A review of carbon cycling and sequestration in urban soils

城市化过程带来的土地利用变化和环境污染是全球变化的重要方面,城市为人们了解人类与自然复合生态系统对全球变化的影响及其对全球变化的响应过程提供一个独特的"天然实验室"。陆地生态系统碳循环是全球变化研究的热点领域之一,然而,人们对城市在全球碳循环中的作用和影响知之甚少,城市土壤碳循环研究处于起步阶段。介绍了城市土壤的主要特性和碳循环特征,指出强烈的人为作用是其最突出的特点;综述了城市土壤碳库、碳通量和碳固持研究方面取得的进展;探讨了城市化过程中土地利用变化、土壤中生物及土壤管护措施、城市小气候、大气污染沉降和土壤污染等对土壤碳循环的影响;提出未来城市碳循环研究需要开展长期系统监测、深化城市土壤碳循环机制研究、创新研究范式和研究方法、并将研究成果与城市景观规划与设计相结合,提升城市土壤碳管理能力。;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.