Effects of Biochar Application on Metal(loid) Solubility and Uptake in Lettuce (Lactuca sativa) Grown in Six Circumneutral Urban Allotment Soils

Abstract The legacy of post-industrial contamination often limits the safe use of urban soils for food production without pre-treatment to reduce metal(loid) solubility and uptake in crops. In this study, a seed residue biochar was applied at two doses (1% and 5%) to six circumneutral urban allotment soils from Denmark and the UK, all exhibiting elevated concentrations of As, Cd, Pb and Zn. Soil pore water was sampled to assess metal(loid) solubility and soil:solution partitioning, and lettuce was grown to determine metal(loid) uptake. Biochar application had varying effects on metal(loid) solubility. For As and Pb, solubility generally increased, particularly at the higher dose. For Zn and Cd, solubility tended to decline. Metal(loid) uptake in lettuce also varied, with no consistent change between control and treatments for As and Pb, despite their increased solubility. However, a clear reduction in Zn and Cd uptake was observed, especially at the 5% application rate. For Cd, this resulted in a marked decrease in the proportion of samples exceeding health-based guideline values, indicating a tangible reduction in dietary risk. These findings show that even in circumneutral soils, biochar can significantly influence metal(loid) behaviour, with the greatest benefit observed for reducing Cd accumulation in edible crops. However, the observed increase in As and Pb solubility underscores the importance of considering potential risks associated with leaching and water contamination.