Trends and drivers of the soil organic carbon stocks along the urban-rural gradients globally

The land use changes and environmental pollution brought by urbanization are important aspects of global change. Cities provide a unique "natural laboratory" for people to understand the impact of human-nature composite ecosystems on global change and their response processes. Unlike natural soils, human activities during urbanization directly or indirectly affect the formation and development of soils. Generally, urban soils have a higher organic carbon content compared to agricultural soils and some natural soils, making them an important carbon reservoir in urban ecosystem carbon cycles. The urban-rural gradient method based on urbanization intensity provides a possibility for studying the response of Soil Organic Carbon (SOC) storage to urbanization on a global scale. Here, we define Urban Intensity (UI) as the proportion of impervious surface area within each 1x1 square kilometer (ranging from 0 to 100%). We first analyzed the trends of SOC storage along the UI gradient globally and in different climate zones, distinguishing the contributions of natural and socio-economic factors through multiple linear regression and residual quantification. The study found that as UI increases, SOC storage undergoes phased changes, with natural and socio-economic factors contributing differently in each urbanization stage and climate zone. For example, globally, when UI is low (UI ≤ 0.25) and high (0.75 < UI ≤ 1), SOC storage tends to decrease with increasing UI, while at medium intensity (0.25 < UI ≤ 0.75), SOC storage shows an increasing trend. Globally, changes in socio-economic factors (population, GDP) are the main drivers of SOC storage changes during urbanization. Particularly at low and medium urbanization intensities, socio-economic contributions reach 98% and 89%, respectively. However, as urbanization intensity increases, the driving role of natural factors becomes more apparent, contributing over 40% in areas of high urbanization intensity.