Use of Piptatherum miliaceum for the phytomanagement of biochar amended Technosols derived from pyritic tailings to enhance soil aggregation and reduce metal(loid) mobility

•Biochar at high rate reduced plant growth.•Technosols showed high organic matter recalcitrance, with low labile compounds.•Technosols showed higher proportion of macro and microaggregates than tailings.•P. miliaceum enhanced microbial biomass/activity, with increased soil aggregation.•Plants showed low metal(loid) uptake in shoots, with no effect of biochar rate.

Tailings and overburdens pose high environmental and human health risks. The development of Technosols from anthropogenic materials and growth of native metal-tolerant plant species could be a sustainable strategy to minimize such risks. In this study, a 10-month pot experiment was carried out with Technosols developed from acidic tailings amended with marble waste (CaCO3) and two rates of biochar (10 g C kg− 1 and 20 g C kg− 1), with and without Piptatherum miliaceum. The main aims were to assess if biochar addition influenced plant metal uptake, increased soil aggregation, decreased metal mobility and enhanced plant growth. In addition, we assessed if the presence of P. miliaceum improved soil structure, stimulated microbial communities, and increased metal immobilization. Biochar and CaCO3 additions were the main factors responsible for acid neutralization. However, the growth of P. miliaceum also contributed to increase soil pH, likely due to increases in electrical conductivity. High organic matter recalcitrance with low content of labile and soluble organic compounds was observed in the developed Technosols. The presence of P. miliaceum positively stimulated microbial biomass and enzyme activities, and contributed to significantly increase the ratio recalcitrant C-to-recalcitrant N. This suggests an effect of rhizosphere in soil organic matter stabilization and degradation. Thus, microorganisms degrade N functional groups in biochar more easily than C functional groups. Greenhouse gas emissions were not increased by addition of biochar. Methane emissions decreased, with no effect of plant growth. The use of biochar and marble waste reduced the mobility of metal(loid)s due to increase in pH, and the presence of plant increased the fraction of Cd retained in Fe/Mn oxides. Plants showed low metal(loid) uptake in shoots, with no effect of biochar rate. Technosols showed higher proportion of macro and microaggregates than original tailings. The presence of plant significantly contributed to increase the proportion of microaggregates. The highest rate of biochar had a negative effect on P. miliaceum biomass which resulted in limited growth compared with the lower biochar rate.