In the bustling heart of our cities, a quiet but crucial battle against climate change is being waged beneath our feet – in the urban soils. Often overlooked, these soils play a pivotal role in mitigating climate change and storing carbon, a critical component in our quest for a sustainable future.

🏙️Urban soils, integral to urban ecosystems amid global urbanization, fulfill vital city functions and offer essential ecosystem services like climate adaptation. However, they face severe degradation from artificialization and contamination, impacting functions and exacerbating natural disasters. Anthropogenic contamination poses risks to human, plant, and soil health. Managing these issues is crucial as 12 million hectares of natural soils are impacted annually by urban expansion. Understanding and sustainably managing these soils are pivotal as they greatly influence city development and human-environmental health.

The term ‘urban soil’ appeared in the 1960s with Zemlyanitskiy, who studied 📚 soils in urban areas and recognized their disturbances. Defining the concept of urban soil is not easy, as there is no consensus among scientific disciplines. There is a frequent distinction between the two definitions. The first one simply defines “urban soils” as “soils located in the urban area”. Such soils are strongly heterogeneous. Some of them could be strongly affected by anthropization and, as such, altered by human activities like soil sealing, compaction, and contamination, but others might be spared by any degradation and be similar to natural soils in some urban forests, parks, or gardens. The second definition is solely focused on “urban soils” as a synonym of “artificial and degraded soils” as previously described. By the way, it is also of true importance to notice that not all artificial soils have been degraded, indeed some soils have been created by human activities to be very fertile (e.g. growth substrates for pots or green roofs). 🔬

The presence of various exogenous materials and the mixing of the original soil result in significant vertical and horizontal heterogeneity, leading to diverse spatial physical and chemical properties.

Urban soils are formed through processes similar to natural soils 🌱, but their pedogenic evolution occurs at a much faster rate due to human interventions. Several days are enough to change entirely the urban landscape instead of centuries! Their formation can be explained with the same stages as natural soils, but with human interventions:

• Weathering: The original material is transformed by mixing, compaction or aeration of material layers.
• Transport: Layers can be dug and eliminated; the original soil profile is partially or totally modified.
• Accumulation: Various exogenous materials are added to the soil.

The result can be completely different from the original soil material.

Soil heterogeneity stems from diverse city uses 🛠️: essential for infrastructure, supporting vegetation, and serving industrial, agricultural, and recreational needs. That multitude of uses gives rise to a variety of soil types. The intensity level and duration of anthropic influence play also a main role in that plurality, such as transformation influence, of which intensity increases from periphery to center in towns. Finally, these uses are succeeding in time ⏰, mostly rapidly, and must be supervised wisely. Indeed, transforming an industrial site into a residential one can be hazardous for both human and natural lives in case of contaminated soil or groundwater. Their management is thus essential.

All these human pressures very often degrade the resource ‘soil’. However, its functions and the ecosystem services a healthy urban soil can provide are priceless for sustainable city development 🏙️.

Urban soils serve as plant substrates, water, and carbon reservoirs, and regulate biogeochemical cycles in urban 🌆 ecosystems. Their functions, a result of interactions between living and non-living elements, define soil quality and health. Poor management and artificialization can degrade soil, affecting its physical, chemical, or biological qualities and hindering healthy plant growth. This impacts the ecosystem services a well-managed urban soil can provide to its environment.

Urban soils serve multiple roles 🌿: nutrient cycling, water storage, structural support for buildings, and biodiversity hubs. They’re reservoirs for vital nutrients like nitrogen, phosphorus, and potassium, often richer than non-urban soils. Their water retention, ranging from 50 to 400 L/m2, is substantial, and they support diverse ecosystems.

Urban soils play a pivotal role in regulating ecosystems, particularly in water 💧 and carbon cycles. They facilitate rainwater infiltration, and control transfers between atmosphere, groundwater, and rivers, thus mitigating flood risks. They also play a crucial role in mitigating urban heat islands through direct evaporation and transpiration of the vegetation they are the basis for. Depending on factors such as soil type, vegetation, management practices and the contribution of organic matter, notably via compost, urban soils can become a major carbon reservoir, containing three to five times more carbon than natural soils. In this way, they could regulate greenhouse gases and mitigate urban heat islands. Healthy urban soils are crucial for climate change adaptation.

Moreover, they offer provisioning services, supporting biomass production for food 🍎, construction materials 🧱, and energy ⚡. Additionally, they provide cultural benefits by enhancing landscape aesthetics and creating recreational spaces like city parks.

Unfortunately, artificialization and human interventions impact the main functions of the soil, inhibiting these crucial ecosystem services.

City expansion 🏗️, construction, loss of green spaces, and shifting land use strain soil properties, degrading organism communities and activities. Urban soils, composed of diverse materials, can introduce pollutants like heavy metals—copper, lead, zinc—threatening nature and human health, and disturbing soil biodiversity.

Artificial sealing, like sidewalks, hampers rainwater infiltration, heightening runoff to streams and flood risks. The absence of green spaces disrupts the carbon cycle, impeding greenhouse gas regulation, impacting air quality, and elevating urban temperatures 🌡️.

Overall soil degradation impedes their crucial roles in climate regulation, heat management, and carbon storage, posing significant challenges, notably in cities.

🌱 To preserve urban soil functions, sustainable management and integrating soil ecosystem services into urban planning are vital. Sustainable management ensures responsible resource use without compromising future needs, considering environmental, social, and economic aspects for long-term viability.

With an estimated 70% of the global population residing in urban areas by 2050, competition for space intensifies. 💡 Innovative urban agriculture methods like green walls, vertical farms, and rooftop gardens optimize existing structures, creating sustainable green spaces. Community gardens, urban beekeeping, and street landscaping contribute to local, sustainable food production and greener cities, enhancing ecosystem functioning.

Managing contamination is crucial, especially in dynamic land-use scenarios. Regular assessment of hazardous element concentrations informs decisions; for instance, avoiding transforming polluted areas into residential districts with gardens to prevent health risks 🤒 from soil and water pollution.

At a larger scale, a tremendous effort is expected to ensure biological connectivity by considering not only the blue and green corridors but also the connectivity of soils in our cities i.e. the brown corridor.

📜 Laws play a pivotal role in soil conservation, promoting existing green space preservation and new establishment while implementing management guidelines. These measures consider urban soil risks and the potential to enhance ecosystem services, safeguarding soil health in cities.

🚧In urbanizing landscapes, healthy soils are vital for liveable cities, supporting air quality, climate regulation, flood control, and local food production. Urbanization threatens soil health, endangering the environment and human well-being. Recognizing soil’s significance is crucial for sustainable cities, demanding strategic soil management and integration into urban planning. Understanding urban soils better is essential for effective policies 🌱.

BIBLIOGRAPHY

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