The effectiveness of sewage sludge biochar amendment with Boehmeria nivea L. in improving physicochemical properties and rehabilitating microbial communities in mine tailings

Biochar amendment can be adopted to improve soil substrate, in turn facilitated phytoremediation. However, improvements to the properties of tailings following different feedstocks of biochar amendment in phytoremediation, particularly the impacts on nitrogen cycle and the related nitrogen-fixing microorganisms remain unclear. In this study, a 100-day pot experiment was designed to determine the co-effects of different combinations of woody and non-woody biochar, namely hibiscus cannabinus core biochar (HB), sewage sludge biochar (SB), chicken manure biochar (MB) and two crops (Cassia alata L., Boehmeria nivea L.). It was found that, on the one hand, biochar amendment directly immobilized heavy metal (loid) contamination in the tailings; on the other hand, biochar amendment, particularly non-woody SB, improved soil properties (i.e., the combination of SB with crops increased the total nitrogen content by 4.7–7.5 times). This indirectly improved phytostabilization (i.e., SB increased crop height 1.5–1.8 fold, root length 3.3–3.7 fold, decreased NH4NO3-extractable Pb, Cu, Cd and also increased the relative abundance of nitrogen-fixing bacteria such as Mesorhizobium, Bradyrhizobium, and Rhizobium). Besides this, redundant analysis shown that the carbon, nitrogen sources, and pH provided by the biochar were identified as the key factors associated with the nitrogen-fixing bacteria. Through the comprehensive evaluation of different biochar amendment in phytoremediation, it was found that the non-woody SB got higher comprehensive score (3.1–3.6) in biochar amendment in phytoremediation, especially in Boehmeria nivea L. Thus, the combination of non-woody SB and Boehmeria nivea L. improved microbial function, while the microorganisms in turn promoted crop growth. Our results revealed the prospect of using non-woody SB assisted Boehmeria nivea L. for phytoremediation in multi-metal mine tailings.