High Cadmium and Mercury Soil Contamination Outweighs the Effect of Soil Amendments When Growing Miscanthus x giganteus

This three-year study evaluated the effects of various soil amendments on growth parameters and heavy metal (HM) accumulation in above- and belowground biomass of Miscanthus x giganteus (MxG), assessing its phytoremediation potential. A randomised complete block design included four treatments: I—control, II—sludge, III—mycorrhiza, and IV—MxG ash. All experimental pots were filled with soil spiked with Cd (100 mg kg−1) and Hg (20 mg kg−1). Aboveground biomass yield ranged from 3.44 to 5.59 tDM ha−1, with Cd and Hg concentrations in biomass varying from 5.98 to 14.62 mg Cd kg−1 and 41.8 to 383.9 μg Hg kg−1, respectively. Belowground biomass mass ranged from 6.90 to 8.30 tDM ha−1, with Cd and Hg concentrations between 44.3 and 57.2 mg Cd kg−1 and 4.24 to 6.05 mg Hg kg−1, respectively. Enrichment coefficients (EC) in aboveground biomass ranged from 0.060 to 0.146 for Cd and 0.002 to 0.019 for Hg. Belowground biomass EC values ranged from 0.44 to 0.57 for Cd and 0.21 to 0.30 for Hg. The translocation factor (TF) varied from 0.104 to 0.145 for Cd and 0.008 to 0.024 for Hg. Our findings suggest that miscanthus is more effective for heavy metal phytostabilisation and biomass production in moderately contaminated soils than for phytoextraction.