Spatial heterogeneity of urban soils: the case of the Beijing metropolitan region, China

Introduction: Urban soils are large pools of carbon, nitrogen, and other elements, supporting plant growth, sustaining biogeochemical cycles, and serving as the foundation for maintaining ecosystem function and services of urban green spaces (UGS). Quantifying urban soil properties is essential for assessing urban ecosystem services and detecting pollution. Characterizing spatial heterogeneity of urban soil properties, which may change with land use or urbanization, is crucial for understanding urban ecosystem functions. Methods: We collected 466 soil composite samples across the Beijing metropolitan region and then analyzed the heterogeneity of soil properties in UGS of Beijing metropolitan region by ANOVA along the urbanization gradient spanning different land uses. Results: Our results show that soil properties vary considerably among land-use types and along the urban-rural gradient. Soil moisture content, soil organic carbon (SOC), available phosphorus, available potassium, total nitrogen (TN), the ratio of SOC:TN (C/N), Pb, and Cu increased notably from suburbs to the urban core, while soil pH, bulk density, TN, Cr, Ni, and Mn were not significantly different across urbanization levels. Most soil properties, except soil TN, Cu, Cr, Ni, exhibited significant differences between different land uses. The highest levels of soil nutrients and heavy metals were found in roadsides and residential areas among all the land uses. Conclusions: The spatial heterogeneity of urban soils in the Beijing metropolitan region is mainly attributable to the different land uses. Soil nutrient content has increased in UGS due to fertilization and pollution. Contamination of urban soils by heavy metals is due largely to traffic emissions and pollution by industrial and household wastes. Although human activities tend to augment urban soil nutrient pools to maximize certain ecosystem services, they may inadvertently compromise these and other services by increasing pollution. Knowledge of urban soil spatial heterogeneity in UGS is indispensable for improving urban ecosystem management.