Direct and indirect carbon losses from urban redevelopment: An overlooked soil carbon stocks and fluxes in the construction sector

Underground expansion is a clear manifestation of urbanization in the 21st century. With the population surge and the improvement of construction technology, underground developments have been widely considered in redevelopment plans in many cities. Underground space offers the benefits of energy efficiency in terms of thermal conservation, reducing the use of fossil fuels. Underground development, however, threatens natural carbon pools like soil ecosystems. Earthworks, such as cutting trees and excavating soils, are preliminary steps towards underground development. Excavated soils are one of the biggest construction and demolition waste, but are also the largest carbon stocks in urban ecosystems. Most studies only focus on embodied and operational carbon, namely carbon emissions during or after construction. As such, the initial carbon loss stored in natural resources is often overlooked. Here, we estimated direct carbon loss to above- and below-ground biomass based on field-measured data from 9,487 trees over 50 years old within six redevelopment sites (100 ha) in Seoul, the capital city of South Korea. We also quantified soil organic carbon (SOC) stocks of the 693 soil samples under impervious surfaces and vegetative surfaces across soil profiles to a depth of 5 m using a portable soil boring machine. To quantify indirect carbon loss, we measured soil respiration (Rs) of excavated soils that were moved outward during underground development. Our results highlight that (i) the loss of SOC stocks (17.66 ± 3.89 Gg) at a depth of 5 m is more than 15 times greater than the loss of above- and below-ground biomass carbon combined (1.12 Gg), (ii) SOC stocks (12.62 ± 3.16 Gg) at subsoil layers (1.0-5.0 m) account for > 70% of total SOC losses, and (iii) the indirect carbon loss of Rs (4.42 ± 0.97 Gg yr-1) outside the construction sites is up to 25% of the total loss of SOC stocks, which is nearly four times larger than the loss of biomass carbon. Collectively, these findings provide baseline information on how urban redevelopment may affect natural carbon pools, which should be taken into consideration when evaluating the carbon cycles and budgets of urban ecosystems.