The reduction of chromium (VI) phytotoxicity and phytoavailability to wheat (Triticum aestivum L.) using biochar and bacteria

Chromium (Cr) is considered a serious environmental pollutant due to its wide industrial use. Toxicity of Cr to plants depends on its valence state. Cr(VI) is highly toxic and mobile whereas Cr(III) is less toxic. Cr accumulation in plants causes high toxicity in terms of alterations in the germination process, reduction in the growth of roots, stems, and leaves, which may affect total dry matter production and yield. We performed a pot experiment to investigate chromium (50 mg kg⁻¹) induced phytotoxicity in wheat (Triticum aestivum L.) and to reduce its phytoavailability by amending the contaminated soil with chromium reducing bacteria (CRB) and 1% or 5% biochar. For the phytotoxicity assay, wheat was grown at different concentrations of chromium (10, 20, 30, 40 and 50 mg L⁻¹). After 3 weeks a subsequent reduction in root and shoot length, fresh and dry biomass, percentage germination, total chlorophyll, and carbohydrates was observed. Our results showed reduction in phytotoxic effects of Cr(VI) mainly due to a reduction of toxic Cr(VI) to Cr(III). Highest reductive transformation of Cr(VI) to Cr(III) was observed in T9 (5% biochar with bacterial consortia) in all three matrices i.e. soil (99%), root (98%) and shoots (97%). The highest (90%) Cr retention within soil was also observed in T9 with the addition of 5% biochar and bacterial consortia. Of the remaining 10% Cr retention (entering into the plant), 3 mg kg⁻¹ and 1.3 mg kg⁻¹ was found in roots and shoots (on dry weight basis), respectively. Soil inoculation with consortia showed 33% higher stabilization than individual strain application. Soil amendment with biochar and bacteria showed an improvement in plant height, biomass production, seed germination, chlorophyll, protein, and carbohydrate content (p < 0.05). Findings of this study may help to reduce food chain availability of potentially toxic Cr by employing cost-effective bioremediation amendments.