Artificialization of soils: state of play of a complex challenge - Why is soil artificialisation a major problem?

Rémi Guidoum, June 2022

Fondation pour la Nature et l’Homme (FNH)

After 30 years of action, the Fondation pour la Nature et l’Homme is giving a new ambition to its Think Tank activity, which was born about ten years ago. With the question « What would a government determined to make the ecological transition do? » as its starting point, the Foundation’s Think Tank focuses on the « blind spots » of public policies by untangling the subjects that are bogged down or even unexplored. Its ambition: to create the conditions for a next five-year period of social and ecological transformation.

Based on a complex notion, recently redefined by the « Climate and Resilience » law and fuelled by historically disparate data, the debates relating to soil artificialisation are often difficult to access for the uninitiated, and sometimes lead to confusion. Despite its new definition, the notion of soil artificialisation remains trapped in a binary vision between artificialised and non-artificialized soil.

This dichotomy is maintained by monitoring tools that focus on the surface of the land, without characterising it in depth or measuring the ecological impacts actually induced on the ground. However, not all areas classified as artificial are equal from an ecological point of view, just as land uses considered as non-artificial are not necessarily favourable to biodiversity. The policy of sober land use must therefore be equipped with tools that allow for a detailed description of situations, in order to be able to reduce the consumption of space while favouring biodiversity throughout the territory. In addition to presenting the definitions and impacts, this contribution provides a summary of the distribution of built-up land in metropolitan France (geography and uses)

To download : tt-contribution-sols.pdf (7.7 MiB), artificialisation-des-sols-synthese-en-francais-1.pdf (4.7 MiB), egs_2021_28_calvaruso_03-29.pdf (2.6 MiB), primeur326.pdf (2.7 MiB)

In order to contextualise the data and take the measure of what the trends in soil artificialisation imply for France, we felt it was important to present the main consequences of this phenomenon. In short, why is soil artificialisation a major problem? Soil, which is the result of « the alteration of rocks outcropping on the surface of the globe » following various processes, depending on biological activity, climate and relief10 , is a « limited and non-renewable resource on human time scales  »11 . More than a resource, soils are in fact the living foundation of terrestrial ecosystems, without which humans and other species cannot thrive.

The agricultural impacts of soil artificialisation

From the point of view of land consumption, i.e. the transformation of ENAF into urbanised areas, the artificialisation of land represents a loss of agricultural land. According to the Teruti-Lucas survey data, two-thirds of the artificial land use between 2006 and 2014 was on agricultural land12. Historically, cities have most often developed, for obvious reasons, near particularly fertile land: urban expansion is thus largely to the detriment of land of good agronomic quality. In a context of demographic growth (INSEE forecasts 76 million French people in 207013), agro-ecological transition and the desire for greater strategic autonomy, the preservation of agricultural land is a crucial issue. In addition, climatic variations and tensions over water are contributing to the levelling off or even the reduction of agricultural yields and require a review of production methods and geography. In the case of livestock farming, the massive loss of grassland due to artificialisation (see below) is a major obstacle to its agro-ecological transition. From this point of view too, preserving as much agricultural land as possible must be a priority.

The ecological impacts of soil artificialisation

In addition to the space available for agricultural use, the artificialisation of land has an impact on its ecological functionality. The various types of urban development modify soil environments, both from a biotic (living organisms) and abiotic (structure, composition and physico-chemical conditions, etc.) point of view, and are thus one of the main causes of biodiversity loss in France. Some types of ecosystems have been particularly affected by development over the last few decades: this is particularly true of grasslands, which accounted for 47% of the natural surface area that was artificially modified (Corine Land Cover data) between 1990 and 201814, and of wetlands, whose surface area was reduced by 50% in France between 1960 and 199015. The issue is crucial.

As a result, artificial soils are more or less able to fulfil the functions on which terrestrial ecosystems depend. The scientific literature generally lists seven distinct soil functions (Calvaruso et al., 2021)16 , including the ability to provide support for plants, habitats and nutrients for biodiversity, the ability to transform and store organic matter, participation in the water cycle, filtering and degradation of pollutants, and participation in climate processes.

From this point of view, it is clear that artificialisation cannot be a binary process, since the ecological functions of soils can be altered in various ways and to different degrees not only in a context of so-called « artificialised » soil, but also within natural, agricultural and forest areas. In urban areas, the composition and structure of soils is highly variable, with important consequences on their capacity to contribute to water infiltration, store carbon, support habitats or fight against the urban heat island effect. In other words, a binary vision of artificialisation should not lead to considering urban and peri-urban environments as necessarily lost for soil and biodiversity, just as it should not lead to considering all agricultural and forest environments as ecologically virtuous. While it is essential to reduce urban sprawl, it is also necessary to work on the health of the soil in each type of area in order to promote biodiversity.

Ecosystems are more than just ecological functions: they are inhabited. Populations of various species inhabit and make up the different ecosystems which, seen on a larger geographical scale, form landscapes. Land artificialisation through urbanisation contributes to fragmenting and simplifying the landscape grid, while at the same time transforming ecological conditions at the local scale, thus applying what scientists call an « environmental filter »17. This filter has a selective effect on animal and plant species, i.e. only those species capable of adapting to the new ecological conditions of the artificial environment can survive. Soil artificialisation therefore contributes to a reduction in species richness (number of species present) at the landscape level, with a selection process that benefits generalist species to the detriment of so-called specialist species, which need specific ecological conditions to thrive. This leads to a homogenisation of animal and plant communities, to which is added a decrease in genetic diversity due to the isolation of populations.

The limits of urban densification policy

Considering the effects of land artificialisation on the landscape, the scientific studies analysed in the framework of the INRA-IFSTTAR collective expertise (2017) seem to indicate that « urbanisation would have a less negative effect than mining and road infrastructures », emphasising in particular that « landscape fragmentation and loss of habitat quality are observed in all works dealing with transport infrastructures […] whereas this is not always the case for urbanisation ». These elements are summarised in the table below, taken from the INRA-IFSTTAR report.

Although the impact of roads appears to be more negative overall than that of urbanisation, the scientific literature studied by INRA-IFSTTAR nevertheless emphasises the « major importance » of urbanisation in the impacts of artificialisation on landscapes. Thus, scientific research establishes an « alarming diagnosis  »18 concerning the impact of landscape transformation on the quality of habitats. Wetlands and grasslands, which are home to many specialist species, are « particularly affected », as mentioned above.

The scientific report stresses that despite this environmental filter effect, artificial spaces are not necessarily unfavourable to all forms of biodiversity. Some so-called « favourable » urban habitats allow generalist species to flourish: the research results seem to indicate, however, that this positive effect is more marked for flora than for fauna, just as poor quality habitats have a more negative impact on fauna than on flora.

The authors of the INRA-IFSTTAR study draw important conclusions for the design of policies to combat artificialisation, specifically urban densification policies: « These results show the limits of urban densification policies, which should be compensated for by preserving quality habitat patches or improving the quality of existing habitats, in order to preserve animal biodiversity in particular […]. These results suggest the importance of the presence of favourable habitats for the preservation of floristic richness and specialist species of flora and fauna at average levels of urbanisation. (p. 439).

In sum, if the densification of buildings in peri-urban areas is a tool for reducing urban sprawl, whose impacts on available agricultural land, soil functionality and ecosystems are particularly harmful, it must be designed in such a way as to maintain quality habitats and ecological corridors.

Conclusion

Far from being a binary phenomenon, soil artificialisation manifests itself in various ways, the consequences of which depend on the nature of the activities implemented and the initial context. It therefore needs to be monitored with tools adapted to this complexity, allowing the study of soils not only at the surface but also at depth, and considering the ecological impacts at the landscape level.

Beyond the necessary reduction in the consumption of space, the policy of land sobriety is a global project, which must integrate at all levels the issues of biodiversity and social justice. In this perspective, the question of the drivers of land artificialisation is central, in order to be able to treat not only the symptoms but also the causes of the phenomenon.

Thus, the FNH Think Tank will soon propose a synthesis of the main drivers of land artificialisation in France, before launching a reflection on possible responses in terms of public policies. This work will seek to propose measures that contribute to achieving the objectives of reducing the consumption of space, without increasing inequalities or harming biodiversity. For example, it will be a matter of thinking about public action capable of reconciling land sobriety, access to housing and services, and the promotion of biodiversity in all areas.

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