A new study has revealed that excavated soils from urban construction sites represent a previously overlooked source of greenhouse gas emissions, while also highlighting practical strategies to significantly reduce their climate impact.
Urban development generates massive volumes of excavated soil, often stockpiled for extended periods. Until now, the carbon emissions associated with these soils have remained poorly understood. In this new field-based study, researchers quantified both carbon dioxide and methane emissions from excavated urban soils and tested mitigation strategies using biochar and soil capping.
"Our findings show that excavated soils are not just passive waste materials. They actively release carbon into the atmosphere and should be considered in urban carbon management strategies," said the study's corresponding author. "The good news is that relatively simple interventions can dramatically reduce these emissions."
The research team conducted experiments on carbon-rich soils excavated during a large-scale urban redevelopment project. When these soils were left exposed at the surface, they emitted approximately 12.78 tons of carbon per hectare per year, primarily as carbon dioxide, with smaller but important contributions from methane. These emissions were driven by increased oxygen exposure and higher temperatures, which accelerate microbial decomposition of soil organic carbon.
Importantly, the study demonstrates that management practices can substantially mitigate these emissions. When excavated soils were buried deeper beneath the surface and amended with a small amount of biochar, emissions dropped significantly. Deep burial combined with biochar reduced carbon dioxide emissions by more than 40 percent and methane emissions by nearly 96 percent.
Biochar, a carbon-rich material produced from biomass, plays a dual role. It stabilizes carbon within soils while also improving soil structure. By increasing porosity and oxygen availability, biochar can suppress methane production, which typically occurs under oxygen-limited conditions.
Even when applied alone at the surface, biochar reduced emissions, though to a lesser extent. The researchers note that the relatively modest application rate used in the study reflects realistic, cost-effective conditions for large-scale implementation.
At the national level, the implications are substantial. The study estimates that excavated soils in South Korea emitted about 0.14 million tons of carbon between 2019 and 2023. However, adopting combined strategies such as deep burial and biochar amendment could mitigate a significant portion of these emissions. In addition, the carbon stored directly within biochar contributes further to long-term sequestration, resulting in a total mitigation potential equivalent to about 15 percent of the country's waste sector emissions over the same period.
The study also highlights the importance of methane, which, although emitted intermittently, contributes disproportionately to climate impact due to its high global warming potential. During wet conditions, methane accounted for up to 22 percent of total greenhouse gas effects in some treatments.
These findings have important implications for urban planning and climate policy. Excavated soils are currently not explicitly included in most greenhouse gas inventories, yet their contribution may be significant, particularly as urbanization continues to accelerate worldwide.
"Urban infrastructure projects are expanding globally, and with them the volume of excavated soils," the author added. "By integrating soil management practices such as capping and biochar application into construction workflows, we have an opportunity to turn a hidden emission source into part of the climate solution."
The researchers emphasize that further work is needed to refine large-scale estimates and develop monitoring frameworks. However, the study provides one of the first real-world datasets demonstrating both the scale of the problem and the feasibility of practical mitigation strategies.
As cities continue to grow, managing the carbon footprint of construction activities will be increasingly critical. This study positions excavated soils as an important, yet manageable, component of that challenge.
===
Journal Reference: Bae, J., Jeong, M. & Yoo, G. Urban excavated soils as an overlooked carbon source: quantifying CO2 and CH4 emissions and mitigation via biochar and soil capping. Biochar 8, 65 (2026).
https://doi.org/10.1007/s42773-026-00587-y
===
About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
