ReviewConsolidating the current knowledge on urban agriculture in productive urban food systems: Learnings, gaps and outlook
Introduction
Increasing urbanization, population growth, deteriorating soil and extreme climate events put pressure on global food security, while a third of the global population suffers from malnutrition, through either obesity, food insufficiency, or nutrient deficiency (UNICEF, 2016; UNCCD, 2017). Additionally, the current food system contributes heavily to our global environmental challenges including biodiversity loss, climate change, fresh water pollution, deforestation, and excess nutrient accumulation (Jaramillo and Destouni, 2015; Lenka et al., 2015). Innovation, dietary pattern change, and rethinking the direction of our global food system are therefore imperative in the 21st century (Tilman and Clark, 2014).
Producing food within urban boundaries has been suggested as a way to simultaneously tackle a breadth of environmental, social, and human health issues. Research has shown that in addition to achieving a certain degree of self-sufficiency (Clinton et al., 2018), potential climate change mitigation (Kulak et al., 2013) and ecosystem benefits (Wilhelm and Smith, 2017), urban agriculture (UA) has the potential to increase resource efficiency and circularity (Mohareb et al., 2017), foster social development and community cohesion (Duchemin et al., 2008), and broaden biosphere (Lin et al., 2015) and dietary diversity (Wilkins et al., 2015).
Urban agriculture as a primary production process can be viewed as a component embedded in the urban food system, which further includes processing, packaging, distribution and retail. These closely interact with urban material and resource streams such as water, energy, and organic waste. The practice of UA also entails various institutional norms, government and private sector policies, and cultural attributes in any city region. Here, the term ‘productive urban food systems’ (PUFS) is used in this work to account for the range of productive activities within the urban boundaries (excluding peri-urban or non-urban local agriculture), the required logistics, and all the related material and other resource flows as well as the social components.
This review has been produced in light of a multitude of recent studies and urban trends that illustrate a way UA could be understood to maximize its benefit to society and environment. In addition, this review endeavors to address: (i) the debate about self-sufficiency with a detailed overview of relevant estimations and their methodologies, (ii) the most recent knowledge of nutrient circularity opportunities and critical sustainability metrics, (iii) empirical research requirements for holistically understanding and propelling PUFS and (iv) the current knowledge of guiding principles about scaling up in a socially and environmentally inclusive way. We believe a particular focus on these elements is warranted in order to shed light on needs for future research and practices on UA.
Other important elements and the general development of UA have already been reviewed in the last several years and are not directly included in this work. Mok et al. (2014) examined the state of UA in the developed world, while Orsini et al. (2013) investigated several development factors required for UA. The current state of global rooftop greenhouse projects was reviewed by Buehler and Junge (2016) and Proksch (2011) summarized the multifold benefits and technicalities of productive green roofs. Specht et al. (2014) reviewed existing literature regarding environmental, social, and economic dimensions. Mohareb et al. (2017) reviewed studies on the potential integration with urban resource flows, such as greywater and waste heat, embedding UA into the broader food system and the food-energy-water nexus. Literature reviews exist also on ecosystem services, such as storm water attenuation and biodiversity (Lin et al., 2015), as well as disservices, such as run-off pollution (Russo et al., 2017). The academic discourse on the climate innovation potential of different UA typologies (e.g. water retention, urban heat island effect, energy consumption, inputs, all by topology) as well as their potential for global climate mitigation has been reviewed by Koegler et al. (2017).
This review considers mainly activities in the Global North for several reasons. The study and quantification of UA practices in the developing world is only loosely defined (Ellis and Sumberg, 1998). Even though there is considerable urban farming activity (Hamilton et al., 2014), Martellozzo et al. (2014) have found the potential for self-sufficiency for most of the developing countries with their densely populated cities to be rather small. The lack of available urban data in the developing world makes accurate spatial analysis and planning difficult. Nevertheless, findings and outlooks from reviewing the development in the Global North could be transferred to other geographies with appropriate consideration of place differences.
Until recently, UA has been used as an all-encompassing term for different practices and strategies, which has attracted both optimistic and pessimistic views.
Research on UA has generally tended to take an advocacy viewpoint (Neilson and Rickards, 2017), which might have led to overstating its potential in some cases, in turn attracting criticism. Badami and Ramankutty (2015) found that the contribution of UA to the food security of the urban poor is quite low, despite the strongest need. A study of community growing practices led the authors to conclude that UA can “produce little more than “nibbles” (Martin et al., 2014). However, these studies look at self-sufficiency for the whole food basket (i.e. including meat and cereal) based on low-yield methods – which is clearly unattainable – and necessitates studies on self-sufficiency in terms of certain types of produce only, such as fruits and vegetables.
A similar narrative is observed for the promises of increased sustainability, where benefits offered by some particular growing practices seem to have been applied to the whole spectrum of UA. An early report by Mougeot (2000) provided a balanced picture of environmental benefits and risks pertaining to UA. However, these environmental risks or challenges have been mostly overlooked, leading to critical research gaps (Wortman and Lovell, 2013). Russo et al. (2017) reviewed the most relevant studies that investigated positive ecosystem services – such as biodiversity and storm water attenuation – and negative ecosystem disservices – such as contaminated run-off and gaseous emissions – with 146 versus 52 papers respectively. A case-specific challenge to the general sustainability claim came from Goldstein et al. (2016a) who employed life cycle assessment (LCA) and found that the high-yield growing practices which significantly increase self-sufficiency have a worse impact on the environment than conventional agriculture. Furthermore, controlled environment UA likely does not provide other often associated ecosystem benefits, e.g. increased biodiversity and reduction of the urban heat island effect.
Due to the stark differences of outcome and impact, it seems obligatory in further scientific discourse to specify the growing techniques and operational modes employed when suggesting the benefits and potential drawbacks of UA.
To cope with the differing production capacity and socioecological impacts, growing practices for UA have been defined and classified depending on the paradigm of interest. Goldstein et al. (2016b) classified practices depending on their location (ground based or building integrated) and the extent of controlling the growing environment (conditioned and non-conditioned) and characterized the four resulting classifications by their operational characteristics, capital inputs and potential for urban symbiosis. Koegler et al. (2017) further sub-divided these four categories depending on their economic model and size (ground based) and technological sophistication and access to sunlight (building integrated), and showed their potential regarding climate change adaptation and mitigation.
A more socioeconomic approach has been employed by looking at different motivational frames (sustainable development, radical, DIY secessionist, educational, eco-centric, entrepreneurial), their funding sources and consequences, thus allowing more intention-matching policy making and support (McClintock and Simpson, 2017). Another study grouped practices by their produce distribution level, the actors involved and the main interest (self-supply, commercial, social-cultural), concluding that diversity of types ensures multifunctionality and policy should not favor one over the other (Krikser et al., 2016). Thomaier et al. (2015) created a typology for vertical farming practices (image-oriented, living quality, commercial, social and educational, innovation incubator) depending on their market orientation and transformative potential.
The studies mentioned in this review have been selected through keyword searches in Google Scholar in December 2017 and again in February and April 2018. The following terms were searched individually and in combinations of up to 3 terms:
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Aquaponics, economics, energy efficiency, fish feed, food waste anaerobic digestion, food waste digestate, GIS, hydroponics, insects, integrated greenhouse, LCA, micro-scale anaerobic digestion, nutrient circularity, organic agriculture, organic waste management, organic waste valorization, policy, resource circularity, rooftop greenhouse, spatial analysis, stakeholder (perception/participation), sustainability, sustainable, urban agriculture, urban farming, urban food-energy-water nexus, urban food systems, urban horticulture, urban self-sufficiency, vermicomposting, vertical farming.
Titles of over 500 papers were scanned and over 300 papers were screened before 145 relevant scientific publications were eventually selected for this review. Studies were screened if they either:
- (i)
reviewed the current state of urban agriculture or aspects of it;
- (ii)
contained research and findings that were not typically found in other reviews;
- (iii)
conducted studies on emerging topics such as organic hydroponics, waste valorization, nutrient circularity, or food waste digestate use in agriculture;
- (iv)
combinations and integrations of current technologies and practices;
- (v)
looked at the broader picture of urban food production such as logistics, economics, environmental impacts, implementation and policy.
Additional studies were found by investigating the references and “cited by” of the most relevant papers and authors, scanning Researchgate for reports and recommendations and the “literature suggestion” function of Mendeley.
The literature included peer-reviewed articles, books, conference proceedings, personal communications with researchers in this area and commercial websites if the technology was relatively new and not yet researched well. Hence, this paper includes a few references to commercial products to show developments in the respective areas; the authors do not endorse the products or attest to the veracity of their analysis. Although part of the urban food system, urban beekeeping is not addressed.
Section snippets
Nutrients versus calories
An important metric for quantifying the self-sufficiency potential of UA is the choice of food items intended to supply. Existing studies have assumed different selections of food types, including animal products, cereals, oils and the more common fruits and vegetables. Martellozzo et al. (2014) compared an estimation for the area required to satisfy global cereal (661.6 Mha) and vegetable (47.2 Mha) demand with the global urban land area (64.3 Mha), which indicates that it is unrealistic to
Missing puzzle pieces
The development of PUFS requires a great number of considerations in different areas. The urban resource integration and sustainability drivers have been discussed earlier and slowly a picture emerges for these topics in the existing literature. On the other hand, several broader considerations regarding the supply chain as well as socioeconomic impacts and development factors have not been sufficiently and systematically covered yet. These are very important for fully understanding and
Summary and outlook: realizing the transformative potential of PUFS
In this review, we have analyzed the existing literature on UA with the attempt to both consolidate existing knowledge and raise important issues that require future attention. Table 2 summarize the learning and recommendations as well as the challenges and research gaps identified. To highlight the key points, the following recommendations are made for future developments with respect to different types of available surface area for growing crops:
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Small to medium urban patches: Ground based
Acknowledgements
Financial support for TW for the duration of his PhD project by the Clarendon Fund is greatly appreciated.
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