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

Rapid urbanization has created large areas of impervious surface areas globally. As there is little carbon input via photosynthesis into soils under impervious surfaces, soil organic carbon (SOC) stocks under impervious surfaces generally have been overlooked in the urban carbon cycle. Here we investigate SOC stocks under impervious concrete surfaces and vegetative surfaces across soil profiles to a depth of 5 m in apartment 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, SOC stocks at depths of 1 m to 5 m were 17% in impervious surfaces and 36% in vegetative surfaces. Stable isotope data combined with historical aerial photographs reveal that cropland that existed until the 1970s formed the 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 will help city planners and policy makers to develop carbon management programs better towards sustainable urban ecosystems.