and S_u. To have a more accurate categorization adapted to the UCC5, the rating was based on the relevance of either food or biomass inputs (I) or outputs (O) (Table 1). Thus, the proposed categories were as follows (Table 1):
- Food and/or biomass production with relevant I and/or O: UA-NBS whose main purpose is food and/or biomass production or that, due to its characteristics, produce a relevant amount of food and/or biomass and/or consume it for their operation;
- Usable for food and/or biomass production: UA-NBS that may produce food and/or biomass, even if it is not their primary purpose, contributing to the UCC5; and
- Food and/or biomass production with no relevant production levels: these UA-NBS can produce plant material or food in small quantities. They are considered as potential contributors that can be scaled up or designed for that purpose.
- Urban agriculture-related NBS-composed list: The NBS_u/i and S_u considered relevant for food and/or biomass production were those addressing, contributing, and/or potentially contributing to the food and/or biomass production (UCC5).
- Classification according to typologies and urban space (implementation): The NBS_u/i related to UA were grouped according to the type of urban space they are associated with: (A) as urban blue infrastructure (urban water); (B) as green infrastructure (GI) in buildings (including containers); (C) as GI on buildings; (D) as GI for parks and landscape; and (E) as GI for the urban farm. NBS_u/i can be located in one or multiple urban spaces. The classification was based on the defining characteristics of the NBS, the expert knowledge of workshop participants, and literature references.
- Selection of representative UA-NBS: To narrow the list and focus on food and biomass production (UCC5), eight UA-NBS were selected as relevant representatives to assess the I/O streams and identify circularity challenges. The selection was made according to the available references, considering that all typologies and urban spaces were covered, and upon the experience of the participants in the workshops. In order to gather information on the selected UA-NBS, a literature search was carried out using the names and synonyms given in Langergraber et al. .
2.2. Linkages between Food and Biomass Production and Other Urban Circularity Challenges
2.3. Urban Agriculture-Related Nature-Based Solutions Circularity: Input and Output Streams
2.4. Identification of Key Challenges and Opportunities of Agricultural Nature-Based Solutions in Circular Cities
3. Results and Discussion
3.1. Nature-Based Agricultural Solutions for Food and Biomass Production towards Urban Circularity
3.2. Relevance of Nature-Based Solutions Related to Urban Agriculture to Address the Fifth Urban Circularity Challenge
- Wall-based green facade (14): Wall-based green facades, as “Vertical Greening Systems”, are known for their ability to mitigate urban heat island (UHI) effect and to enhance building energy savings in the urban environment, e.g., increasingly, the possibilities for crop production and wastewater treatment, particularly greywater [39,40,41]. They mostly consist of a modular vertical support structure with vegetation, substrate, irrigation, and drainage systems. Depending on the purpose of the system, different plants are used, with low-maintenance plants being the most common option to minimize costs. This NBS_tu can produce ornamental plants (low maintenance) as well as horticultural crops. When designed for food production, they are generally used for self-consumption and local supply (e.g., restaurants, schools, or hospitals) . The yield depends on the crop/plant, type of substrate, management, irrigation and drainage systems, and the climate and orientation when it is placed outdoors. Indoor, wall-based green facades under controlled conditions at buildings or greenhouses are mostly used to produce high-yielding crops. In order to address circularity, it is relevant to characterize drainage water, which can be reused since it is rich in nutrients. Additionally, wall-based green facades can be designed as modular treatment systems when irrigated with wastewater, resembling constructed wetlands, where plant matter can be harvested and used as biomass [39,43].
- Intensive green roof (18): Green roofs can be used to cultivate agricultural products, and their importance for this purpose has increased in recent years, as they provide additional land space in urban centers [44,45]. Intensive green roofs are characterized by a substrate depth between 15 and 70 cm, which requires more maintenance than extensive ones and allows for a wider choice of plants . As an engineered structure, a green roof requires prefabricated materials to be constructed, such as protection and drainage layers, substrate, etc. Such structures may be built in residential buildings but also in commercial ones. For example, a supermarket in Brussels, Belgium (Delhaize chain), has a 360 m2 urban farm on the rooftop for greenhouse and open-air vegetables, with a certified organic production system . The aim is to control the production system and sell the products in the supermarket on the ground floor, avoiding transportation and the need for a cold chain. The residual heat from the refrigeration systems is used to heat the greenhouse, improving energy efficiency (UCC6). Since the farm is small, and the impact is thus limited, it serves as a demonstrator of possibilities for professional UA.
- Green corridors (37): According to Castellar et al. , green corridors aim to re-naturalize areas along derelict infrastructure, such as railways or along waterways and rivers, by transforming them into linear parks. Green corridors can play an important role in urban GI networks and can offer shelter, food, and protection for the urban wildlife while enabling migration from one green patch to another. Back-up irrigation may be provided by reclaimed wastewater, and the biomass produced can be used for energy generation and composting. As for the vegetation planted, it depends on the site and the objectives set. Forest species, fruit trees, and fruiting shrubs or ornamental species are generally used. Lisbon (European Green Capital 2020) is an example of a network of nine green corridors that are part of the urban GI. They cover an area of about 200 ha and contribute to ecological connectivity, create spaces for UA, revalorize abandoned spaces by increasing soil permeability, and improve the connection to other NBS specialized in rainwater retention and infiltration . Other cities, such as Montreal, Mexico City, Seoul, London, or Singapore, also have green corridors that provide ecosystem services to the city . At each site, this NBS_su is adapted to the local context, from the use of plant species and the reuse of the available resources to the using of space according to social needs.
- Hydroponic and soilless technologies (45): In hydroponics, plants grow in water containing necessary macro- and micronutrients that are supplied by mineral fertilizers dissolved in water according to the plant-specific recipe. In ebb and flow systems and in grow beds, the plants grow in different media, like mineral-/rockwool, vermiculite, sand, gravel, etc., which also offer mechanical support [4,50,51]. Other soilless technologies, such as nutrient film technique, aeroponics, and deep flow technique, do not involve media. Recently, soilless technologies are being innovated by implementing artificial intelligence to learn the best way of composition of synthetic, mineral, or organic fertilizers to grow the crop, often together with artificial light in greenhouses or plant factories.
- Organoponic/Bioponics (46): In contrast to hydroponic that relies on mineral fertilizers, bioponics is an emerging soilless technology for nutrients recovery that links (organic) vegetable production to organic effluent remediation or organic waste recycling . The plants in growing media derive nutrients from natural animal, plant, and mineral substances that are released by the biological activity of microorganisms . Bioponics allows closing nutrient cycles by using organic waste streams, such as urine , biogas digestate , chicken manure [52,56], and others, thus reducing the use of mineral fertilizers and the greenhouse gas emissions. Aquaponics  could also be considered as a form of bioponics, as it utilizes waste streams (process water, sludge) from an aquaculture. Synonyms used for bioponics are “organic hydroponic” [58,59], digeponics , or anthroponics . Beside the source of nutrients, the key difference between organic and conventional soilless culture is the active promotion of microorganisms in bioponics to enhance nitrification, mineralization, and disease suppression and thus contribute to productivity and plant quality similar to soil-based systems .
- Aquaponic farming (47): Aquaponics is a technology that couples tank-based animal aquaculture with hydroponics by using water from aquaculture for plant nutrition and irrigation. Trans-aquaponics extends this technology to tankless aquaculture and/or non-hydroponic plant cultivation. Aquaponic farming comprises both aquaponic types, whereby aquaponic farming does not imply a specific size but the fact that such this generic NBS_tu can embody both aquaponics types . The NBS_tu can be established in very different setups: while aquaponics is often implemented as controlled environment agriculture, trans-aquaponics includes, e.g., pond-aquaponics [63,64], outdoor aquaponics [65,66], aquaculture using constructed wetland for sludge removal , and other integrated aqua-agriculture systems  that exploit the aquaponics principle. Both technologies are often used for food production, but aquaponics cannot be eco-certified because it exploits hydroponics and is thus not soil-based, a precondition for eco-certification—at least in the European Union. However, it is possible to meet a large city’s demand for tomatoes, fish, and lettuce through aquaponic production, as shown in a case study related to Berlin .
- Productive garden (49): Productive gardens are found around the world and contribute significantly to food security. Vegetables, fruits, herbs, and, occasionally, small livestock are produced in reduced spaces for the market, private consumption, or educational purposes. The productivity of urban gardens depends on climate conditions and type of crops and can exceed that of rural farms ; if correct cultures are selected and machine-based crop treatment technologies are replaced by manual work, it results in higher cropping density and higher biodiversity of crops to be grown together . Different types of cultivation can be selected for horticultural crops, both in open fields and/or under cover.
- Urban farms and orchards (51): Urban farms and orchards are part of the city’s GI and are intended for food and biomass production. They are large enough to grow cereal crops, fruit and vegetables, and even big livestock . This NBS_su can seek an economic profit or have social and educational purposes. It is common to find urban farms located in public areas and managed by a community (e.g., neighborhood). While other NBS_u are more specific, with a defined configuration, this unit encompasses a wide range of possibilities that make it very versatile. It is the NBS_su that most resembles the rural farms, with the advantage of having the urban streams nearby to tap into. For example, food waste—which has a high nutritional value—can be used to feed animals and lower the production costs; on the other hand, treated water can be used for irrigation . Within its boundaries, several other NBS can be implemented to close loops, such as composting (23) or constructed wetlands for wastewater or runoff water treatment for on-farm resource recovery and reuse.
3.3. Interfaces between Food and Biomass Production and the Other Six Urban Circularity Challenges
- “Restoring and maintaining the water cycle (by rainwater management)”—UCC1: Several NBS_u/i and S_u identified as relevant for the UCC5 also address the UCC1. The nature of the NBS_u/i, with a significant vegetation component and located in different urban spaces, such as the UA-NBS classified as “Vertical Greening Systems and Green Roofs” and “(Public) Green Space”—e.g., green corridors (37) and large urban parks (40) —, enable the restoration and maintenance of the water cycle at different scales. These NBS_u/i facilitate processes, such as water retention, infiltration, transport, treatment, and evapotranspiration . The UA-NBS_su from the category of “Food and Biomass Production”, i.e., productive garden (49), urban forest (50), and urban farms and orchards (51), are also relevant for the UCC1, as they enable the same processes as the above. The implementation of these UA-NBS_u is seen as an opportunity to regulate the water cycle and not a barrier to be overcome in the sector of UA.
- “Water and waste treatment, recovery, and reuse”—UCC2: NBS_u/i and S_u addressing the UCC2 are crucial for UA, as water is a continuous input stream to most UA-NBS. In general, a minimum quality is required to use reclaimed water for irrigation and fertigation. In addition, some UA-NBS may require a certain quality depending on the crop or culture. Furthermore, the effluent water from UA-NBS, e.g., aquaculture (44) and photo bio reactor (48), needs to be treated, and for this purpose, other NBS_u/i and/or S_u, such as circular systems like aquaponic farming (47), can be implemented .
- “Nutrient recovery and reuse”—UCC3: Nutrient recovery, reuse, and recycling is key to achieving a circular metabolism of cities [31,72]. For this purpose, it is necessary to identify and analyze the nutrient-rich flows generated in the city, such as wastewater or organic waste. Urban agriculture harnesses the recovered nutrients and keeps them in the urban system. Besides, the NBS_u and S_u from the category of “Remediation, Treatment and Recovery” [6,7] (cf. Table 2) comprise anaerobic treatment (26), phosphate precipitation (for P recovery) (S3), and ammonia stripping (for N recovery) (S4), and they are not considered as relevant for food and biomass production because they do not generate food and/or biomass to a significant extent nor require it to operate. However, they may be crucial for nutrient recovery from urban streams to be used in UA-NBS. In addition, the recovered nutrients must be able to meet the needs of crops or living organisms, considering the macro- and micronutrients required for production. It is therefore seen as both a challenge and an opportunity to recover and reuse nutrients.
- “Material recovery and reuse”—UCC4: Material recovery is seen as an opportunity for UA. Urban agriculture can provide a considerable amount of biomass that can be used for several purposes, e.g., building materials, soil amendment, or energy production. For example, biochar/hydrochar production (S6) and composting (23), classified as S_u and NBS_is, respectively (“Remediation, Treatment and Recovery”), can be obtained from the biomass produced in vertical greening systems and agricultural waste. Biodegradable materials, such as wood, can be used directly to build structures. One challenge would be to replace stable insulating materials, such as plastic and glass, or materials used in irrigation pipes. This could be accomplished by using recovered and/or recycled materials.
- “Energy efficiency and recovery”—UCC6: Mitigation of UHI effect is one of the strengths of UA-NBS in urban outdoor spaces such as infrastructure, i.e., NBS_u/i and S_u located in/on buildings, in parks and landscape, and/or urban farms. At the building scale, green roofs or vertical greening systems can improve energy efficiency by reducing rooftop and walls’ surface temperature during summer, improving insulation and decreasing heat losses during the cold season . On the other hand, NBS_u that include greenhouses or are located indoors may require energy to regulate room temperature and to provide artificial lighting. However, high-yield crops or indoor urban vertical farming using hydroponics and soilless technologies (45) can substantially increase energy efficiency [73,74]. In addition, within the urban system, there is the possibility of recovering heat sources for food and biomass production that would otherwise be lost.
- “Building system recovery”—UCC7: An urban system is multi-stakeholder and space-constrained; therefore, the essential planning to achieve circularity is challenging. Both the design of new spaces and the retrofitting and adaptation of old ones require planning for the effective implementation of the NBS_u/i. By using UA-NBS, urban spaces can be revalorized, although the complexity of the urban system makes it a challenge for food and biomass production, as there are different ownerships, available spaces, and regulations to consider. Achieving circularity may require new approaches.
3.4. Contribution of Input and Output Streams to Urban Circularity in Nature-Based Agricultural Solutions
- Biomass and Living organisms (cf. Table 3): Biomass refers to the total mass of all living organisms in an area. In a circular city, that means all organic materials derived from produced plant mass together with all microorganism and animals, important in a CE point of view . Biomass is an important resource for technologies like pyrolysis—conversion of biomass to biochar—heat transfer , Fe2/biocarbon composite derived from a phosphorous-containing biomass , and several other biomass-derivate methods. Biomass concerns to materials including soil conditioners, such as wood chips or biochar; organic fertilizers, such as manure or compost; different types of organic waste, ranging from food waste to crop residues or pruning residues; and to organic crop-protection products (Table 3, Figure S2). Cultivation of plants, mushrooms, and insects may positively influence the air and soil quality. Plants take up essential nutrients from the soil; however, they can also absorb metals like lead (Pb), cadmium (Cd), arsenic (As), tin (Sn), chromium (Cr), and nickel (Ni). This makes certain plants, together with other living organisms, effective phytoremediators .
- Water: Irrigation water is required whenever precipitation is not sufficient. Using tap water may lead to competition with other urban users ; therefore, alternative water sources should be preferred. These could be subterranean water, stored rainfall water, or treated wastewater. Urban agriculture provides an opportunity to reuse (waste)water wherever it is generated, as opposed to rural agriculture, because there are no or less costs associated with transport. The use of water is minimized in soil-independent production systems with a closed circuit for water, as exemplified by Rufí-Salís et al. , who found daily water savings up to 40% for such systems. However, soilless systems mostly require higher energy inputs .
- Nutrients: Nutrient-rich urban waste for the primary production can be recycled from wastewaters of different provenance, e.g., domestic wastewater, urine, feces, greywater; wastewaters from food production, e.g., milk, tea, coffee, brewery; and nutrient-rich solid waste streams, e.g., composting, biogas, biochar. The nutrient-rich streams usually need to be subjected to one or several stages of treatment before use in UA. As Jurgilevich et al.  pointed out, the demand for nutrients, especially phosphorous (P), is growing drastically faster than the human population. This is coupled to large nutrient losses on one side  and increasing global nutrient imbalance  on the other. While the soils of rich countries accumulate nutrients, the soil in developing countries experience P deficit . Schoumans et al.  argued that the European P cycle could be completely closed if imported chemical P fertilizers were replaced by P fertilizers recovered from waste streams.
- Energy: Energy flows can be optimized, too. Mohareb et al.  proposed co-location strategies of agricultural operations and waste streams in order to increase energy efficiency; this is the sixth urban circularity challenge (UCC6) proposed by Atanasova et al.  on “Energy efficiency and recovery” that is mainly addressed. Such a strategy can be, for instance, to locate greenhouse food production next to waste heat or waste nutrient sources, such as from biogas or refrigeration equipment. Another possibility would be using phase-change technologies  to mediate between the locations that emit waste heat and locations that require heat, therefore obliterating the need for close proximity of these operations.
3.5. Challenges of Circular City Resource Flows
3.6. SWOT Analysis of Urban Agriculture Related Nature-Based Solutions
- Weaknesses identified in UA-NBS are the lack of professional experience that can lead to inappropriate use of phytosanitary products, thus aggravating pollution problems in the city. In addition, the risk of contamination is higher when treated water or materials obtained from waste are used instead of sources, such as mainstream water or freshwater. Traceability of products by means of regular monitoring and digital tools, e.g., internet of things (IoT) and blockchain technology (BCT), would facilitate both food safety and environmental risk mitigation (Figure 5).
- Opportunities include that the implementing UA-NBS as part of a sustainable bioeconomy in cities facilitates the reuse of resources stemming from urban metabolism, e.g., building materials, water, and nutrients reduce the environmental footprint of the final products . For this purpose, Langergraber et al. [6,7] proposed supporting units that enable nutrients and carbon to be recovered and directed back into the system. In this regard, regulations like the recently approved European Union Circular Economy Fertilizing Products Regulation (EU 2019/1009) may facilitate the use of fertilizers that are produced in the same city, fostering circularity. Urban metabolism and industrial synergy provide multiple streams of different characteristics that can be harnessed for food and biomass production.
Institutional Review Board Statement
Conflicts of Interest
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|Mark||General Category (UCC)||Food and Biomass Production Category (UCC5)|
|●||Addresses directly the UCC||Food and/or biomass production with relevant I and/or O|
|●||Contributes to the UCC||Usable for food and/or biomass production|
|○||Contributes potentially depending on specific design||Food and/or biomass production with no relevant production|
|Classification 1,2||(#) UA-NBS_u/i and UA-S_u 3||Food||Biomass||UCC5||Implementation 4|
|●||NBS_tu||(1) Infiltration basin||○||○||A|
|(5) (Wet) Retention pond||○||○||A|
|(7) Bioretention cell||○||○||A|
|(9) Dry swale||○||○||A|
|(10) Tree pits||○||○||○||A,D|
|(11) Vegetated grid pavement||○||○||A,D|
|(12) Riparian buffer||●||●||A|
|●||NBS_tu||(13) Ground-based green facade||●||●||●||B,C|
|(14) Wall-based green facade||●||●||●||B,C|
|(15) Pot-based green facade||●||●||●||B,C|
|(16) Vegetated pergola||●||●||●||B,C|
|(17) Extensive green roof||●||●||●||C,D|
|(18) Intensive green roof||●||●||●||C,D,E|
|(19) Semi-intensive green roof||●||●||●||C,D|
|(20) Mobile green and vertical mobile garden||●||●||●||B,C|
|●||NBS_tu||(21) Treatment wetland||●||●||A,D|
|S_u||(S6) Biochar/Hydrochar production||●||●||―|
|(S7) Physical unit operations for solid/liquid separation||●||●||―|
|(S11) Chemical and biological methods||●||●||―|
|●||NBS_ir||(28) River restoration||●||●||A,D|
|(32) Coastal erosion control||○||○||A,D|
|●||NBS_is||(33) Soil improvement and conservation||○||●||●||D,E|
|(34) Erosion control||○||○||D,E|
|(36) Riverbank engineering||○||○||A,D|
|●||NBS_su||(37) Green corridors||○||●||●||D,E|
|(38) Green belt||○||●||●||A,D|
|(39) Street trees||●||●||●||D|
|(40) Large urban park||●||●||●||D,E|
|(41) Pocket/garden park||●||●||●||D,E|
|(42) Urban meadows||○||●||●||D|
|(43) Green transition zones||○||●||●||D|
|(45) Hydroponic and soilless technologies||●||●||●||A,B,C,E|
|(47) Aquaponic farming||●||●||●||A,B,C,E|
|(48) Photo Bio Reactor||●||●||B,C|
|NBS_su||(49) Productive garden||●||●||●||D,E|
|(50) Urban forest||●||●||●||D|
|(51) Urban farms and orchards||●||●||●||D,E|
|Stream Type||Category||Subcategory||I in UA-NBS_u 1||O from UA-NBS_u 1|
|Biomass||Organic fertilizer||Compost||(18) (37) (49) (51) 2|
|Organic crop protection||Mulch||(18) (37) (49) (51)|
|Food waste||Vegetables, fruits||―||(18) (37) (49) (51)|
|Crop residues||(18) (37) (49) (51)||(14) (18) (37) (45) (46) (47) (49) (51)|
|Pruning remains||(14) (18) (37) (49) (51)|
|Living organisms||Plants||Edible||(14) (18) (37) (45) (46) (47) (49) (51)|
|Algae||(45) (46) (47)|
|Poultry||(18) (37) (49) (51)|
|Livestock||(37) (49) (51)|
|Other||(18) (37) (49) (51)|
|Auxiliary||(14) (18) (37) (45) (46) (47) (49) (51)|
|Microorganisms||Mycorrhiza, Bacteria||(18) (37) (47) (49) (51)|
|Fungi||(18) (37) (49) (51)|
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Canet-Martí, A.; Pineda-Martos, R.; Junge, R.; Bohn, K.; Paço, T.A.; Delgado, C.; Alenčikienė, G.; Skar, S.L.G.; Baganz, G.F.M. Nature-Based Solutions for Agriculture in Circular Cities: Challenges, Gaps, and Opportunities. Water 2021, 13, 2565. https://doi.org/10.3390/w13182565
Canet-Martí A, Pineda-Martos R, Junge R, Bohn K, Paço TA, Delgado C, Alenčikienė G, Skar SLG, Baganz GFM. Nature-Based Solutions for Agriculture in Circular Cities: Challenges, Gaps, and Opportunities. Water. 2021; 13(18):2565. https://doi.org/10.3390/w13182565Chicago/Turabian Style
Canet-Martí, Alba, Rocío Pineda-Martos, Ranka Junge, Katrin Bohn, Teresa A. Paço, Cecilia Delgado, Gitana Alenčikienė, Siv Lene Gangenes Skar, and Gösta F. M. Baganz. 2021. "Nature-Based Solutions for Agriculture in Circular Cities: Challenges, Gaps, and Opportunities" Water 13, no. 18: 2565. https://doi.org/10.3390/w13182565