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 CO 2 and CH 4 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 (CO 2 : 12.54, CH 4 : 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 CO 2 and CH 4 fluxes were significantly reduced by 42.5% and 95.8%, respectively. Separately, under surface-exposed conditions, biochar amendment alone reduced annual CO 2 and CH 4 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.