Integrating soil organic carbon loss into building life cycle assessment and urban planning: implications for urban sustainability practice.

Urban expansion depletes soil organic carbon (SOC) stocks through topsoil disturbance, removal, and sealing, contributing to carbon emissions and climate change. However, SOC considerations are largely absent from urban sustainability practice. This study integrates SOC loss into two distinct assessment domains-urban planning and building life cycle assessment (LCA)-to evaluate its magnitude at both the urban and building levels and investigate potential pathways to mitigate it. At the urban level, a business-as-usual urban expansion scenario for Auckland, New Zealand, is modelled and compared with a SOC-aware alternative that prioritizes development on low-SOC soils. The SOC-aware scenario was found to conserve 14.6% of SOC (0.092 MtCO2-eq), equivalent to 20-46% of the national embodied impact reduction target for the construction sector, demonstrating the significance of SOC-aware planning. At the building level, SOC loss is introduced as a potential LCA module, examining four residential typologies-apartment, townhouse, double-storey detached, and single-storey detached-to evaluate its magnitude relative to conventional embodied impacts. The results indicate that SOC loss can account for up to 70.9% of conventionally assessed embodied impacts in extreme cases (for single-storey detached house built on Organic Soils), while remaining a substantial contributor in all other cases-on average representing 0.9%, 4.6%, 9.3%, and 21.5% of conventional embodied impacts for apartment, townhouse, double-storey detached, and single-storey detached houses, respectively. Overall, integrating SOC loss into decision-relevant assessments reveals its substantial magnitude and offers an opportunity to reduce it, while underscoring the need for improved soil characterization in areas zoned for urban expansion.