Does biochar application in heavy metal-contaminated soils affect soil micronutrient dynamics?

Biochar has been practically used as a soil conditioner in degraded or contaminated soils. Whether biochar would simultaneously affect soil micronutrient bioavailability in heavy metal-contaminated soils warrants research. A 90-d incubation of rice straw biochar with either Pb- or Cd-contaminated soils was conducted to investigate the biochar effect on Fe, Mn, Cu and Zn bioavailability. Biochar amendment decreased the DTPA-extractable, acid extractable and reducible Pb and Cd but increased the oxidizable and residual Pb and Cd. It decreased the DTPA-extractable Fe by 73.8% and 23.1% but increased the DTPA-extractable Mn by 9.8% and 44.3% in the neutral and acidic soil, respectively. The lower Fe availability was attributed to the liming effect and the increased numbers of oxygen-containing groups and organic-mineral phases of biochar over time. Biochar amendment increased soil bacterial richness and diversity in both soils. Rice straw biochar can effectively immobilize heavy metals and reduce soil Fe bioavailability but increase Mn bioavailability, which would potentially affect soil micronutrient fertility and productivity.