Fertilisation effects on metal uptake in lettuce and soil geochemical properties in mine-affected soils investigated through a pot experiment

The aim of this study was to determine how organic and inorganic fertilisers influence the accumulation of metal contaminants in lettuce grown in mine‑affected soils, and to identify the soil properties that govern metal uptake and how fertilisation alters these conditions. Lettuce was grown in mine-affected soils treated with NPK or organic fertilisers. Soil and plant samples were analysed for metal concentrations, pH, SOM, CEC and texture, and bioconcentration factors (BCFs) were calculated as indicators of soil‑to‑crop metal transfer. Linear mixed models were used to assess fertilisation effects on BCF. Soil variables influencing metal uptake identified using a Least Absolute Shrinkage and Selection Operator (LASSO) approach, to enable variable selection within the hierarchical design. The overall treatment effects on BCF was significant for Cu, Pb and Zn (p = 0.006, 0.007 and 0.003, respectively), while close to significant for Cd (p = 0.052). Organic fertilisation significantly reduced the soil-to-crop transfer of Cu and Zn, compared to inorganic NPK fertilisation (p = 0.008 and 0.002, respectively). Key predictors of metal uptake included soil concentrations of metals, pH, SOM, K, P, Mg, clay content, and exchangeable Mg²⁺. Fertilisation significantly affected pH, SOM, K, P, and exchangeable Mg²⁺, which in turn influenced metal phytoavailability. The effect of fertilisation varied by soil type, with site-specific geochemical properties playing a dominant role. When modelling uptake with the LASSO regression approach, Pb was less predictable ( $$R^{2}_{\text{adj}} $$ = 0.42) than Cd, Cu, and Zn (0.79, 0.60 and 0.72, respectively), reflecting its complex behaviour in soil matrices. Overall, fertilisation altered both metal uptake and the geochemical conditions controlling it. Fertilisation affected both soil‑to‑crop metal transfer and the controlling soil properties, indicating that organic fertilisers may be more suitable than inorganic fertilisers in contaminated environments, where tailored fertilisation is essential because it can either increase or reduce metal transfer to crops. The findings supports safer cultivation practices in areas with legacy pollution.