Effect of Rice Straw Biochar on Rhizosphere Microbiome and Groundwater Quality During Phytoremediation of Alum Mines

Alum mine pollution is becoming a great threat to the health of ecosystem and human beings. Phytoremediation is an economic and green strategy to treat mine pollution, but its efficiency is frequently compromised by the mine hazardous pollutant-induced attenuation of plant growth. In this study, we introduced rice straw biochar to investigate its effect on plant growth, rhizosphere microbiome composition and the quality of soil and groundwater. Indoor culturing experiments showed that the biochar improved seed germination and plant heights in the soils sampled from the alum mine. In situ phytoremediation was then performed in the long-term alum mining area, i.e., the Fanshan alum mine in Anhui Province, China. As revealed by microbiome analysis, the biochar with multiple plants drastically changed the composition of rhizosphere soil microbiome, leading to the increase in the relative abundance of rhizosphere beneficial bacteria, e.g., Rhizobiales, Xanthomonadales and Bacillales. Furthermore, the biochar and the remediation plants improved the quality of both mine soil and groundwater. Under this multiple treatment for 1 year, the soil total nitrogen levels increased from 0.038 g/kg to 1.08 g/kg, and the soil organic matters from 1.72 g/kg to 9.50 g/kg. Meanwhile, the soil heavy metal levels were remarkably reduced. More strikingly, in the mine groundwater after 1-year or 2-year of treatment, the levels of hazardous metal elements, e.g., copper, fluorine, and cadmium, drastically decreased. This study developed a biochar-based phytoremediation strategy to efficient treat the pollution of alum mine area for in situ application.