Magnetic response of sampling depth to heavy metal pollution in urban topsoil in China

Abstract As urbanization accelerates, the issue of heavy metal contamination in urban topsoil has garnered significant attention. To investigate the soil layer where magnetic and heavy metal characteristics peak, this study analyzes the magnetic signatures of urban topsoil heavy metal pollution across various Chinese regions using literature data. It assesses the relationship between magnetic susceptibility (MS) and heavy metal concentrations at different sampling depths. Findings reveal diverse heavy metal pollution levels in urban topsoils, with pronounced spatial variability in contamination severity. The magnetic signatures of heavy metal pollution fluctuate with sampling depth. Specifically, the 0–5 cm layer typically reflects peak magnetic values. Heavy metal concentrations initially rise then fall with increasing depth, reaching peak levels at slightly varying depths for different metals. Pb, Zn, Cr, and Mn tend to concentrate within the top 5 cm, while Cu, Ni, and Fe show enrichment up to 10 cm. Mass magnetic susceptibility (χlf) demonstrates the strongest correlation with heavy metals at 0–5 cm, suggesting this as the optimal sampling depth for studying magnetic responses to soil heavy metal pollution, unless specific requirements dictate otherwise. Moreover, χlf is a reliable indicator of urban topsoil heavy metal pollution. These insights are crucial for determining sampling depths in related studies, thereby improving urban soil quality research and assessment methodologies.