Regulation of exogenous substances and intercropping are effective methods to improve the efficiency of phytoremediation of heavy metal contaminated soil. A pot experiment was used to study the effects of earthworms, straw, and citric acid on the remediation of Zn, Pb, and Cd contaminated soil by monocropping and intercropping of Solanum photeinocarpum and Pterocypsela indica. The results showed that the bioaccumulation factors (BCF) of earthworms for Zn, Pb, and Cd were 0.07-0.13, 0.10-0.26, and 5.64-15.52, respectively. The addition of straw in the soil increased the biomass of earthworms by 22.29%-223.87% but reduced the heavy metal concentrations by 8.15%-62.58%. Straw and citric acid showed passivation and activation effects, respectively, but earthworms had no significant effect on the available concentrations of heavy metals in the soil. Earthworms had no significant effect on the heavy metal concentrations of P. indica but reduced the heavy metal concentrations of S. photeinocarpum. Straw showed an inhibitory effect on the concentrations of heavy metals in P. indica but promoted the concentrations of Cd in S. photeinocarpum. Citric acid had no significant effect on the heavy metal concentrations in S. photeinocarpum but significantly increased the Pb concentrations in P. indica. Intercropping significantly reduced the soil available heavy metal concentrations and increased the heavy metal concentrations in plant roots; however, it had no significant effect on heavy metal concentrations in plant shoots. The total extraction amounts of Zn, Pb, and Cd by plants were mainly manifested as P. indica>intercropping>S. photeinocarpum. The addition of earthworms increased the total extraction amounts of Zn, Pb, and Cd by 12.49%, 35.89%, and 29.01%, respectively, and the addition of straw+earthworms increased the total extraction amounts of Pb by 87.21%. The results indicated that straw significantly promoted the growth of earthworms and reduced their accumulation of heavy metals, and the addition of earthworms alone or in combination with straw can effectively improve the remediation potential of P. indica of Zn, Pb, and Cd contaminated soil.
Keywords: citric acid; earthworm; heavy metal pollution; intercropping; phytoremediation; straw.