Using compost and technosol combined with biochar and Brassica juncea L. to decrease the bioavailable metal concentration in soil from a copper mine settling pond

One of the most important sources of pollution caused by metals, if not the most important, is mining. Metal pollution is covert, persistent and irreversible. For this reason, soil metal pollution has become a severe problem in many parts of the world. The aim of this study was to observe which combination of amendments (compost + biochar or technosol + biochar) combined with Brassica juncea L. was best at reducing the assimilable contents of Cu, and which also increased to a lesser extent the contents of other metals (Ni, Pb, Zn) found in these amendments. We also studied the phytoremediation capacity of brassicas in these amendments. The experiment was carried out using 45-cm-deep cylinders over and 11-month period, with soil from the settling pond in the depleted copper mine located in Touro (Galicia, north-west Spain). At depth 0–15 cm, the settling pond soil (S) had a higher CaCl2-extractable Cu, Pb, and Ni concentration, at the three time periods measured (time 1 = 3 months, time 2 = 7 months, time 3 = 11 months). The settling pond soil + technosol + biochar and vegetated with Brassica juncea L. (STBP) had the highest CaCl2-extractable concentrations of Zn over time. In general terms, the most effective treatment for reducing the phytoavailable contents of Cu, Pb, Ni and Zn was the treatment using compost +b iochar + Brassica juncea L. In the two treatments applied, Brassica juncea L. had a good phytostabilisation capacity.