Biochar and Black Carbon Reduce the Bioavailability and Phytotoxicity of Heavy Metals.

Biochar has attracted research interest due to its ability to increase the soil carbon pool and improve crop productivity. However, little research has been done to evaluate the influence of biochar application to soil on the bioavailability of heavy metals to plants. The objective of this study was to evaluate the metal immobilizing effect of black carbon and chicken manure- and green waste-derived biochars, and their plant growth promoting effect. Application of biochar significantly reduced NH4NO3 extractable Cd, Cu and Pb concentrations of soils, indicating the immobilization of these metals. Chicken manure-derived biochar increased plant dry biomass by 353 and 572% for shoot and root, respectively with 1% of biochar addition, which may be attributed to reduced toxicity of metals and increased availability of nutrients such as P and K. Both biochars significantly reduced Cd, Cu and Pb accumulation by Indian mustard (Brassica juncea), and the reduction increased with increasing amount of biochar application except Cu concentration. Biochar application significantly reduced Cd and Pb concentrations in pore water, but Cu concentration increased with chicken manure-derived biochar application. Metal sequential fractionation data indicated that biochar treatments substantially modified the partitioning of Cd, Cu and Pb from the easily exchangeable phase to less bioavailable organic bound fraction. The results clearly showed that biochar application was effective in metal immobilization, thereby reducing the bioavailability and phytotoxicity of heavy metals.