Contribution of arbuscular mycorrhizal fungi and soil amendments to remediation of a heavy metal-contaminated soil using sweet sorghum

Owing to their potential advantages such as waste reduction, recycling, and economic attributes, fast-growing bioenergy crops have the capacity to effectively phytoremediate heavy metal-contaminated soils. However, little is known about the role of microbial and chemical amendments in phytoremediation using bioenergy crops. Here, we studied the contributions of inoculation with the arbuscular mycorrhizal fungus (AMF) Acaulospora mellea and three soil amendments, i.e., hydroxyapatite (HAP), manure, and biochar, at doses of 0.1% and 1% (weight:weight) to heavy metal phytoremediation using sweet sorghum grown on an abandoned agricultural soil, with environmentally realistic contamination (2.6 mg kg−1 Cd, 1796 mg kg−1 Pb, and 1603 mg kg−1 Zn), in a plant growth chamber. Mycorrhizal colonization, plant biomass and metal accumulation, metal availability, and soil pH were determined in harvested seedlings 12 weeks after sowing. The results showed that root colonization by indigenous AMF decreased by 28%-46% with HAP, but increased after manure and biochar applications as compared to the no amendment control (CK). The AMF inoculation increased root colonization rates by 16%-128% and in particular, alleviated the inhibition of HAP. The remediation effects were highly dependent on the amendment type and dose. Among the three soil amendments, HAP was the most effective in promoting plant growth and phytostabilization of Cd, Pb, and Zn and phytoextraction of Cd, particularly at a dose of 1%. Compared to CK, 1% HAP decreased DTPA-extractable Cd, Pb, and Zn concentrations in soil by 31%-43%, 30%-38%, and 22%-23%, respectively. Manure and biochar also exerted positive effects on heavy metal immobilization, as indicated by lower DTPA extractability, but only the 1% manure treatment showed plant growth-promoting effect. The AMF inoculation did not affect plant growth, but increased soil pH and induced synergistic interactions with amendments on the immobilization of Cd and Pb. In conclusion, soil amendments, particularly HAP, produced positive impacts and synergistic interactions with AMF on the phytostabilization of heavy metals using sweet sorghum. Accordingly, sweet sorghum combined with soil amendments and AMF may be an effective strategy for heavy metal phytoremediation.