Does micro/nano biochar always good to phytoremediation? A case study from multiple metals contaminated acidic soil using Salix jiangsuensis '172'

Biochar amendments have shown great superiority in reduction of heavy metals (HM) toxicity and soil amelioration in severely contaminated mining areas, which suggested to facilitate vegetation rehabilitation using fast-growing trees. Bone biochar (BC) and ball-milled bone biochar (MBC) were fabricated by pyrolyzing bone meal at 600°C followed by eco-friendly wet-milling techniques. The effect of BC and MBC application (0, 0.5, 1.0, 2.0 and 4.0 wt%) on soil properties, plant growth and metal accumulation of Salix jiangsuensis '172' (SJ-172) in multi-metal (Cu, Pb, Cd and Mn)-contaminated acid clay soil was investigated in a 150-day pot experiment. The results showed that BC and MBC considerably improved the survival rate of SJ-172 when the application rate was larger than 1.0%. MBC displayed outstanding performance in reducing HMs bioavailability and improving N and P soil fertility, while BC enhanced the reduction in acidity and increase in K supply. Interestingly, BC enhanced HMs accumulation (Cd 115.23%, Pb 161.82%, Mn 285.23% and 219.29% Cu at 4% application rate) by SJ-172 compared with MBC. Taken together, the good performance of BC in enhancing HMs accumulation in SJ-172 indicated that it is a promising amendment for phytoremediation of clay soils, while MBC can be utilized as an excellent amendment for HMs stabilization. Graphical Abstract