Effects of biochars on bioavailability of Pb，As，and Cd in the rhizosphere and non-rhizosphere soil of corn and wheat seedlings

Biochar is a carbon-rich composite derived from the slow pyrolysis of biomass feedstock in the absence of oxygen. It has been widely investigated for carbon sequestration，soil remediation/restoration，and soil fertility. In this study，biochars were prepared through the slow pyrolysis of scrap paper，wood chips，and dehydrated sludge under limited oxygen at 600 ℃. Different levels（0.5% and 5%）of the re⁃ sulting biochars were added to soil from a lead-zinc mining area of Qixia in Nanjing；the soil was contaminated with heavy metals. Laborato⁃ ry experiments were then conducted on corn（Zea mays L. cv. Suyu 44）and wheat （Triticum aestivum L. cv. Nongmai 88）seedlings via rhi⁃ zo-bag pot cultures. The bioavailabiliy of Pb，As，and Cd in the rhizosphere and non-rhizosphere soil of the seedlings，and the accumulation of the former in the roots of the seedlings，were investigated. The results showed that the CaCl2-extractable，EDTA-extractable，and dilute-HCl-extractable fractions of As，Cd，and Pb in the rhizosphere and non-rhizosphere soils of the seedlings in the groups treated with the bio⁃ chars were lower than those in the control group，to which the biochars were not added. With the increase in biochars levels from 0.5% to 5%，the extractable As，Cd，and Pb content decreased significantly；however，the reduction in the extractable fractions of As，Cd，and Pb in the non-rhizosphere soils was generally clearer than that in the rhizosphere soils. The effects of biochars with different physicochemical properties on the extractable As，Cd，and Pb in soils were related to the extractants（CaCl2，EDTA，and dilute HCl）and the extracted ele⁃ ments（As，Cd，and Pb），but no unified rules were observed. There were no significant differences between the rhizosphere and non-rhizo⁃ sphere soils from which the water-soluble As，Cd，and Pb had been extracted by CaCl2，but some of the exchangeable and carbonate-bound，iron/manganese-bound，and organic matter-bound fractions extracted by EDTA and dilute HCl differed significantly. The As，Cd，and Pb content in the roots of the seedlings in the groups treated with biochars were lower than those in the control group. With the increase in bio⁃ chars from 0.5% to 5%，the decrease in contents of As，Cd，and Pb was more significant；wood chip- and sludge-based biochars were better at reducing As，Cd，and Pb enriched in the roots of the seedlings than the scrap paper-based biochar.