Comparative effects of Trichoderma harzianum and Bacillus subtilis on the growth of Canavalia ensiformis (L.) DC. and phytoremediation potential in soils with excess copper (Cu)

High levels of copper (Cu) in agricultural soils, resulting from intensive input use, negatively affect crop growth and physiology. This study evaluated whether the beneficial microorganisms Trichoderma harzianum and Bacillus subtilis can mitigate Cu toxicity in Canavalia ensiformis. The experiment followed a completely randomized design in a 5 × 3 factorial scheme, with five Cu doses (0, 50, 150, 250, and 350 mg kg−1) and three microbial treatments (inoculation with T. harzianum, B. subtilis, or a non-inoculated control), totaling 75 experimental units. Cu reduced plant growth and physiological performance; however, microbial inoculation mitigated these effects. B. subtilis was more effective at lower Cu doses, increasing shoot length by up to 24% and root fresh biomass by 65% compared with the control, while maintaining higher photosynthetic rates and stomatal conductance under Cu stress. In contrast, T. harzianum performed better at higher Cu doses, increasing root dry biomass by more than twofold (112%; p < 0.05) and enhancing Cu accumulation in shoots and roots. It also improved plant water use efficiency under higher metal stress. Under controlled conditions, C. ensiformis tolerated Cu levels up to 250 mg kg−1. The effectiveness of microbial inoculation was dose-dependent, with B. subtilis performing better at lower Cu concentrations and T. harzianum at higher levels. These findings highlight the potential of microbial inoculation as a sustainable strategy to enhance phytoremediation of Cu-contaminated soils.