Global patterns of urban soil degradation: revealing the impacts of urbanization on natural and sealed urban soils

Urban expansion and intensification have caused dramatic changes in urban land cover, significantly increasing the area of sealed soils. Understanding the distribution of urban soil quality is urgently needed, as global soils are undergoing moderate to severe degradation due to human activities. However, information on urban soil quality beneath sealed soil is relatively limited. Moreover, whether the effect of soil sealing on urban soil quality can be modulated by urbanization, climatic, and biotic factors remains unclear. By conducting a comprehensive meta-analysis of 48 published articles, we found that urban sealed soils significantly reduced soil quality, including a 20% reduction in the total soil quality index (SQI), a 43% reduction in soil organic carbon (SOC), and a 19% increase in soil bulk density. We also found that urbanization and climatic factors were stronger predictors of soil quality responses to sealing than the available biotic variables (such as plant presence). Specifically, the total SQI of both urban sealed and natural soils was negatively related to the human footprint index. Meanwhile, the response of SOC under sealing was negatively associated with population density and mean annual precipitation (MAP). The response of total SQI was negatively associated with urbanization areas, while positively associated with MAP. In conclusion, our study showed that soil sealing negatively affected soil quality and that local climatic and urbanization factors could modify these adverse effects. Our findings elucidate the significant impact of soil sealing expansion on the degradation of urban soil quality. Meanwhile, these findings may inform urban soil management and planning strategies by highlighting the vulnerability of sealed soils to degradation under increasing urbanization pressure. • Soil quality index (SQI) was lower in urban sealed than in urban natural soils. • Negative relationships between SQI and human footprint index in both urban soils. • Soil organic carbon in sealed soils was negatively linked with urbanization factors. • Urbanization and climatic were stronger predictors of soil quality responses to sealing than the biotic factors.