Urban soils as hot spots of anthropogenic carbon accumulation: Review of stocks, mechanisms and driving factors

Urban soils and cultural layers may accumulate C over centuries and consequently large C stocks may be sequestered beneath cities. Processes and mechanisms leading to high C accumulation in urban soils remain unknown. Data on soil organic carbon (SOC), soil inorganic carbon (SIC), black (pyrogenic) carbon (BC), and nitrogen contents and stocks in urban soils were collected from 100 peer‐reviewed papers. The database (770 data points for SOC, SIC, and BC stocks from 116 cities worldwide) was analysed considering the effects of climate and urban‐specific factors (city size, age, and functional zoning) on C stocks. The processes of C accumulation specific for urban soils were analysed, and C sequestration rates were assessed. For the wide range of climatic conditions, total C content in urban soils was 1.5–3 times higher, and C accumulation was much deeper compared with natural soils, resulting in 3–5 times larger total C stocks. Urban SOC stocks increased with latitude, whereas SIC stocks were less affected by climate. City size and age were the main factors explaining intercity differences in C stocks. The intracity variability of C and N stocks was dominated by functional zoning: Large SOC and N stocks in residential areas and large SIC and BC stocks in industrial zones and roadsides were consistent across all climates and for cities of various sizes and ages. Substantial amounts of SOC, SIC, and N are sequestered in the subsoils, cultural layers, and sealed soils, underlining the importance of these hidden stocks for C assessments. Long‐term С input from outside the cities and associated C accumulation coincided with upward soil growth of ~50 cm per century, and continuous accumulation of 15–30 kg C/m2 per century in urban soils and cultural layers. We conclude that, despite the relatively small area of cities, urban soils are hot spots of long‐term soil C sequestration worldwide, and the importance of urban soils will increase in future with global urbanization.