Large but overlooked carbon fluxes (CO2 and CH4) from excavated soils by urban development: Magnitudes, causes and possible mitigation strategies

Globally, excavated soils are the biggest construction wastes and are simultaneously the largest carbon repositories in terrestrial ecosystems. As urbanization continues to drive underground development, handling excavated soils has gained growing importance. The reuse or transfer of excavated soils has direct and long-term effects on regional and cross-border soil carbon dynamics. Soil carbon fluxes, the second largest in terrestrial ecosystems, play a crucial role in regulating the global carbon cycle. However, quantifying carbon fluxes in excavated soils and technologies to mitigate these overlooked carbon emissions have yet to emerge in the construction sector. In our study, we quantified annual soil carbon fluxes (CO2 and CH4) at varying soil capping depths (0, 20, and 40 cm) following the reclamation of excavated soils. Additionally, we investigated the impact of biochar, a commonly utilized agricultural amendment, on soil carbon fluxes. Our findings revealed that implementing a 40 cm soil capping depth led to a substantial 20.52% reduction in CO2 flux (from 11.46 to 8.87 tonC ha-1 yr-1) and a substantial 79.58% decrease in CH4 flux (from 0.219 to 0.045 tonC ha-1 yr-1). Furthermore, the incorporation of biochar resulted in a significant reduction in annual CH4 flux, with reductions of up to 28.62% (from 0.219 to 0.157 tonC ha-1 yr-1), while no significant differences were observed in annual soil CO2 flux. In summary, our study offers essential insights into the impact of excavated soils on regional carbon cycles and proposes viable strategies for mitigating excessive soil carbon emissions within the construction sector. Keywords: urban soils, excavated soils, soil CO2 flux, soil CH4 flux, soil capping, biochar