Synergistic effect of biochar and intercropping on lead phytoavailability in the rhizosphere of a vegetable-grass system.

The effects of engineered steam exploded biochar on the phytoavailability of toxic elements in the shared- and nonshared-rhizosphere of vegetable-grass intercropping system have not been investigated yet. Therefore, we explored and elucidated the synergistic effect of pristine rape-straw biochar (BC), steam exploded BC (BCSE), KMnO-modified BCSE (BCSEMn), and hydroxyapatite-modified BCSE (BCSEHA) on the solubility, fractionation and phytoavailability of lead (Pb) in a vegetable-grass intercropping system. In a rhizosphere box, Brassica chinensis L. (pakchoi; PC, as a vegetable) and Pennisetum polystachion L. (mission grass; MG, as a Pb hyperaccumulator), were grown in the biochar treated soil with (non-shared rhizosphere) or without (shared rhizosphere) root separation. Addition of BCSEMn and BCSEHA, particularly BCSEMn, significantly improved plant growth, photosynthetic pigment levels, and positively influenced the gas exchange attributes by suppressing oxidative stress and boosting antioxidant enzymes activities. Both biochars altered a proportion of Pb in the acid soluble to the immobile fraction and thus significantly decreased its leachability (TCLP-Pb) and bioavailability (CaCl-extrcated Pb) by 32.7 %-33.9 % and 48.5 %-53.5 %, respectively, as compared to the control. Both biochars, particularly BCSEMn, reduced significantly the Pb content in shoots and roots of PC and MG with a significantly higher efficiency in the PC than in the MG; this was the case more in the shared than in the non-shared rhizosphere. These findings indicate the synergistic effect of BCSEMn and BCSEHA and intercropping for enhancing the grass phytostabilization capacity for Pb and reducing its uptake by edible plants in a vegetables-grass system, which could be used as a promising approach for the phytomanagement of Pb contaminated soils.