Effects of biochar immobilization Bacillus subtilis on the heavy metal accumulation, rhizosphere microorganisms and metabolism in rice.

Aim: The mining of heavy metals leads to environmental pollution in the soil of surrounding farmland. A method for reducing heavy metal concentrations in crops grown on heavy metal enriched soils is utilizing functional microorganisms and biochar. The 9311 rice variety was used as a model plant to investigate the effects and underlying mechanisms of biochar immobilized functional microbial materials on reducing the accumulation of heavy metals (Sb, Cr, Pb, and Cd) in plants.

Materials and methods: Three remediation materials including Bacillus subtilis (BS), coconut shell biochar (bc), and biochar immobilized B. subtilis (MIX) were evaluated for their effects of heavy metal accumulation, rhizosphere microbial diversity, and metabolism in rice. All three treatments (BS, bc, and MIX) significantly increased the yield of rice plants and reduced the accumulation of heavy metals in rice. The MIX group reduced heavy metal enrichment the most in all three test treatments while also increasing crop yield. The 16S rRNA analysis revealed that the MIX treatment significantly altered the abundance of some non-dominant bacteria (e.g. Spirochaeta, RB41, S0134, and Bryobacter). The LC-MS analysis showed that compared to the control group, the BS, bc, and MIX treatment groups had 35, 94, and 74 differential metabolites respectively, mainly including esters, amino acids and their derivatives, ketones, and terpenoids. The MIX group had a significant effect on the metabolic pathways involved in purine metabolism, glyoxylate and dicarboxylate metabolism, and phenylpropanoid biosynthesis in rice. The overall effect of the MIX treatment was superior to that of applying BS and bc separately.

Conclusion: The application of BS, bc, and MIX treatments in Sb-contaminated farmland can increase rice yield and reduce the absorption of heavy metals. The overall effect of the MIX treatment is superior to applying each separately (BS and bc). These three treatments had no significant effect on the rhizosphere biodiversity or the dominant bacterial structure of the rice rhizosphere but had an impact on non-dominant bacteria and root metabolism in rice roots.