Effects of biochar and root inoculation with Bacillus subtilis on plant growth and phytoremediation efficiency of Phalaris arundinacea and Festuca arundinacea in highly zinc and cadmium-polluted soils

In highly metal-contaminated soils, the application of phytoremediation techniques is often constrained by the limited number of suitable plant species that can tolerate the adverse conditions. To overcome these limitations, plant growth is often supported by the application of growth-promoting rhizosphere bacteria (PGPR) and biochar. Still, information on the effects of combined biochar and PGPR applications on soil metal availability and plant responses beyond the most profoundly studied accumulator species is very scarce. In a greenhouse experiment, we cultivated the commercially available bioenergy grasses Phalaris arundinacea and Festuca arundinacea on soil contaminated with four concentrations of Zn, Cd, Cu, and Pb, in the absence or presence of Bacillus subtilis and biochar in the growth substrate (quartz sand). In the highest metal treatment, the plants received 1000 mg/kg Zn, 6 mg/kg Pb, 5 mg/kg Cu, and 0.5 mg/kg Cd in readily plant-available forms. The other metal treatments accounted for 50%, 25%, and 0% (reference) of the highest concentrations. Within each concentration level, the soil was either left untreated (reference) or additionally treated with Bacillus subtilis, 10% biochar or a combination of B. subtilis and biochar. Each treatment was fourfold replicated.  After 5 weeks of plant growth, morphometric plant parameters of roots and shoots were measured. The rhizosphere soil was characterized regarding available element fractions and B. subtilis cell density. Root and shoot accumulation of elements was evaluated by ICP-MS. When metal treatment was low, the application of biochar and B. subtilis had no significant effect on plant growth. However, when exposed to high metal concentrations, plants treated with B. subtilis showed more than 60% higher biomass, irrespective of plant species. In contrast, the application of biochar and the combined treatment with biochar and B. subtilis had no significant effects on plant development. Preliminary results on root and shoot element concentrations indicated that B. subtilis did not influence net shoot and root uptake of elements. Ongoing analysis will elucidate the processes involved. We conclude that the application of B. subtilis is a promising strategy for enhanced plant tolerance and phytoremediation efficiency on highly polluted soils. We cannot rule out that combinations of B. subtilis with biochar might have synergistic effects when other soils and plant species are considered. Nonetheless, our data show that these combinations do not enhance phytoremediation per se when the plants are exposed to high metal concentrations in the growth medium.