Enhanced phytoremediation of Cd-contaminated soil by Solanum nigrum L. through the addition of earthworm and organic wastes

Phytoremediation is considered to have great development potential to deal with soil Cd contamination, and assisted measures are necessary to improve the efficiency of Cd extraction from soil by hyperaccumulator plants. Based on this, a pot experiment was conducted to determine the growth indices, physiological indices, Cd content of Solanum nigrum L. and soil bacterial community under single and combined application of earthworm and organic wastes [chili straw and spent mushroom substrate (SMS)]. Pot experiments revealed that adding earthworms, chili straw, or SMS alone did not affect the growth of S. nigrum. However, the combination of earthworms + chili straw and earthworms + SMS, significantly increased the total dry weight of S. nigrum by 20.94% and 74.69% compared to the control. The chlorophyll content of S. nigrum in the treatment groups with earthworms, SMS, and earthworms + SMS was also significantly higher than that in the control. Meanwhile, adding earthworms and organic wastes also reduced the malondialdehyde content in the leaves of S. nigrum and increased the activity of superoxide dismutase and peroxidase. In addition, applying earthworm + chili straw and earthworm + SMS also increased the availability of Cd in the soil, promoted the absorption of Cd by S. nigrum, and significantly increased its total extraction amount of Cd by 41.55% and 92.83%, respectively. The diversity of soil bacterial communities increased when earthworms and organic wastes coexisted. Adding earthworms and organic wastes also increased the relative abundance of Verrucomicrobia at the phylum level and of Sphingomonas, Ohtaekwangia, Saccharibacteria_genera_incertae_sedis, and Aridibacter at the genus level in the soil. At the same time, this process reduces the relative abundance of Proteobacteria and Firmicutes at the phylum level and Devosia and Bacillus at the genus level. The combined application of earthworms and SMS enhanced phytoextraction of Cd-contaminated soil by S. nigrum. It also improved soil nutrient conditions and reduced external environmental stress on bacteria, increasing their diversity and restructuring the community structure, which was conducive to forming a good soil microecological environment. The results of this study can provide a scientific basis for the treatment of Cd contaminated soil, but field experiments need to be conducted for verification and optimization before practical application in the future.