Dynamic Responses of Soil Organic Carbon to Urbanization: A Global Perspective

Rapid global urbanization has a complex impact on soil organic carbon (SOC) stocks. Through its direct and indirect impacts on soil formation and development, urbanization greatly influences SOC stocks. However, the extent to which urbanization affects SOC stocks globally remains unclear. In this study, we utilized an urban-rural gradient approach to assess the effects of urbanization on SOC stocks at both global and national scales. First, we calculated the urbanization intensity (UI) at a 1 km scale globally, categorizing urbanization into three stages: low (0 ≤ UI ≤ 25), medium (25 < UI ≤ 75), and high (75 < UI ≤ 100). Additionally, we distinguished the contributions of natural factors and human activities and analyzed the effects of land-use changes in eight representative cities. We found the following: (1) The SOC stocks exhibit distinct trends with increasing UI, but when UI is low or high, an increase in UI is associated with decreasing SOC stocks (reductions of 6.8% and 5.4% at a depth of 30 cm; 6.4% and 3.2% at a depth of 100 cm, respectively). (2) Changes in human activities are the main drivers of SOC stock changes during urbanization. At low and medium urban intensities, the contributions of human activities reach 98% and 89%, respectively. Additionally, land-use transitions are closely correlated with SOC stock changes, particularly in areas near the urban core, across different climate zones. (3) The response of SOC to urbanization varies across climatic zones. In water-scarce arid climates, attention should be given to the negative effects of urbanization, and more targeted measures should be taken to enhance the carbon sequestration capacity of urban soils. This study provides valuable insights into the dynamic interplay between urbanization and SOC stocks, underscoring the need for tailored strategies to manage soil carbon in urban environments.