High soil organic carbon stocks under impervious surfaces contributed by urban deep cultural layers

Abstract

Rapid urbanization has created large areas of impervious surface areas globally. As there is little carbon input by plants into soils under impervious surfaces, soil organic carbon (SOC) stocks under impervious surfaces generally have been overlooked in the urban carbon budgets. Here we investigate SOC stocks under impervious concrete surfaces and vegetative surfaces across soil profiles to a depth of 5 m in urban housing complexes in Seoul, Republic of Korea. In the top 1 m of the profile, SOC stocks under vegetative surfaces were three times greater than those under impervious surfaces. However, we discovered that unexpectedly high SOC stocks appeared in deeper soil layers under both surface types, which led to comparable SOC stocks at a depth of 5 m beneath the impervious surface (16.9 ± 1.9 kgC m−2) and at the vegetative surface (22.3 ± 2.2 kgC m−2). Consequently, the ratio of SOC stocks at depths of 1 m to 5 m were 16% in impervious surfaces and 34% in vegetative surfaces, suggesting conventional soil sampling at 1 m depth could miss large SOC. Stable isotope data (δ13C and δ15N) combined with historical aerial photographs revealed that cropland that existed until the 1970s formed the high SOC cultural layer in deeper soils. Our results highlight that deep soils under impervious surfaces could be overlooked carbon hotspots in urban ecosystems. We believe this finding could help city planners and policy makers to assess regional carbon budgets and to reduce carbon footprint by recycling the deep SOC excavated from various construction projects towards sustainable urban development.