Combined remediation effect of ryegrass-earthworm on heavy metal composite contaminated soil

Combining plants and soil organisms offers an effective strategy for managing heavy metal contamination. A pot experiment was conducted by using ryegrass (Lolium perenne L.) and earthworms (Eisenia fetida) under five levels of heavy metal contamination, applied individually and in combination. After 80 days, plant and earthworm growth, heavy metal accumulation, and residual soil metal concentrations were analysed to assess the potential of ryegrass-earthworm for remediation of heavy metal contaminated soil. The results indicated that the combined treatment significantly enhanced ryegrass and earthworm growth, increasing both the density and biomass yield of ryegrass and earthworms compared to the individual treatments. This approach also improved heavy metal accumulation, achieving the maximum decreases Cd, Pb, and Cu in soil of 38 %, 42 %, and 34 %, respectively. Structural equation modeling analysis revealed mutual growth promotion between ryegrass and earthworms, though competition for heavy metal accumulation was noted. The combined treatment achieved the highest subordinate function value (0.50), outperforming ryegrass alone (0.48) and earthworms alone (0.43). These findings highlight the superior effectiveness of the combined ryegrass-earthworm strategy, particularly at higher heavy metal concentrations, making it a promising strategy for remediating contaminated soils.