In the last decade, urban agriculture has been a growing topic for urban stakeholder’s worldwide. Urban agriculture is perceived as one way to counter some of the negative impacts of urban growth and development. Indeed, urban agriculture has potentially important and diverse functions, such as improving social cohesion [1
], food production [2
], urban waste valorization [3
], improved nutrient cycling etc. The multi-functionality of urban agriculture could help to tackle several societal challenges of urbanization [5
Whether urban agriculture can significantly contribute to feeding urban citizens has raised debate and promoted a number of empirical studies. Weidner et al. [6
] showed that a wide range of methodologies are used to estimate productive surface areas, yields and other production parameters in urban agriculture, which results in a great variability of estimates. From 1.87% to 150% of vegetables and fruit demands could be met by urban agriculture in different cities depending on the type of space considered as being available for urban agriculture, the growing system used (low and/or high tech system) and on the type of production (vegetables and/or fruits and/or other multiple food categories) [6
]. MacRae et al. [4
] estimated that cropping all the potential growing space of the city of Toronto, including rooftop farming, would provide for 10% of the city needs in fresh vegetables, but stressed the need for more agronomic knowledge regarding this type of production. It is clear that current published estimated are more based on estimated and extrapolated yields rather than based on field/case studies [6
At the ground level, urban agriculture projects are often limited by the access to land and by soil contamination. To face that, one solution is to use vertical space, e.g., the Z-Farming project (zero-acreage farming, see [7
]). Rooftop food production also has inherent constraints, when compared to ground level food production, such as the need to limit the weight on the buildings, the need to create soils and the meteorological condition. Meteorological conditions on rooftops can be harsher compared to ground level with potentially higher access to light and more windy conditions. Depending on the context this can represent an opportunity (light) or a constraint (wind). However, little is currently known about how the constructed soil characteristics or Technosols characteristics for rooftop gardening determine the food production potential.
Only a few studies have investigated the productivity of rooftop (~12 studies, see Table 1
). Whittinghill et al. [9
] showed the feasibility of food production on a fertilized and irrigated extensive green roof with 10.5 cm thickness of substrate. They report a satisfying level of production except for pepper, and do not find much difference between yields at the ground and roof level. Contrarily, [2
] showed a greater food production on rooftop than at ground level. Orsini et al. [3
], in Bologna (Italy) demonstrated a higher production on a fertilized substrate than on a hydroponic system with simplified management. These studies highlighted that producing food on rooftop is a challenge and raised research questions regarding soil fertility, contamination of the food products as well as regarding the parent materials used to design a productive substrate on rooftops.
Rooftop agriculture implies creating soils on the roof. The growing medium used can be called in this case isolatic Technosol [10
] as it aims to stay for several years. Indeed, soil creation implies arranging and mixing different parent materials in order to obtain a functional isolatic Technosol. This functionality is directly linked to the expected ecosystem services (food production, water retention, carbon storage etc.) as well as the creation of an ecosystem design by human action and relying on ecological and biological process (biodegradation, mineralization, lixiviation, nutrient uptake etc.). Human action, through the choice of parental materials, plants, technical operations etc., should aim to maximize the functionality of such an ecosystem regarding its specific constraints. For weight constraints, natural soils are often avoided. Several experiments used peat or potting soil, which can be seen as non- or little renewable resources, while urban wastes represent a large and not sufficiently used resource. Life cycle assessment (LCA) analysis showed that a classical substrate based on peat generates a higher environmental impact than growing media based on urban wastes [11
]. However, only a few studies aimed to design Technosols for productive rooftops based mostly on by-products derived from organic urban wastes, which have the advantage of being a local renewable resource and of being light (Table 1
]. Eksi et al. [2
] used green waste compost mixed with expanded clay pellets, finding an optimal ratio of around 60%–80% of green waste in the Technosol. Aloisio et al. [14
] compared on a rooftop of New-York (USA) two commercial mixes for extensive green roofs to a potting soil and measured a higher production with the potting soil over 45 days. These studies show the key influence of the nature of the parent material that directly impacts soil fertility and in consequence food production. However, among the 10 studies published on productive green roofs (Table 1
), none of them looked at the spatial arrangement of the used parent materials and the study time was in average of 16 months, while such set-ups are expected to be implemented for several years. Technosols made of organic materials that are decomposable can be hypothesized to change with time, e.g., decreased particle size and porosity, changes in the organic C and N content [15
], raising the question of the sustainability of urban agriculture based on the use of by-products generated from organic wastes from the city. Compared to classical green roof substrates such as peat or pozzolan, the used of by-products derived from urban waste as Technosol parent materials could result on more intense biological and ecological processes because of biodegradation.
Urban food production could be contaminated via two major pathways: root plant uptake and/or atmospheric fallout. Contamination of food products with trace metals is of particular concern in urban agriculture [23
]. These studies show the risk of vegetable contamination due to historical pollution of urban soils and agricultural practices (such as the use of contaminated wastes). Several factors have been reported to influencing food products contamination: climate, soil characteristics (pH, organic matter etc.) [30
], type of pollutant, type of plant and soil organisms [31
]. An effect of urban pollution is described for different forms of urban agriculture using uncontaminated soils at natural levels trace metals concentrations [32
], hydroponic system on rooftop [33
] or urban contaminated soils [34
]. However, to our knowledge the quality of vegetables grown on Technosols made with by-products derived from urban wastes was not informed.
The use of by-products derived from urban wastes, in particular of organic urban wastes, to construct Technosols appears as an opportunity for the development of rooftop agriculture, but requires an extensive evaluation. Here, we aimed to test constructed Technosols made of different by-products derived from organic urban wastes and with different layouts, in terms of (i) their fertility and (ii) fertility sustainability with time and (iii) the associated risks of food contamination with trace metals. For this, we monitored Technosols characteristics, yields and food products trace metals concentrations for Technosols made of various parent materials and with two layouts, either as layers or homogeneous, in a rooftop experiment over five years, i.e., a much longer time period than considered previously in the literature.
Developing rooftop farming requires to design specific growing systems fitted to the constraints of this environment: weight constraints, specific meteorological conditions with potential higher rates of evaporation and dealing with potentially small surface areas that force to maximize the use of space and adapt the cropping system. Using by-products from urban organic waste for rooftop farming requires to select organic wastes that are not contaminated or only contaminated at a low level, and that are biodegradable and hence can provide the necessary nutrients to the plants. A balance has to be found between fast enough decomposition of the urban by-products, to ensure provision of nutrients and slow enough decomposition, to ensure that stability of the constructed Technosol, i.e., that the physical environment of roots persist long enough, a balance that requires an appropriate selection of parent materials. In the present system, the parent materials provided ample amounts of nutrients, but it was hence necessary to add large amounts of new by-product after the first year and smaller amounts the following years, which is a constraint. It is nevertheless a way to favor circular economy while creating soils of a possibly known quality regarding soil contamination and fertility. We showed that by-products from urban organic wastes such as crushed wood, green waste composts and spent mushroom substrate were valuable substrates to build highly fertile Technosols leading to acceptable and even consequent yields per square meter with vegetables respecting the European norms for regulated trace metals (Cd and Pb). Certainly, the quality of urban vegetables remains a public health concern and needs to be further investigated for a wider range of vegetable species and also for other pollutants present in urban environment such as polycyclic aromatic hydrocarbons. Our study clearly showed the feasibility of using by-product from urban organic waste as parent materials of Technosol. Thus, questions relative to the access to these by-products in terms of volume, quality and cost needs to be investigated to ensure the feasibility of this practice in different urban context.
Regarding Technosol design, a layered layout of the Technosol, mimicking the A and B horizons of a mineral soil, was more favorable to plant growth, while the reasons behind still need to be identified. While a pedogenesis was clearly taking place in the Technosols, their fertility was maintained at sufficient level over five years to maintain the yields, demonstrating the sustainability of this rooftop organic wastes vegetable cropping system. The characteristics of the material (high content of organic matter and low density) as well as the yearly input of fresh organic material seem to promote pedogenetic processes such as biodegradation of the organic matter and subsidence, partly caused by the mineralization of the organic matter. Furthermore, the increased C content and N content of lower layers can only be explained by transport processes from the upper layers downwards. Illuviation and/or lixiviation are hence also taking place in the profile of the constructed Technosols. This pedogenesis may compromise the sustainability of the constructed Technosols for growing vegetables, if the consolidation leads to a compaction and reduction of soil porosity that compromises root growth. However, we did not observe such a porosity loss over five years (Table 4
). Soil structure formation was not investigated here, but may take place, in particular due to the activity of earthworms. The biodegradation may also compromise the sustainability of the fertility of the constructed Technosols, if, eventually, all labile organic components of the Technosols having been mineralized only recalcitrant or stabilized organic compounds remain and not enough mineral elements are released by mineralization to meet the plants demands. However, it is likely that the remaining organic matter would have a large CEC, retaining nutrients and changes could be implemented in the cultural system to manage plant nutrition, such as adding urine as a fertilizer, inserting legume crops in the rotation.
In the future, to up-scale urban agriculture in dense cities, the conquest of roof space (mostly unused for the moment) is necessary. To design proper and suitable growing systems, a wider variety of urban wastes could be considered and further questions need to be addressed, such as the pest management, cost analysis and the water management. Rooftop farming could be of interest for the city of tomorrow only if it functions in symbiosis with the urban environment: minimizing the use of natural resources and maximizing the use of space available to create and circular and multifunctional ecosystem.