Fertilizer-driven heavy metal contamination in tomato soils: Insights from multivariate source apportionment

Excessive use of chemical fertilisers has increased the accumulation of heavy metals in agricultural soils, posing serious threats to soil health and food security. However, multi-index ecological risk assessments and depth-wise contamination profiles of soils under intensively fertilised tomato systems in India remain poorly characterized. This study quantified the concentrations of Fe, Mn, Zn, Cu, Cd, Cr, Pb and As, along with associated physicochemical and biological properties, in one hundred soil samples collected from intensively cultivated tomato fields in Chintamani, Karnataka, at both surface (0-20 cm) and subsurface (20-40 cm) depths. Pollution levels were assessed using the  geo-accumulation index (Igeo), enrichment factor (EF), pollution index (PI) and ecological risk index (Ei and RI), owing to their ability to account for geogenic variability, anthropogenic inputs and ecological hazards. Potential sources of metals were identified using multivariate statistical methods.  Most soil attributes and metal concentrations showed highly significant depth-wise differences (p < 0.001), indicating pronounced anthropogenic enrichment in surface soils. Surface soils exhibited elevated concentrations of Cd (0.66-5.40 mg kg-1), As (9.44-30.80 mg kg-1),  Pb (18-39.40 mg kg-1), Mn (525-1870 mg kg-1) and Cu (38.41-83.29 mg kg-1), while Fe, Zn and Cr were below levels of concern. EF and RI identified Cd, As and Pb as priority pollutants, with Cd accounting for more than 60 % of the total ecological risk, despite classification of most soils as "uncontaminated" by Igeo. Subsurface soils had reduced anthropogenic influence, with Cd, Pb, As and Mn remaining the concern. Principal component and cluster analysis distinguished anthropogenic (Cd, As, Pb, Mn and Cu) from geogenic (Fe, Al, Zn and Cr) sources. Depth-specific management, region-specific background values and future work on metal speciation, bioavailability testing and isotopic source tracing are critical for improving ecological risk assessments and safeguarding soil sustainability and food safety in the study area.