Community-based phytoremediation of multi-metal soils: Overyielding effects and microbial mediation

According to the ecological niche theory and the stress gradient hypothesis, diverse communities consisting of hyperaccumulators, nursing plants, and pioneer plants may enhance community productivity and more efficiently extract potentially toxic elements (PTE) from the soil. Pteris vittata L. (hyperaccumulator, E), Medicago sativa L. (nurse plants, M), and Parthenocissus semicordata (Wall.) (pioneer plants, P) were selected for the primary investigation of this hypothesis. Seven monoculture and intercropped treatments (E, M, P, EM, EP, MP, EMP) were carried out to explore the ecological effects and interspecific relationships within plant communities, and to assess their impact on the efficiency of PTE (As, Cd, Pb, Cu, and Zn) combined pollution in-situ remediation. All intercropping communities exhibited an overyielding effect, with the plant aboveground PTE accumulation (mg/m²) exceeding that of monoculture communities, in the order of EM > EP > EMP > MP. The overyielding effect in EM was significant and dominated by complementary effects, with the aboveground biomass of P. vittata (EM-E) being 3.04 times greater than that of E. The accumulation of PTE in aboveground plant of EM was significantly highest, especially for As and Cd, reaching 1861.36 and 48.01 mg/m², respectively. The microorganisms (Candidatus Solibacter, Candidatus Udaeobacter, Nitrosovibrio, Kaistobacter, Thiobacillus, etc.) involved in nutrient cycling in the rhizosphere soil of P. vittata at EM, EP, and EMP communities accounted for a large proportion, showing a notably higher abundance compared to the monoculture community. Structural equation model analysis revealed that the structure and function of the rhizosphere microbial community and soil PTE content mediate the response of phytoremediation efficiency to ecological effects. The findings suggest a viable new concept for in-site bioremediation of PTE contamination in soil.