Application of magnetic proxies for characterizing black carbon contamination of urban soil in Xuzhou, China

Black carbon (BC) pollution significantly impacts urban soil ecosystems and public health, highlighting the need to systematically evaluate its concentration gradients, spatial distribution, and emission sources. To address this, a dual-layer soil investigation was conducted in Xuzhou, a representative Chinese industrial city. A total of 142 topsoil samples from 71 sites (with one sample taken from the 0–2 cm layer and another from the 3–10 cm layer at each site) was collected. The mean BC concentrations were 17.12 g·kg⁻¹ and 11.90 g·kg⁻¹ for the 0–2 cm and 3–10 cm layer soils, respectively, reflecting 4.5 and 3.1 times the background levels. These findings indicate significant contamination from anthropogenic sources in the urban soils of Xuzhou. Particle size fractionation experiments showed preferential BC enrichment in coarse soil fractions. Environmental magnetic analysis indicated elevated magnetic susceptibility (χ) in urban soils, with ferrimagnetic minerals primarily occurring as coarse-grained particles, further evidencing human-induced inputs. Correlation analysis validated significant relationships between BC concentrations and magnetic parameters. These strong correlations establish magnetic parameters as reliable proxies for BC monitoring. The integration of geochemical and magnetic techniques offers an efficient framework for assessing BC contamination and mapping polluted areas in urban soils. Scanning electron microscopy (SEM) observations identified anthropogenic spherical magnetic particles. Geospatial mapping revealed overlapping distributions between BC and magnetic parameters (χ, SOFT, SIRM, χARM), with pollution hotspots concentrated along major roads and industrial areas. This spatial covariation confirmed shared sources of BC and magnetic minerals linked to transportation and industrial activities.