Objectives The aims were to investigate the effects of adding Bacillus polymyxa and woody biochar to cadmium (Cd) contaminated soil on Cd uptake by Perilla frutescens, in order to provide technical support and theoretical basis for safe cultivation on Cd contaminated farmland.

Methods A pot experiment was conducted using Cd-contaminated soils (light contaminated, LC; heavy contaminated, LG) from Ganzhou, China. Eight treatments were set up, including blank control ( LCCK, LGCK) , single application of woody biochar (LCB, LGB) , single application of Bacillus Polymyxa (LCJ, LGJ) , application of Woody Biochar + Bacillus polymyxa (LCBJ, LGBJ). The aboveground and underground biomass of perilla frutescens were tested. The total Cd contents in organs of perilla frutescens, effective Cd contents and pH in soil were determined.

Results In heavy Cd-contaminated soil, LGBJ treatment had the most significant effect on perilla's "growth promotion". The biomass of aboveground and belowground parts increased by 48.26% and 36.77%, respectively. In light Cd-contaminated soil, the biomass of aboveground and belowground parts increased by 14.15% and 17%, respectively. In heavy Cd-contaminated soil, effective Cd contents in soils were follows: LGBJ < LGB < LGCK < LGJ. Meanwhile, effective Cd content was reduced by 56.97% under the LGBJ treatment compared with the control, and the Cd contents in the above-ground and below-ground parts of perilla were significantly reduced by 39.84% and 33.03% respectively. But in the lightly Cd-contaminated soils, the effect of LCBJ treatment on effective Cd contents and total Cd contents of the above-ground and below-ground parts of perilla were not significant (P > 0.05). In addition, under different levels of Cd pollution, the combined treatment of woody biochar and Bacillus polymyxa significantly increased soil pH (P < 0.05), showing the contents of Cd in the soil active state was negatively correlated with soil pH, while it was positively correlated with the Cd in the aboveground and belowground parts of perilla (P < 0.01).

Conclusions The joint application of woody biochar and Bacillus polymyxa could reduce the uptake of Cd by Perilla in heavily Cd-contaminated soil by regulating pH and Cd effective state in soil, so as to alleviate the growth inhibition of Perilla by Cd.