Distribution Characteristics and Impact Factors of Surface Soil Organic Carbon in Urban Green Spaces of China

As a key component of urban green spaces, which provide sustainability-relevant ecosystem services such as carbon sequestration, soils support plant growth and represents an important carbon pool in urban ecosystems. However, surface soil organic carbon (SSOC) in urban green spaces can be highly heterogeneous due to the combined influences of natural conditions and human activities. To quantify national-scale patterns and major correlates of SSOC in China’s urban green spaces, we compiled published surface (0–20 cm) SSOC observations from 154 field studies and synthesized SSOC density and stocks across 224 Chinese cities, providing a nationally comparable assessment at the city scale. Measurements were harmonized to a consistent depth, and a random forest gap-filling approach was used to extend estimates for data-poor cities. The mean SSOC density and total SSOC stock of urban green spaces were 3.22 kg C m−2 and 57.87 × 109 kg C, respectively, and SSOC density showed no obvious latitudinal gradient across the 224 cities. Variable importance from the random forest analysis indicated that soil physicochemical properties (e.g., bulk density, total nitrogen, and texture) were the strongest predictors of SSOC density, whereas climatic and topographic variables showed comparatively lower importance. This pattern may suggest that anthropogenic modification and management dampen macro climatic signals such as temperature and precipitation at the national scale.