Biochar addition to an arsenic contaminated soil increases arsenic concentrations in the pore water but reduces uptake to tomato plants (Solanum lycopersicum L.).

Arsenic (As) concentrations in soil, soil pore water and plant tissues were evaluated in a pot experiment following the transplantation of tomato (Solanum lycopersicum L.) plantlets to a heavily As contaminated mine soil (~6000 mg kg−1 pseudo-total As) receiving an orchard prune residue biochar amendment, with and without NPK fertiliser. An in-vitro test was also performed to establish if tomato seeds were able to germinate in various proportions of biochar added to nutrient solution (MS). Biochar significantly increased arsenic concentrations in pore water (500 μg L−1–2000 μg L−1) whilst root and shoot concentrations were significantly reduced compared to the control without biochar. Fruit As concentrations were very low (b3 μg kg−1), indicating minimal toxicity and transfer risk. Fertilisation was required to significantly increase plant biomass above the control after biochar addition whilst plants transplanted to biochar only were heavily stunted and chlorotic. Given that increasing the amount of biochar added to nutrient solution in-vitro reduced seed germination by up to 40%, a lack of balanced nutrient provision from biochar could be concluded. In summary, solubility and mobility of As were increased by biochar addition to this soil, but uptake to plant was reduced, and toxicity-transfer risk was negligible. Therefore leaching rather than food chain transfer appears the most probable immediate consequence of biochar addition to As contaminated soils.