As urban landscapes continue to sprawl vertically and extend beneath the surface, subterranean construction projects have become increasingly prevalent worldwide. These underground expansions—encompassing new roads, tunnels, housing developments, and critical infrastructure—often necessitate extensive excavation of soil. While such excavated soils are typically perceived as mere byproducts or inconveniences to be managed as construction waste, groundbreaking research published recently in the journal Biochar illuminates their hidden contribution to climate change. This study reveals that these excavated urban soils are active sources of greenhouse gas emissions, specifically carbon dioxide (CO2) and methane (CH4), with significant implications for future carbon management strategies in urban planning and construction.
The team of researchers from Kyung Hee University in South Korea undertook a comprehensive experimental investigation to quantify greenhouse gas emissions associated with carbon-rich urban soils excavated during redevelopment activities. Focusing their field studies on a large residential redevelopment zone in southern Seoul, they identified that soils originating from buried, organic carbon-rich agricultural layers beneath the city, once disturbed and exposed above ground, undergo accelerated carbon decomposition. Environmental factors such as elevated temperatures and increased oxygen availability at the surface stimulate microbial activity, breaking down soil organic matter and releasing CO2 at notable rates. Conversely, after rain events, the temporarily saturated soils create anaerobic microenvironments favorable to methanogenic microbes, leading to episodic bursts of CH4 emissions.
Quantitatively, the exposed excavated soils emitted a total of 12.78 tons of carbon per hectare annually. Notably, 12.54 tons derived from CO2 fluxes and an additional 0.24 tons from methane emissions. This carbon flux corresponds to an annual decomposition rate of 1.45% of soil organic carbon. Methane’s role, though quantitatively smaller, is disproportionately climatically potent due to its high global warming potential relative to CO2. Thus, these emissions cumulatively represent a substantial and previously underappreciated carbon source within the urban environmental context.
The findings emphasize that excavated soils, traditionally relegated to considerations of logistics and waste management, constitute a neglected node in urban carbon cycles. Professor Gayoung Yoo, lead correspondent of the study, underscores the urgency of revising current practices. She advocates for the integration of carbon management strategies addressing excavated soil handling amid increasing global subterranean development. Such interventions, the researchers argue, could serve as viable avenues for mitigating construction-sector contributions to greenhouse gas inventories.
To explore mitigation, the research examined two primary interventions: soil capping and biochar amendment. Soil capping involved re-burying excavated soils beneath layers of uncontaminated in-situ soil at depths between 40 and 60 centimeters. This approach limits soil exposure to oxygen and temperature fluctuations. Biochar amendment entailed mixing excavated soils with 2% by weight wood-derived biochar before burial or surface exposure. Biochar—a form of charcoal produced through pyrolysis of biomass—possesses recalcitrant carbon structures, enhancing carbon sequestration potential alongside altering soil physicochemical properties.
Results were striking. Deep burial alone significantly curtailed greenhouse gas emissions by creating more stable, less aerated conditions. When combined with biochar, annual CO2 emissions fell by 42.5%, and methane emissions plummeted by 95.8%, relative to surface-exposed soil conditions. These reductions demonstrate a synergistic effect where biochar not only serves as a direct carbon sink but also facilitates improved soil structure and aeration, thereby suppressing methanogenesis during wet conditions.
Biochar’s beneficial effects extended even under surface exposure. Amendments reduced CO2 emissions by 8.9% and CH4 by 25%, suggesting immediate mitigation potential without necessitating soil burial. This could be particularly valuable in urban settings where deep burial is logistically challenging or cost-prohibitive. Furthermore, biochar’s inherent chemical stability contributes to persistent carbon storage within soils, offering climate mitigation dividends spanning decades.
Scaling their findings nationally, the investigators estimated that unused excavated soils in South Korea emitted approximately 0.14 million tons of carbon over the five-year period from 2019 to 2023. Implementing deep burial combined with biochar amendment could have averted about 0.06 million tons of carbon emissions during this timeframe. Beyond emissions avoidance, biochar itself could sequester approximately 3.78 million tons of carbon long term. Together, these mitigation effects aggregate to an impressive potential reduction of 3.84 million tons of carbon, equivalent to nearly 15% of South Korea’s total waste sector emissions in those five years.
This study casts light on an overlooked facet of urban carbon cycling with practical implications. Excavated soils are not inert detritus but dynamic sources of greenhouse gases influenced by environmental and soil management conditions. Despite this, greenhouse gas inventories and construction-sector protocols generally exclude these emissions, omitting a considerable source of urban carbon flux. Inclusion of these emissions in urban greenhouse gas accounting is imperative to accurately gauge and mitigate sectors’ climate impacts.
While the research offers compelling evidence for climate-smart management of excavated soils, the authors call for further investigations. Diverse soil types, variable climatic regimes, potential contamination risks, economic feasibility, and long-term monitoring requirements warrant comprehensive study before widespread adoption. Yet the clear results reported provide a strong rationale to begin incorporating strategies like soil capping and biochar amendment into mainstream urban construction and waste management.
The implications extend beyond South Korea, as rapid urbanization and underground development proliferate globally. Proactive soil management techniques thus represent an innovative lever to curb carbon emissions arising from urban growth infrastructure demands. Professor Yoo envisions a future in which recognizing excavated soils as both a carbon source and a mitigation opportunity transforms construction practices and advances climate resilience.
Ultimately, this research exemplifies how re-examining ostensibly mundane construction byproducts can uncover significant environmental challenges and solutions. By quantifying and mitigating the greenhouse gas emissions emanating from excavated urban soils, the study provides a blueprint for integrating carbon-conscious practices into urban development. In doing so, it opens a promising new front in the battle against climate change—one rooted quite literally beneath our feet.
Subject of Research: Quantification of CO2 and CH4 emissions from urban excavated soils and evaluation of mitigation strategies via biochar amendment and soil capping.
Article Title: Urban excavated soils as an overlooked carbon source: quantifying CO2 and CH4 emissions and mitigation via biochar and soil capping.
News Publication Date: 1 March 2026
Web References:
Biochar Journal
Article DOI
References:
Bae, J., Jeong, M., & Yoo, G. (2026). Urban excavated soils as an overlooked carbon source: quantifying CO2 and CH4 emissions and mitigation via biochar and soil capping. Biochar, 8, 65. https://doi.org/10.1007/s42773-026-00587-y
Image Credits: Jeehwan Bae, Minseop Jeong & Gayoung Yoo
Keywords: Urban excavated soils, greenhouse gas emissions, carbon dioxide, methane, biochar, soil capping, carbon sequestration, climate mitigation, soil organic carbon, urban infrastructure, environmental engineering, climate-smart construction
