As cities expand underground through new roads, housing, tunnels, and infrastructure, huge volumes of soil are dug up, moved, and often left exposed at construction or storage sites. A new study published in Biochar suggests that these excavated soils may carry a largely overlooked climate cost.
Researchers from Kyung Hee University quantified carbon dioxide (CO2) and methane (CH4) emissions from carbon-rich excavated urban soils at a field site in South Korea. They also tested whether biochar amendment and soil capping could reduce those emissions. Their findings show that excavated soils are not just passive construction waste. When left exposed, they can release measurable amounts of greenhouse gases into the atmosphere.
The study focused on soils excavated from a large residential redevelopment site in southern Seoul. These soils came from carbon-rich subsurface layers created by past land use and burial of former agricultural soils. Once excavated and exposed to air, warmer temperatures and improved oxygen availability can accelerate decomposition of soil organic carbon, producing CO2. During heavy rainfall, temporary water saturation can also create anaerobic conditions that promote CH4 emissions.
The researchers found that surface-exposed excavated soils emitted 12.78 tons of carbon per hectare per year, including 12.54 tons as CO2 and 0.24 tons as CH4. This corresponded to an annual soil organic carbon decomposition rate of 1.45%. Although methane emissions occurred mainly after rainfall events, they made an important contribution to total climate impact because CH4 has a much stronger warming effect than CO2.
"Excavated soils are often treated as a logistical or waste-management issue, but our results show that they should also be considered in carbon management," said corresponding author Professor Gayoung Yoo. "As underground development increases worldwide, better handling of these soils could become a practical opportunity for climate mitigation."
The team tested two mitigation approaches: burying excavated soils under cleaner in-situ soil layers, and mixing the excavated soils with 2% wood-derived biochar. Deep burial at 40 to 60 cm substantially reduced carbon fluxes by limiting exposure to warm, oxygen-rich surface conditions. When combined with biochar, deep burial reduced annual CO2 emissions by 42.5% and CH4 emissions by 95.8%, compared with surface-exposed excavated soils.
Biochar alone also helped under surface-exposed conditions, reducing annual CO2 and CH4 fluxes by 8.9% and 25%, respectively. The authors suggest that biochar may improve soil structure and aeration, making conditions less favorable for methane production during wet periods. At the same time, biochar itself provides long-term carbon storage because much of its carbon remains stable in soil for decades or longer.
At the national scale, the researchers estimated that unused excavated soils in South Korea emitted 0.14 million tons of carbon between 2019 and 2023. If deep burial and biochar amendment had been applied, about 0.06 million tons of carbon emissions could have been avoided. In addition, the carbon stored directly in biochar could contribute 3.78 million tons of long-term carbon sequestration, giving a total mitigation potential of 3.84 million tons of carbon. This is equivalent to about 15% of South Korea's waste sector emissions over the same five-year period.
The study highlights a carbon source that is rarely included in greenhouse gas inventories or construction-sector planning. The authors note that more research is needed to evaluate different soil types, contamination risks, costs, and long-term monitoring needs. However, the findings suggest that simple practices such as soil capping and biochar amendment could be integrated into existing construction and waste-management workflows.
"Our work provides field-based evidence that excavated soils can be managed more climate-smartly," Yoo said. "Recognizing these soils as a carbon source is the first step toward turning an overlooked problem into a mitigation opportunity."
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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
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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.
