Researchers at the Technical University of Munich (TUM) have demonstrated a method for repurposing excavated construction soil by converting it into “constructed soils.” This project, involving samples from Munich and Augsburg, Germany, focuses on enriching degraded excavated material with urban organic waste products. By blending soil with green-waste compost and biochar—often derived from biogas production residuals—the team has created a functional substrate that aligns with the EU Soil Strategy’s goal of zero net “land take” by 2050. This research positions constructed soils as a pivotal element in urban landscape planning and waste management.

The primary challenge addressed by the TUM team is the massive volume of excavated soil generated by construction, which currently accounts for approximately one-fourth of all waste in the European Union. Traditionally, this material is transported to landfills, while high-quality topsoil is concurrently stripped from areas surrounding cities to support urban parks and green spaces. This dual inefficiency results in high transportation emissions, the depletion of peri-urban land, and the loss of potentially valuable mineral resources buried in construction waste. Furthermore, degraded urban soils often lack the fertility and filtration capacity required for modern green infrastructure.

To solve this, the TUM researchers developed a customization framework for constructed soils using biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More as a primary additive. By adjusting the ratios of biochar and compost, the team can tailor the soil’s properties to specific urban use cases. For instance, biochar’s high adsorption capacity is leveraged to bind up to 90% of heavy metals and other pollutants, making it ideal for roadside swales that protect groundwater. Simultaneously, the inclusion of biochar and compost quadruples nitrogen content and improves carbon accrual, providing the necessary fertility for urban wetland plants to thrive under stressors like heat and flooding.

The outcomes of the TUM study reveal a robust “win-win” scenario for urban circularity and climate resilience. The resulting constructed soils not only divert millions of tons of excavated material from landfills but also create high-performance substrates that outperform traditional soil in urban settings. Greenhouse tests conducted by the Chair of Restoration Ecology confirmed that plants grown in these biochar-enriched substrates exhibit superior tolerance to extreme weather events. Ultimately, this research provides city planners with a technical blueprint to close the resource cycle for urban soils while enhancing groundwater protection and carbon sequestration.