Effects of iron-modified biochar and AMF inoculation on the growth and heavy metal uptake of Senna occidentalis in heavy metal-contaminated soil

S. occidentalis can be used in pharmacology and vegetation restoration. A pot experiment was conducted to study the effects of biochar and mycorrhizal fungus inoculation on plant growth and heavy metal accumulation. The mycorrhizal infection rate was increased by apple branch biochar but decreased by coconut husk iron-modified biochar. Conversely, soil pH was not affected by mycorrhizal inoculation but was increased by biochar. Compared to the uninoculated control, the combination of apple branch biochar and mycorrhizal inoculation significantly increased the growth of stems, leaves and roots by 226.46%, 163.15% and 86.00%, respectively. The application of apple branch biochar increased the root Pb content, while root Cd, Cr, Cu and Fe were decreased by 36.30%, 13.63%, 3.09% and 7.66%, respectively. Furthermore, the content of all elements in the stems and leaves also decreased. The application of iron-modified biochar alone increased the content of all the elements in the roots by 4.23-109.33%. But their contents in stems and leaves were decreased by iron-modified biochar and mycorrhizal inoculation alone. The combination of biochar and mycorrhizal inoculation most effectively promoted plant growth, enhanced heavy metal uptake by the roots and produced a barrier effect that reduced the transfer of heavy metals from the roots to the shoots. This might constitute a feasible means of promoting the safe utilization of S. occidentalis in phytoremediation.