Phytoavailability of Cd and Pb in crop straw biochar-amended soil is related to the heavy metal content of both biochar and soil

• Heavy metal free- (FC) and rich- (RC) biochars were used as soil amendments.
• FC application immobilized Cd and Pb in both lightly and heavily contaminated soils.
• RC application may increase Cd accumulation risk in lightly contaminated soil.
• RC application could immobilize Cd and Pb in heavily contaminated soil.
• Biochar effect on heavy metals related to their concentrations in soil and biochar.

Crop straw biochar incorporation may be a sustainable method of amending soil, but feedstock-related Cd and Pb content is a major concern. We investigated the effects of heavy metal-rich (RC) and -free biochar (FC) on the phytoavailability of Cd and Pb in two acidic metalliferous soils. Biochar significantly increased soil pH and improved plant growth. Pb in soil and plant tissues significantly decreased after biochar application, and a similar pattern was observed for Cd after FC application. RC significantly increased NH4NO3-extractable Cd in both lightly contaminated (YBS) and heavily contaminated soils (RS). The Cd content of plants grown on YBS increased, whereas it decreased on RS. The Cd and Pb input–output balance suggested that RC application to YBS might induce a soil Cd accumulation risk. Therefore, identifying heavy metal contamination in biochar is crucial before it is used as a soil amendment.

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