After the fourth flood event in Emilia-Romagna (Italy) in only 18 months, the problem of soils consumption is once again on the tables for debate.
2016 FAO data said that worldwide 17 ha of soil are sealed every minute under expanding infrastructure. Agenda 2030, in Goal 15, remarks “By 2030, combat desertification, restore degraded land and soil, including land affected by desertification, drought and floods, and strive to achieve a land degradation-neutral world”.
But 2024 data show a worldwide urbanization of 57%, and Europe accounts for 75%. This is in the face of a much slower demographic growth. When considering a longer time frame, the population of Europe grew from 354.5 million in 1960 to 449.2 million on 1 January 2024, an increase of 94.7 million people. The rate of population growth has gradually slowed down in recent decades: the Europe population increased on average by about 0.6 million persons per year during the period 2015–2024, whereas the average increase in the 1960s was 2.9 million people per year (Eurostat data and Statista data).
Previsions are no better: the European Commission shows that Europe’s level of urbanization is expected to increase to approximately 83.7% in 2050. The migration of population to cities is one of the factors driving agricultural land abandonment, which is expected to reach 4.2 million ha net over the period 2015-2030, bringing the total abandoned land to 5.6 million ha by 2030, the equivalent of 3% of total agricultural land.
Among the prospectives for soil recovery, soil de-sealing is emerging, even though this this practice raises concerns.
What is soil sealing?
Sealing of soil consists in impermeable layers that separate it from the atmosphere and hydrosphere. It reduces its ecological functionality related to water infiltration, reduces microbial diversity and microbial activity due to reduced inputs of organic matter and contributes to the adverse effects on urban microclimate. Biomass and activity of soil microorganisms depend on the quantitative and qualitative inputs of litter and organic matter, as soil microorganisms obtain energy from their decomposition and mineralization. The impacts of sealing on soil microbially driven soil functions have been well documented.
Moreover, soil sealing fragments the land-scape and leads to an overall loss of and functioning soils, through modifications of the physical and chemical conditions of the soils. Imperviousness is considered to be the main cause of the reduced capacity of soils to provide expected ecosystem services in cities such as flood risk and stormwater runoff management, or the contribution to microclimate regulation and thermal comfort.
What is soil de-sealing?
Creation of new green space in cities can increase the urban resilience to the extreme climatic events and improve the citizens’ life quality. De-sealing consists in the removal of the impermeable surfaces. In sealed areas, urban greening interventions require as a preliminary step the soil de-sealing and the adoption of appropriate agro-environmental practices for restoring the quality of de-sealed soils, which generally require backfill soil for leveling and allowing the optimal plant rooting. The soil material is conventionally taken from the first layers (topsoil) of agricultural land, with high impacts on both soil consumption and transport environmental and economic costs. In this context, urban soil restoration and management can be hampered by the lack of available topsoil with suitable physicochemical properties and adequate fertility. This practice is generally adopted because data on physico-chemical and biological fertility of de-sealed soils are still scarce, and the potential fertility of de-sealed soils is still poorly known.
Two interesting case study on de-sealed soils
The first study (by Institute of BioEconomy, National Research Council and collaborations) was done in three municipalities of Italy, in Emilia Romagna Region (Northern Italy), Carpi, San Lazzaro di Savena and Forlì, which planned a de-sealing action in their urban fabric as a compensation measure for new building sites. Two cars park and local waste management have been de-sealed. It is quite interesting that the topsoil used in the study was collected along a channel in a depression of the alluvial plain.
The results showed that de-sealed soils could restore their quality and fertility without the using exogenous topsoil. In all the experimental plots, the presence of ornamental shrubs enriched the soil organic carbon through leaf. De-sealed soils showed high chemical fertility, in terms of bulk density, pH values and total and organic carbon. The metal concentrations lie in the background ranges of the soils of the Emilia Romagna region plain. Heavy metal concentrations in soils of the Emilia-Romagna alluvial plain are chiefly controlled by texture and less by the parent material composition, and under such conditions, Cu, Pb and Zn are preferentially bound to the silt and clay fractions and characterized by low bioavailability. The metal enrichment over time in all sites and plots in our experiment is due to the accumulation of pollutants derived by exposition on anthropogenic activities in urban areas. This is a demonstration that de-sealed soils, after accurate management (shrub planting, irrigation), can improve their fertility reach in a short time, as well as functional and biological stability.
The second study is a survey by the Institut Agro, EPHOR, France and collaborators. In this study, the identified de-sealing methods usually implied replacing the sealing cover and subbase by newly imported materials, and any soil characterization or monitoring of the de-sealed sites has been performed. The primary objective of de-sealing soils was to regulate water run-off in the city. But the survey showed that a wider range of ecosystem services were sought, such as reducing the heat island effect through greening. De-sealing often consisted in small but multiple projects when greening was targeted, and resulted in vegetated patches that could significantly contribute to urban biodiversity if they were close enough to each other. The results of this study provide quite an exhaustive view of current French good de-sealing practices and could provide guidelines for improving soil functions by applying soil engineering processes to construct sustainable fertile soils for urban greening.
The New York High-line case
Unfortunately soil de-sealing isn’t so easy, especially in metropolis. Green areas are more and more required, but when demolitions are not possible, the redemption of old infrastructure is possible.
It was once an unused railway destined for demolition. Today it is an elevated park that hosts millions of people every year; you can stroll through its gardens, admire works of art, attend a show and enjoy the view of New York City. We are talking about the famous High Line Park, which in the center of Manhattan was created to be much more than a park: it is in fact a project that lives thanks to the care of citizens and the visionary ability to see an opportunity for redemption in an abandoned space.
References:
[1] Biological restoration of urban soils after de-sealing interventions; https://doi.org/10.3390/agriculture11030190
[2] Impacts of soil de-sealing practices on urban land-uses, soil functions and ecosystem services in French cities; https://doi.org/10.1016/j.geodrs.2024.e00854
