Constructed soils for greener cities tomorrow

By 2020, 55% of the world's population will be living in urban areas, and this number is rising steadily, particularly as a result of urban sprawl. In addition to global changes, these expanding urban areas are today the focus of a large number of environmental challenges (e.g. urban heat island, disruption of the urban water cycle, lack of ecological connectivity). It is therefore essential to develop and implement solutions to make tomorrow's cities more sustainable.

Green infrastructures are urban environmental constructions designed by man as interconnected natural spaces capable of providing multiple ecological functions. Because of their ability to provide a wide range of ecosystem services in concrete cities, green infrastructures are receiving increasing attention in Europe and beyond.

Soils are the fundamental component of green infrastructure, playing a key role in plant growth, water infiltration, microbial activities responsible for nutrient cycling and pollutant management. Taking the most fertile surface soil horizons from rural areas and transporting them to urban areas for use is not a sustainable solution. Soil is a non-renewable and therefore finite resource, requiring thousands of years to form in optimal climates. What's more, excavating and transporting soil from rural to urban areas is costly and generates significant carbon dioxide emissions into the atmosphere. In the other direction, transporting soil excavated for building foundations from cities to other, usually rural, sites degrades the agricultural or natural receiving area. This affects not only the local landscape, but also the well-being of surrounding populations, both human and animal (noise, olfactory and visual nuisance). Finally, even if the soil transported is clean, this activity can affect air quality through the emission of dust and fine particles harmful to human health.

For the past decade, local authorities, waste management companies and the scientific community have been looking at the recycling of waste in short circuits to create soils and growing substrates with characteristics suitable for plant growth. Such artificial soils are known as "constructed technosols". This approach offers two major advantages: it enables the recycling of waste materials that are difficult to recycle, such as building rubble, and it limits the extraction of natural resources, such as soil taken from far-flung urban areas. But this can't be done without considering the many implications, not least environmental, of the choices made at the design stage. What waste materials can be used? How can they be combined, and in what quantities, to obtain the desired characteristics for a given use? Will the soil thus created be conducive to plant growth, and which plant...