Beyond green and blue belts: why we should also look at the soils

 

By Joshua Le Bourhis

Blue and Green networks

Blue and green belts are a well known tool to allow natural ecosystems to flourish even in urban environments, to favor their environmental quality. Their implementation aims at limiting the fragmentation of natural environments and so improving biodiversity thanks to a better circulation of the different species through a greater connectivity between these natural areas. However, these networks are often only considered in two dimensions: the connectivity is only analyzed through maps, omitting to consider the vertical dimension which includes the air, but also, obviously the soil.

The soil: an overlooked resource of biodiversity

Indeed, the land can be an incredible underground life reserve: they are home to about a quarter of our planet’s biodiversity. A rich soil can be full of life. They contain bacteria, archaea, fungi, but also soils animals such as springtails, mites, millipedes, earthworms or termites and finally some vertebrates as well (moles, field mice, reptiles…).

 

 These organisms are involved in many processes which allows the soil to perform several ecosystem services, contributing to enhance human well-being:

●      Biodiversity: The soil is richer and more diverse in species than the surface even in a thin layer.

●      Water cycle: Contrary to impervious soils which are very present in urban landscapes, soil in the open ground containing a good biodiversity enables the infiltration of water, its circulation and water then becomes available for plants.

●      Nutrient cycle: the organisms living in the ground are recycling natural wastes into nutrients to be consumed by plants.

●      CO² absorption and storage: Atmospheric CO² is captured by organic matter deep into the ground, preventing it from being released again.

●      Remediation: The soil filters water and organisms can also degrade toxic components.

●      Soil health: Significant biodiversity allows the different species to help each other through symbioses or cooperation, limiting the impacts of parasites and soilborne diseases.

 

            This rich biodiversity highly depends on the nature of the soil and its physical and chemical characteristics (pH, humidity, density, depth…) allowing the different species to live in an adapted environment. Contrary to common misconceptions, the urban unsealed soils actually contain a lot of biodiversity: the multiplicity of soil types in cities is an asset for living beings and the cities are now often spared of the use of pesticides. Thus, springtails - which are often used as a biodiversity indicator - are 8 times more prevalent and with 1.6 times more species in vegetable gardens than in agricultural lands. In the same way, parks and roofs are home to high levels of microbial biomass.

 

            Urban soils are valuable allies when it comes to managing water flows and inhibiting urban heat island, thus they must be protected and extended.

How to connect the land: the brown networks

            In order to improve the quality of the soil in urban areas a new tool is being developed, following the model of blue and green belts: the brown network. It aims at improving the continuity of the soil by connecting places in open ground and creating ecological corridors in the city. As animals on the surface, species living underground must be able to move to survive potential changes in their habitats, to reproduce or to colonize other lands. Plants also benefit from it: trees exchange nutrients through their roots and support each other by sharing fungi. 

            However, brown networks are very little implemented in our city. The high rate of soil sealing in our cities is a first indicator of this assertion. Indeed, underground spaces are highly colonized by human activities: building foundations, means of transportation (subway) and various networks (water, energy, information…). But even when the soil is not entirely used and there are plants along the streets, trees are often contained in large soil containers, making them unable to communicate with each other and thus to share resources or biodiversity, weakening their resilience and ability to grow.

            Soil fragmentation and the scattering of biodiversity reserves is the main issue limiting the development of those same biodiversity areas and the brown network can be an adequate solution to tackle this problem.

 

 

 

 

In practice: Implementing brown networks in the urban environment

            The next step is to consider how to implement brown networks in our already dense urban environments. We can consider this issue at two different scales.

            At the local scale, we should promote the restoration of soil permeability when it is possible. Sealed soils are a problem when it comes to water runoff while in the other hand soils with high biodiversity are very good at allowing water infiltration: removing artificial surfaces when they are not a necessary seems like an easy solution, in addition to the fact that we can easily find places in public space that does not require a concrete floor.

            At a larger scale, the other goal is to connect the soil with the pre-existing green and blue networks: a brown network is not enough on its own and needs to be associated with other natural areas to truly enhance the development of life in and on the ground. In Europe, those networks are often already implemented so the addition is just an extra step for the return of biodiversity to the city. However, a specific analysis of the needs of these ecosystems is required: we need further studies and data on the quality of our soil to better connect the unsealed areas and their wildlife. 

 

            Brown networks could be a very good improvement to the current blue and green structure, but it is simply just considering a single other aspect among the needs of animals and plants. Light pollution, pesticides, noise, wastes are as many problems for the nature with each a potential specific network to add to the green, blue and brown ones, free of these nuisance, but isn’t this just a temporary fix which does not question the roots of the problem which are urban sprawl and economic and demographic growth of cities?

 

Sources:

●      Blanchart, E., Cortet, J., & Gers, C. La biodiversité des sols, un bien commun au service de tous.

●      Brussaard, L. (1997). Biodiversity and ecosystem functioning in soil. Ambio, 563-570.

●      Joimel, S., Grard, B., Vieublé Gonod, L., & Chenu, C. (2021). Le fonctionnement écologique des villes : et si on pensait aux sols ?. métropolitiques. https://metropolitiques.eu/Le-fonctionnement-ecologique-des-villes-et-si-on-pensait-aux-sols.html

●      SOLS, D. (2022). les bénéfices environnementaux des revêtements de sols perméables.

●      Sordello, R. R72: Trame verte, trame bleue et autres trames, par Romain Sordello.

●      Trame Brune. Lichen. https://agencelichen.fr/trame-brune/

●      VILLES, L. (2022). RENATURER.