Simplified performance assessment methodology for addressing soil quality of nature-based solutions

Purpose In urban areas, soil functions are deeply impacted by all human activities, e.g., water infiltration, carbon storage, and chemical substances degradation potential. In this context, nature-based solutions (NBS) are assumed to deliver multiple environmental benefits for soil quality improvement. The H2020 Nature4Cities project (N4C) offers the framework to develop physical, chemical, and microbiological indicators to the performance assessment for addressing NBS soil quality (performance assessment of soil quality) to be included in a tool-box designed for architects or municipalities. Materials and methods A simplified performance assessment methodology was developed for addressing NBS soil quality. It is based on the comparison of physical, chemical, and biological characteristics to soil reference baseline. In this setting, we present here the results obtained from case studies selected in three European cities (Nantes (F), Nancy (F), Bustehrad (CR)) to test the methodology. The case studies correspond to three different NBSs: former market turned into gardening areas (FMG), green roofs (GR), and urban allotment gardens (UAG). The performance assessment was based on two criteria: (1) soil fertility and (2) soil contamination. Results and discussion The basic soil properties (e.g., pH, bulk density) and soil fertility (e.g., soil organic matter (SOM)) for the two open soil NBS (FMG and UAGs) are equivalent to cultivated soils. Those of GR are highly controlled by the type of natural materials used in the substrate. Concerning contamination, the soil quality of FMG was shown to be significantly impacted by former agricultural practices (pesticide residues, trace metals (TE)). Measured molecular biomass of FMG was compared with predictive molecular biomass (determined according to the soil physicochemical properties). Data showed that 12 of the 14 measured plots are classified as altered or very altered soils with regard to this parameter. TE in UAGs soils exhibit various concentrations, depending on former land use, cultural practices, and geological contexts. Conclusions In conclusion, the study showed that soil fertility is a rather interesting tool in the evaluation of urban soil quality. Nevertheless, basic soil properties seem to be influenced by the effects of trace element and pesticide contamination. The presence of NBSs seems to have a favorable impact (e.g., filtration of pollutants). Inappropriate management of urban soils can lead to a decrease in soil quality and thus influence the current major issues (e.g., carbon sequestration, contamination of urban spaces by organic, and inorganic pollutants).