Proper selection of substrates and crops enhances the sustainability of Paris rooftop garden

Abstract

Rooftop gardens are a promising way to supplement the growing demand for local food production, and are especially relevant in large cities with acute space constraints. However, they face the challenge of achieving viable food productivity while minimizing their impacts on the environment, two priorities that often oppose one another. Also, the actual impacts of management practices, which are deemed environmentally friendly in principle, are rarely quantified. Therefore, evaluations that encompass all components of urban gardens and a comprehensive range of environmental issues are necessary to reveal potential trade-offs and provide guidance in the design of these systems.In this study, we evaluated the environmental and economic impacts of rooftop gardening practices, focusing on crop and substrate selection, which are key parameters in system design but whose consequences have seldom been evaluated so far. Life cycle assessment (LCA) and life cycle costing (LCC) were used to analyze a case study in the center of Paris (France). The production systems considered involved crop rotations of tomato and lettuce each grown in three different substrate types: compost and wood chips; compost, wood chips, and earthworms; and conventional potting soil.Despite the large environmental burdens of compost production, systems with compost performed better environmentally and economically than the system involving potting soil, specifically having 17-47% less greenhouse gas emissions per kg of product. Across systems, length of cultivation and yield appeared to be the most influential determinants of the environmental impacts. Within the compost systems, the most impactful component was the material used for garden infrastructure, and substrate production for the potting soil systems. This is the first study that considers compost as a substrate, weighs its benefits and impacts, incorporates it into a complete garden, and compares it to potting soil. Our results demonstrate that careful system design could significantly abate environmental impacts. They provide critically needed information to people implementing urban rooftop agriculture and considering the trade-offs involved in each decision.

Revisiting the Sustainability Concept of Urban Food Production from a Stakeholders’ Perspective

“…Finally, the economic dimension of UFP has rarely been evaluated in the literature. Sanyé-Mengual et al [16,17] and Dorr et al [31] combined LCA with life cycle costing (LCC) to integrate the environmental and economic dimensions of sustainability for case studies of rooftop agriculture. The economic profitability of emerging types of UFP have also been evaluated for vertical farming [32].…”

“…Studies in warm climate areas highlighted the efficiency of UFP cases and the potential environmental benefits from minimizing the distance between producers and consumers (i.e., reduced transport, packaging and food losses) [16,17,21,45,50]. On the contrary, studies in mild climate cities found that the UFP typology and the productivity of the system would determine whether UFP has environmental benefits compared to conventional food systems [18,31]. Finally, Forchino et al [44] and Maucieri et al [22] investigated the environmental impacts of aquaponics, where the production of fish and vegetables are integrated, paying attention to different production techniques [44] as well as the potential use of aquaponics as an educational tool [22].…”

“…The studies evaluating the life cycle costs of urban food mostly revealed that the cost of UFP falls into the market price range of local or organic products, although some crops can have higher costs due to low crop yield or the cost of the auxiliary equipment [17,31], or because the total cost at the consumption point is cheaper when considering the entire supply-chain [16]. Eaves and Eaves [32] simulated the profitability of a vertical farm in Quebec and obtained slightly more positive values compared to a conventional greenhouse production.…”

“…A recent study [62] evaluated the presence and abundance of ladybirds as a useful biodiversity tool in integrated pest management in an urban garden in Bologna (Italy) where the plant species that had a higher abundance of ladybirds were identified for future urban garden design. UFP has been proved to be a tool that can increase the recycling rate of organic waste, which can be used as fertilizer and substrate for food production [31,46]. Moreover, some entrepreneurs are seeking technical solutions to employ urban waste in food production, such as the use of coffee grounds for fungi production (e.g., La Boite à Champignons, Paris, France or Rotterzwam, Rotterdam, The Netherlands).…”

“…Gasperi [63] evaluated the effects on the micro-climate of an experimental rooftop garden in the city center of Bologna, which had a positive effect on human thermal comfort. Finally, a debate around the increased resource efficiency of UFP exists in the literature, as the comparison between urban food and conventional food strongly depends on the production system and the geographical context [16][17][18]31].…”