Penny bun mushrooms (Boletus edulis) growing on granite-rich soil absorbed and stored trace elements like magnesium, copper, zinc, and cadmium, showing clear isotope fractionation patterns between soil and mushroom. The fungi preferred lighter magnesium and heavier copper, zinc, and cadmium isotopes, indicating selective uptake. These findings help explain how soil composition influences both the nutritional and potential toxic metal content of wild-foraged mushrooms.