Urban excavated soils as an overlooked carbon source: quantifying CO₂ and CH₄ emissions and mitigation via biochar and soil capping

Urbanization generates vast quantities of excavated soils, yet their carbon emissions remain poorly quantified. This study presents the first field-based quantification of CO₂ and CH₄ fluxes from excavated urban soils stockpiled at a vacant site in South Korea, and evaluates the effectiveness of biochar amendment and soil capping as carbon mitigation strategies. Surface-exposed excavated soils emitted 12.78 ton C ha⁻¹ yr⁻¹ (CO₂: 12.54, CH₄: 0.24), corresponding to an annual SOC decomposition rate of 1.45%. When excavated soils amended with 2% (w/w) biochar were deeply buried (40–60 cm), annual CO₂ and CH₄ fluxes were significantly reduced by 42.5% and 95.8%, respectively. Separately, under surface-exposed conditions, biochar amendment alone reduced annual CO₂ and CH₄ fluxes by 8.9% and 25%, respectively. At the national scale, unused excavated soils are estimated to have emitted 0.14 ± 0.44 Mt C between 2019 and 2023, of which around 0.06 Mt C could have been mitigated through deep burial combined with biochar amendment. Furthermore, biochar contributes an additional 3.78 Mt C of long-term carbon sequestration, resulting in a total mitigation potential of 3.84 Mt C, equivalent to 15% of South Korea’s waste sector emissions over the same five-year period. These findings establish excavated soils as an overlooked but substantial carbon emission source and demonstrate that biochar amendment and soil capping offer scalable, nature-based solutions with relevance to urban planning and national carbon mitigation efforts.