Long-term fertilization regimes are associated with multi-contaminant accumulation in agricultural soils and earthworms

Agricultural soils receive a wide range of organic and inorganic contaminants through long-term fertilization, yet field-based evidence linking complex contaminant mixtures to biological exposure of soil organisms under realistic agricultural conditions remains limited. Here, soils and endogeic earthworms (Aporrectodea cf. caliginosa) were sampled from two adjacent agricultural fields in Finland after more than two decades of contrasting fertilization with sewage sludge–based products and broiler litter. A broad suite of organic contaminants together with multiple metals was analyzed in both matrices, and bioaccumulation factors were used to assess soil–biota transfer. Distinct fertilization-specific contaminant profiles were observed in soils, with higher concentrations of several organic contaminants under sewage sludge fertilization. In earthworms, per- and polyfluoroalkyl substances (PFAS) dominated contaminant profiles and showed pronounced bioaccumulation, particularly for long-chain PFAS including perfluorooctane sulfonate (PFOS). Other contaminants showed variable, compound-specific transfer, and biological exposure was not consistently reflected by bulk soil concentrations. These results provide field-based evidence of distinct soil contaminant profiles under contrasting long-term fertilization histories, but biological exposure cannot be reliably inferred from bulk soil concentrations alone. Instead, contaminant transfer to soil organisms was more consistent with bioavailability-related and substance-specific properties reported in previous studies, highlighting the importance of integrating biota-based measurements into assessments of agricultural soil contamination.