How Mineral Wastes are Building Biodiverse Soils
A systematic mapping of evidence reveals how engineered soils from construction waste can become vibrant ecosystems, offering sustainable solutions for our cities and a lifeline for global biodiversity.
Explore the ResearchImagine a city undertaking a massive construction project for a new metro line. Millions of tons of earth are excavated, treated as worthless waste, and hauled to already overflowing landfills. At the same time, that same city imports countless tons of fertile topsoil from the countryside to create new parks and green spaces. This paradox of wasting one resource while importing another is a global reality 1 .
But what if the excavated earth and demolition rubble could themselves be transformed into the fertile ground needed for urban greening?
This is not a futuristic dream. A quiet revolution in soil science is turning this vision into reality. Researchers are now constructing new soils—"Technosols"—from mineral wastes like excavated earth, crushed concrete, and bricks. The goal is not just to grow plants but to create thriving, living ecosystems 1 . This systematic mapping of existing evidence reveals how these engineered soils can become vibrant habitats, offering a sustainable solution for our growing cities and a lifeline for global biodiversity.
In the framework of the World Reference Base for Soil Resources, Technosols are soils whose properties and formation are dominated by human technical activity 1 . They contain significant amounts of "artefacts"—materials recognizably made or drastically altered by humans, like construction debris.
While many Technosols are the unintentional byproduct of urbanization, "constructed Technosols" are different. They are the product of pedological engineering—the deliberate and careful mixing of various mineral and organic wastes to create a new soil profile tailored to support life 1 . This transforms the linear economy of "dig, consume, and discard" into a circular one, where waste becomes the foundation for new ecosystems.
A recent comprehensive analysis, a systematic map published in Environmental Evidence, sifted through over 9,000 scientific articles to distill the existing evidence on the potential of these soils to support biodiversity 1 4 . The findings from 153 relevant articles paint a clear picture of an emerging and exciting field:
The main drivers for building these soils are mine rehabilitation (35%), general waste recycling (16%), and pure experimental research (15%) 1 .
The data reveals a strong emphasis on plants, especially herbaceous species. More importantly, a critical gap exists: only 6 out of the 153 studies investigated the response of plants, invertebrates, and microorganisms simultaneously 1 . This means we have limited understanding of how the entire soil food web develops in these constructed environments.
Many urban trees struggle to survive, not from lack of water overall, but from the intense drought between rainfalls in compacted urban soils. A two-year field study investigated whether Construction and Demolition Waste (CDW), particularly bricks, could improve the water holding capacity of tree substrates 5 .
Researchers created six different soil substrate mixtures. Some contained a standard CDW mix, while others were enhanced with a brick content of either 30% or 60% 5 .
These substrates were tested in real-world conditions over two years, moving beyond a controlled lab environment 5 .
Scientists continuously monitored the soil water content in each substrate type 5 .
They calculated key metrics from the data: Plant Available Water Content (PAWC), Relative Extractable Water (REW), and Days with Water Stress & Water Stress Intensity (WSI) 5 .
The findings were clear and significant. The substrates with enhanced brick content showed a higher Plant Available Water Content (PAWC), with a minimum of 30% bricks required for a statistically significant effect 5 . The pore structure of the bricks themselves was confirmed as the reason, acting like a sponge that retains water for the tree's roots.
This experiment is a powerful example of "closing the loop." It shows how a waste product (bricks from demolished buildings) can be engineered into a solution for another urban problem (tree survival), reducing waste and enhancing green infrastructure simultaneously.
Creating a functional Technosol is not as simple as piling waste together. It requires careful selection of materials, each serving a specific purpose in mimicking the complex structure of natural soil.
Forms the bulk matrix, providing mineral content and structure.
Can be used as a soil parent material, but requires assessment for contaminants.
Provides structure, aeration, and crucially, improves water retention.
The "life starter" - adds crucial organic matter and nutrients to feed microbes and plants.
| Material | Primary Function in Soil Construction |
|---|---|
| Excavated Soil | Forms the bulk matrix, providing mineral content and structure. |
| Mine Waste | Can be used as a soil parent material, but requires assessment for contaminants. |
| Demolition Waste (Crushed Concrete, Bricks) | Provides structure, aeration, and crucially, improves water retention. |
| Compost or Organic Sludges | The "life starter" - adds crucial organic matter and nutrients to feed microbes and plants. |
Table: Key materials used in constructing Technosols, based on prevalent materials found in the systematic map 1 .
The systematic map points the way forward. Three key knowledge clusters have been identified for future research 1 :
A quantitative synthesis of exactly how well plants grow in these waste-derived soils.
A broader assessment of their overall potential to support biodiversity, beyond just plants.
A deeper understanding of how microbial communities assemble and function over time.
The path forward is clear. As a scientific community, we must move beyond studying single groups of organisms in isolation. The ultimate goal is to create self-sustaining ecosystems 1 8 . This requires a holistic understanding of how plants, earthworms, microbes, and countless other organisms interact to form a healthy soil food web.
The transformation of mineral waste into living soil is more than a technical achievement; it is a fundamental shift in our relationship with the planet. It represents a move away from extraction and depletion and toward regeneration and circularity. By building soils, we are not just managing waste or planting trees—we are actively constructing the ecological foundations for more resilient and biodiverse cities for future generations.