A review of carbon cycling and sequestration in urban soils

The world urban population and urbanized land areas have increased dramatically in the past several decades. Land use/cover change and environmental pollution due to urbanization represent a major aspect of global change. Cities also provide a unique "natural laboratory" for understanding how socioecological systems affect, and respond to, global change. Carbon cycle in terrestrial ecosystem is one of the key topics in global change research, and urbanization alters carbon cycle as well as other biogeochemical cycles on local, regional, and global scales. However, the role of cities in the global carbon cycle is still poorly understood, and urban soil carbon biogeochemistry is still in its infant stage. In this paper, we review the major properties of urban soils and the dynamic characteristics of carbon cycling in cities, all of which are strongly influenced by anthropogenic impacts. Specifically, we discuss urban soil carbon pools, carbon fluxes and carbon sequestration and various effects on soil biogeochemical cycling of a suite of urbanization-related factors, including land use change, soil organisms, management practices, urban microclimate, atmospheric pollution deposition, and soil pollution. Because of the complex interactive effects of human activity and environmental factors, urban soil carbon pools and fluxes are highly variable in space and time. This spatiotemporal heterogeneity in urban soil carbon dynamics is attributable mainly to parent materials and anthropogenic disturbances, but little is known about the underlying causes and mechanisms. Human activities (e.g., soil mixing, compaction, soil sealing, and importing man-made substrates, and soil management practices inputs) directly alter soil carbon stocks and carbon cycling. Urban environmental factors, including urban heat island effect, elevated CO₂ dome, atmospheric deposition of pollutants, soil pollution, and introduction of exotic species, may also affect soil carbon cycling. Existing studies show a wide range of variations in urban soil C densities among cities. Urban soils may have higher or lower soil organic carbon (SOC) stocks, depending on their sources and treatment. It remains unclear whether urbanization increases or decreases soil C pools. Some previous studies show that urban soils accumulate more SOC at a higher rate than agricultural fields or native grasslands, suggesting that urban soils have a high potential for carbon sequestration, especially in urban greenspace where horticulture management practices promote SOC accumulation. But greenhouse gases emissions generated by management practices also can be a significant factor that offsets the C sequestration potential of urban greenspace. To maximize benefits of urban soil carbon sequestration, better soil management practices need to be designed and implemented. Better understanding interactions between natural and anthropogenic components of the urban carbon cycle and the whole spectrum of urban soil carbon pools and fluxes is necessary for develop more appropriate management policies to reduce urban carbon footprints. So we urge for further studies on urban soil carbon biogeochemistry, particularly, in rapidly urbanizing areas of China. Four priority areas for future research are proposed: (1) establishing long-term study sites for monitoring and studying urban soil carbon cycling; (2) focusing on mechanistic studies of urban soil carbon cycling, (3) developing new methods for urban soil ecology; and (4) incorporating urban soil carbon chemistry into urban landscape planning and design to promote urban soil carbon management.