Trifolium repens L. regulated phytoremediation of heavy metal contaminated soil by promoting soil enzyme activities and beneficial rhizosphere associated microorganisms

Highlights: • Trifolium repens L. could accumulate both anions and cations effectively. • Trifolium repens L. planting compensated soil enzyme activity lost caused by HM. • Ecologically beneficial rhizosphere and endosphere microorganisms was promoted. • Rhizosphere micro-ecological characteristics significantly correlated with phytoremediation. • Rhizosphere associated microbes played different roles in regulating phytoremediation. Rhizosphere soil physiochemical properties, enzyme activities and rhizosphere associated microbial communities are of the central importance for modulating phytoremediation in heavy metal contaminated soil. In this study, the rhizosphere micro-ecological characteristics of phytoremediation in seven groups of contaminated soil with different heavy metal species and concentrations were examined. The results showed that heavy metal-enrichment inhibited plant growth, but enhanced both anions (Cr₂O₇²⁻) and cations (Cd²⁺ and Pb²⁺) uptake with corresponding mean values ranging from 19.37 to 168.74 mg/kg in roots and 10.89–86.53 mg/kg in shoots. Trifolium repens L. planting was able to compensate the lost caused by the heavy metal on the soil organic matter, available N, available P, available K and enzyme activities as well. According to the cluster, some species like Lysobacter, Kaistobacter and Pontibacter, was significantly related to heavy metal accumulation while others such as Flavisolibacter, Adhaeribacter and Bacillus promoted plant growth. The importance of root-associated microbial community could relatively regulate plant growth and heavy metal uptake. Our study not only illustrated the correlation among rhizosphere micro-ecological parameters, and the possible mechanisms of phytoremediation regulation, but also provide clear strategy for improving the phytoremediation efficiency.