Application of Compost and Biochar with Brassica juncea L. to Reduce Phytoavailable Concentrations in a Settling Pond Mine Soil

After they are closed, mines impact the environment by contaminating air, water, soil, and wetland sediments from the scattered tailings, and by polluting the groundwater with discharged leachate. Touro mine is depleted copper mine (in Galicia, north-west Spain), the settling pond soil has high bioavailable metal concentrations (mainly high Cu concentrations), extreme pH values, a low cation exchange capacity and organic matter. This study aimed to determine the capacity of biochar to fix metals (particularly in the reduction of Cu concentrations) and enhance the positive effects of compost, supported by the phytoremediation capacity of Brassica juncea L. In this experiment, brassica was chosen because several authors used this plant satisfactorily for similar purposes. The greenhouse experiment was carried out in 45-cm cylinders, and the effects of the treatments were studied at different soil depths. The study lasted a total of 11 months and was carried out in the settling pond of Touro mine. At depth 0–15 cm, the treatments applied exhibited the best effect on the reduction of the phytoavailable metal concentrations. At depth 15–30 cm, the treatments showed better results at Time 2 than at Times 1 and 3. Only at depth 0–45 cm and at Time 3, a better behaviour of the treatment elaborated with compost + biochar + B. juncea L was observed. The TC and TF values revealed that the cultivated B. juncea L. presented good phytostabilising capacity for Cu, Pb, Ni, and Zn.