How Cities Function Ecologically: The Importance of Urban Soils

The diversity of urban soils

Are you a regional planner or researcher? Urban planner, architect, gardener or soil scientist? The answer to this question will determine how you define and perceive the term “urban soil.” For some, it’s any soil located in an urban area (Blanchart et al. 2017). Others are interested in the occupation of built or industrial land. Soil scientists, on the other hand, classify them according to their genesis, based on their degree of “anthropic disturbance”: in other words, the degree to which these soils have been transformed or constructed by human activities (Lehmann and Stahr 2007). Most soils have properties strongly influenced by these activities—they are known as anthropized soils—due to their intense use and disturbance (Béchet et al. 2009). There is therefore a wide range of urban soils: sealed soils (infrastructure support), reworked soils with very heterogeneous histories (from industrial wasteland to garden, for example); soils constructed from a variety of materials and, in a small proportion, soils that are said to be pseudo-natural, i.e. relatively untouched by human activities. Constructed soils, sometimes consisting solely of urban residues, may be referred to as Anthroposols or Technosols, depending on the pedological frame of reference chosen (Baize and Girard 2009; IUSS Working Group WRB 2014). Combined with the different uses to which they are put, this diversity of soils creates a spatial heterogeneity that can be observed both horizontally and vertically (Béchet et al. 2009; Morel et al. 2005) (Figure 1).

Putting soil at the heart of how cities function

In the face of urbanization and global change, cities, which are very “mineral,” must evolve to become multifunctional and resilient. This need for change manifests itself in particular in the promotion of more “nature” in the city. This “nature,” often reduced to vegetation (“greenery”), overlaps and intersects with the related concepts of environment or landscape (Arnould et al. 2011), Yet soil, with its physicochemical characteristics and its little-seen but potentially abundant biodiversity, plays a vital role in the functioning of cities. Considered since the industrial era and until recently as mere building supports, soils are becoming central due to their role in the direct or indirect provision of benefits, also known as ecosystem services, which can be divided into three categories: provisioning (food production), regulating (climate) and cultural services. Natural soils are a non-renewable resource, taking thousands of years to form. They are part of our natural heritage, and their degradation irreparably damages the services provided today, as well as the capital of tomorrow.

The ability of a soil to provide ecosystem goods and services is determined by its quality (Morel et al. 2015), defined by its physical, chemical and biological properties. In the case of urban soils, there is a very high variability in these properties (Joimel et al. 2016). The variability results from a diversity of origins (e.g. former market gardening area), uses (e.g. parks) and practices (e.g. composting). This diversity leads us to categorize urban soils according to the ecosystem services they provide (Table 1).

To guarantee the provision of ecosystem services, we need to minimize the artificialization of soils, and especially their sealing, and optimize the management of pseudo-natural soils according to their properties.

As several of these services have been described (Blanchart et al. 2017; Vidal-Beaudet and Rossignol 2018), we will focus here on three of them, objects of emerging research: food supply, biodiversity maintenance and urban waste recycling. As sealing is a major drawback for the provision of supply and regulation services, sealed soils will not be discussed.

Urban soils and food production

The development of urban agriculture directly raises the question of soil quality. High levels of organic matter have been recorded in allotments (up to 10% compared with 1 to 4% for agricultural soils), phosphorus (with sometimes excessive values) and basic pH levels (between 7 and 8.5) that can be accompanied by metal contamination (Joimel et al. 2016). These high levels are the result of successive and historical inputs of soil improvers and fertilizers, and may involve transfers to cultivated plants. Work carried out on AgroParisTech’s rooftop vegetable garden (figure 1b) has demonstrated that vegetables grown in soils constructed solely from organic waste products comply with current regulations on pollutant levels thanks to contamination control of the materials used (Grard 2017).

In the case of soils with high levels of contaminants, local authorities can draw on a number of guides, such as the one on “presumption of soil pollution,” to find the tools and knowledge available. At the individual level, the application of good consumer practices, such as washing and peeling vegetables, can eliminate surface pollutants and reduce the potential risk of contamination.

Urban soils as refuges for biodiversity

The Global Assessment Report on Biodiversity and Ecosystem Services (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), 2019) estimates that around one million plant and animal species are threatened with extinction over the coming decades. Yet soils are home to around a quarter of our planet’s biodiversity. Soil organisms play an essential role in soil fertility, water quality and carbon storage. Given the physicochemical characteristics of urban soils (contamination, extreme pH values, compaction, etc.), we might expect them to harbor low levels of biodiversity, with effects on organisms of all sizes (earthworms, beetles, microorganisms, etc.). On the contrary, unsealed urban soils seem to provide a refuge for urban biodiversity. For example, springtails or Collembola (Figure 2)—well-known indicators of soil quality—present 1.6 times more species and are 8 times more abundant in urban vegetable gardens than in agricultural environments (Joimel et al. 2017). Similarly, parks and rooftops, though highly artificial, harbor high levels of microbial biomass (McGuire et al. 2013).

However, organisms must adapt to the constraints of urban soils, and springtails then display specific characteristics linked to their life on the surface rather than in the depths of the soil (Joimel et al. 2018).

Built-up rooftop soils are home to a motley mix of common springtail species but also rarer species with extreme abundances, which are brought there via composts, with which the soils are made, or by the wind from surrounding green spaces (Joimel et al. 2018).

These results underline the fact that, to promote biodiversity in cities, it is necessary to maintain natural soils in urban and periurban areas, create urban green spaces and connect these spaces to each other via ecological corridors. These should be integrated into the design and layout of cities. While green and blue corridors are now taken into account at various scales, right down to the local level in town planning schemes, thought could be given to brown corridors in order to create soil continuity in cities.

Making best use of waste

As major producers of mineral and organic waste, cities could find new local ways of recycling these residues. How can vegetation be allowed to grow when the soil on site is non-existent or of too poor quality? One option is to build a fertile soil (Vidal-Beaudet and Rossignol 2018), which is inspired by a natural soil with the superposition of two or three layers (Figure 1b showing a soil built with two layers; one fertile, the other support) (Grard 2015, Grard et al. 2017). The use of topsoil and peat-based planting substrate, currently the majority in soil construction, should be greatly reduced: these are non-renewable resources and their use in cities induces the destruction of soils and natural environments outside the city (e.g. peat bogs).

However, few references are available today on the “creation of soil” from various (generally organic) materials and wastes, although this has been implemented in a number of green roof projects in the Paris region (Figure 3, Grard et al. 2017) or on former urban wastelands (Séré 2007). This waste can be organic (compost from household refuse or green waste, etc.) or inorganic (crushed brick, crushed aerated concrete, etc.). These Technosols, which are set to become increasingly common in the city, enable good growth of both food (Grard 2017) and non-food (Séré 2007) vegetation. For the time being, soil construction is confined to small surfaces, such as roofs or aligned tree roots, due to the cost and difficulty of implementation. A change in regulations and professional rules therefore appears necessary to enable better use of urban waste (SITERRE project).

Preserving and managing urban soils

Urban soils are subject to numerous pressures (land pressures, pollution, sealing) that need to be drastically reduced if we are to build and live in sustainable cities, providing a wide range of "ecosystem services" and helping to maintain biodiversity. Changes in the biological, physical and chemical quality of these soils due to human activities (such as depletion or excess of chemical fertility) are likely to modify their functions. Urban soils are not just an inert support for vegetation or buildings, they also host a high level of biodiversity, produce plant biomass (food and non-food) and play a part in regulating climate, waste, water and pollution. This is why, in addition to appropriate public policies, we need to put in place concrete tools to better appreciate their qualities. The road may still seem long, since only 1% of scientific studies on soils concern urban sectors (Guilland et al. 2018). However, knowledge and tools are already available to integrate this component into urban planning, such as the Destisol tool. While the number-one priority is to limit the degradation of natural soils during development by taking their potential into account, it is also possible to build soils in the city from urban waste.

In this way, urban soils are an essential component in the city’s capacity to cope with global change.

Bibliography

• Arnould, P., Le Lay, Y.‑F., Dodane, C. and Méliani, I. 2011. “La nature en ville : l’improbable biodiversité”, Géographie, Économie, Société, vol. 13, no. 1, pp. 45–68.
• Baize, D. and Girard, M.‑C. 2009. Référentiel pédologique 2008, Versailles: Quæ.
• Béchet, B., Carré, F., Florentin, L., Leyval, C., Montanarella, L., Morel, J.‑L. and Raimbault, G. 2009. “Caractéristiques et fonctionnement des sols urbains”, in C. Cheverry and C. Gascuel (eds.), Sous les pavés la terre, Montreuil: Omniscience, pp. 45–74.
• Blanchart, A., Séré, G., Cherel, J., Warot, G., Stas, M., Consales, J. N. and Schwartz, C. 2017. “Contribution des sols à la production de services écosystémiques en milieu urbain – une revue”, Environnement urbain, vol. 11.
• Damas, O. and Coulon, A. (eds.). 2016. Créer des sols fertiles : du déchet à la végétalisation urbaine, Antony: Le Moniteur.
• Dorr, E., Sanyé-Mengual, E., Gabrielle, B., Grard, B. and Aubry, C. 2017. “Proper selection of substrates and crops enhances the sustainability of Paris rooftop garden”, Agronomy for Sustainable Development, vol. 37, no. 5.
• Grard, B. 2017. Des Technosols construits à partir de produits résiduaires urbains : services écosystémiques fournis et évolution, thèse en sciences de l’environnement, PhD dissertation, Université Paris-Saclay.
• Grard, B., Bel, N., Marchal, N., Madre, N., Castell, J.‑F., Cambier, P., Houot, S., Manouchehri, N., Besancon, S., Michel, J.‑C., Chenu, C., Frascaria-Lacoste, N. and Aubry, C. 2015. “Recycling urban waste as possible use for rooftop vegetable garden”, Future of Food: Journal on Food, Agriculture and Society, vol. 3, no. 1, pp. 21–34.
• Guilland, C., Maron, P. A., Damas, O. and Ranjard, L. 2018. “Biodiversity of urban soils for sustainable cities”, Environmental Chemistry Letters, vol. 16, pp. 1267–1282.
• IUSS Working Group WRB. 2014. World Reference Base for Soil Resources 2014. International soil classification system for naming soils and creating legends for soil maps, Rome : FAO.
• Joimel, S., Grard, B., Auclerc, A., Hedde, M., Le Doaré, N., Salmon, S. et Chenu, C. 2018. « Are Collembola “flying” onto green roofs? », Ecological Engineering, vol. 111, p. 117‑124.
• Joimel, S., Cortet, J., Jolivet, C. C., Saby, N. P. A., Chenot, E.-D., Branchu, P., Consalès, J.-N., Lefort, C., Morel, J.-L. et Schwartz, C. 2016. « Physico-chemical characteristics of topsoil for contrasted forest, agricultural, urban and industrial land uses in France », Science of The Total Environment, vol. 545-546, p. 40‑47.
• Joimel, S., Schwartz, C., Hedde, M., Kiyota, S., Krogh, P. H., Nahmani, J., Pérès, G., Vergnes, A. et Cortet, J. 2017. « Urban and industrial land uses have a higher soil biological quality than expected from physicochemical quality », Science of The Total Environment, vol. 584‑585, p. 614‑621.
• Lehmann, A. et Stahr, K. 2007. « Nature and significance of anthropogenic urban soils », Journal of Soils and Sediments, vol. 7, n° 4, p. 247‑260.
• Madre, F., Vergnes, A., Machon, N. et Clergeau, P. 2013. « A comparison of 3 types of green roof as habitats for arthropods », Ecological Engineering, vol. 57, p. 109‑117.
• McGuire, K. L., Payne, S. G., Palmer, M. I., Gillikin, C. M., Keefe, D., Kim, S. J., Gedallovich, S. M., Discenza, J., Rangamannar, R., Koshner, J. A., Massmann, A. L., Orazi, G., Essene, A., Leff, J. W. et Fierer, N. 2013. « Digging the New York City Skyline: Soil Fungal Communities in Green Roofs and City Parks », PLOS ONE, vol. 8, n° 3
• McKinney, M.L. 2008. « Effects of urbanization on species richness: A review of plants and animals », Urban Ecosystems, vol. 11, p. 161‑176.
• Morel, J.-L., Chenu, C. et Lorenz, K. 2015. « Ecosystem services provided by soils of urban, industrial, traffic, mining, and military areas (SUITMAs) », Journal of Soils and Sediments, vol. 15, p. 1659-1666.
• Morel, J.-L, Schwartz, C. et Florentin, L. 2005. « Urban soils », Encyclopedia of Soils in the Environement, vol. 335, p. 202‑208.
• Séré, G. 2007. Fonctionnement et évolution pédogénétique de technosols construits, thèse en sciences agronomiques, INPL.
• Vidal-Beaudet, L. et Rossignol, J.-P. 2018. « Les sols urbains : artificialisation et gestion », in C. Valentin (ed.), Les Sols au cœur de la zone critique 5 : Dégradation et réhabilitation, ISTE Group.