Spatial distribution, pollution assessment, and controlling factors of heavy metals in tropical agricultural soils of Hainan Island, China

Abstract Hainan Island is China’s largest tropical crop-production base, making its agricultural soil quality important for national food security. We determined total concentrations of eight heavy metals (Hg, As, Cd, Pb, Cr, Cu, Zn, Ni) in 300 surface soils collected across the island in late 2025. Soils were predominantly acidic (mean pH 5.18; 77% below pH 5.5), and mean concentrations were low to moderate (e.g., Cd 0.21, Pb 58.19, and Ni 28.50 mg kg⁻¹). Against pH- and land-use-stratified GB 15618 − 2018 screening values, single-metal exceedance was most frequent for Pb (28.0%) and Ni (14.7%); separately, the Nemerow comprehensive index exceeded its pollution threshold ( P n > 1.0) at 23.7% of sites. The geo-accumulation index ( I geo = log 2 [ C i /(1.5 × B i )]) showed the strongest enrichment for Cd (mean 1.48; 99.3% above 0), moderate enrichment for Pb (0.33), and at or below background levels for the remaining metals. Soil pH correlated negatively with total Cd, Cr, Hg, and Pb ( r = − 0.47 to − 0.28). Parent material, land use, and pH jointly explained about 45% of the variance, separating lithogenic Ni-Cu-Zn from mixed Cd-Pb-Hg-Cr associations. Together, these results show that fixed-threshold assessment understates contamination in acidic tropical soils, providing an island-wide baseline for targeted risk management.