Which soil quality indicators to promote vegetation within urban development and planning? | Quels indicateurs de qualité des sols pour promouvoir la nature en ville en planification et aménagement ?

International audience

English. There is an increasing demand of nature in cities worldwide. However, urban soils can be degraded by, for example, pollution or loss of organic matter. In addition, the European strategy to achieve no net land take by 2050 promotes the renaturation of artificialized soils. There is a lack of knowledge and tools however to help optimise the management of urban soils within redevelopment and planning strategies. To address this lack, one of the objective of the R&D project SUPRA was to prepare a decision aid tool to help optimise the qualification of urban soils that could be considered for vegetation growth, including food production and other purposes. Within this framework, we developed a set of indicators to assess the soil functions linked to 3 services associated with vegetation growth: production of biomass, contaminants regulation and climate regulation. This work is based on a literature review and a working group of experts on urban soils. The first step linked services, functions and indicators of soil condition. A set of functions and subfunctions was then decided. For each of them, we listed the corresponding parameters, the existence or not of normative methods and the range of reference values. Discussions were held to select a set of minimum indicators. The existing range of reference values was used to set threshold that would allow the evaluation of each indicators. In case there was no reference, and the indicators were considered essential, new threshold value were proposed. A first version of decision aid tool was also developed and first tests allowed comparing the results obtained for some characterised soils.The following 6 functions were selected: i) Storage, recycling and transformation of organic matter ; ii) Retention and supply of nutrients / Supply-recycling of nutrients (decomposition of organic matter) ; iii) Physical support – housing ; iv) Water retention, circulation and infiltration ; v) Retention, filtration, degradation of contaminants ; vi) Support of biodiversity. The minimum data set includes 30 indicators of which 11 correspond to field observations and others to measurements (in situ or in the laboratory). The level of expertise and an assessment of the cost of acquisition are also described. The first tests of the tool need to be continued with more soils to verify the scoring of soil quality with the assessment of soil experts. A longer-term objective would be to map the results of soil quality scoring that could be used within urban strategies for biomass production.