Biochar properties can be significantly influenced by feedstock source and pyrolysis conditions; this warrants detailed characterisation of biochars for their application to improve soil fertility and sequester carbon. We characterised 11 biochars, made from 5 feedstocks [Eucalyptus saligna wood (at 400°C and 550°C both with and without steam activation); E. saligna leaves (at 400°C and 550°C with activation); papermill sludge (at 550°C with activation); poultry litter and cow manure (each at 400°C without activation and at 550°C with activation)] using standard or modified soil chemical procedures. Biochar pH values varied from near neutral to highly alkaline. In general, wood biochars had higher total C, lower ash content, lower total N, P, K, S, Ca, Mg, Al, Na, and Cu contents, and lower potential cation exchange capacity (CEC) and exchangeable cations than the manure-based biochars, and the leaf biochars were generally in-between. Papermill sludge biochar had the highest total and exchangeable Ca, CaCO3 equivalence, total Cu, and potential CEC, and the lowest total and exchangeable K. Water-soluble salts were higher in the manure-based biochars, followed by leaf, papermill sludge, and wood biochars. Total As, Cd, Pb, and polycyclic aromatic hydrocarbons in the biochars were either very low or below detection limits. In general, increase in pyrolysis temperature increased the ash content, pH, and surface basicity and decreased surface acidity. The activation treatment had a little effect on most of the biochar properties. X-ray diffraction analysis showed the presence of whewellite in E. saligna biochars produced at 400°C, and the whewellite was converted to calcite in biochars formed at 550°C. Papermill sludge biochar contained the largest amount of calcite. Water-soluble salts and calcite interfered with surface charge measurements and should be removed before the surface charge measurements of biochar. The biochars used in the study ranged from C-rich to nutrient-rich to lime-rich soil amendment, and these properties could be optimised through feedstock formulation and pyrolysis temperature for tailored soil application.

Biochar has gained importance due to its ability to increase the long-term 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 trace elements to plants. A pot experiment was conducted to investigate the influence of biochar on the availability of As, Cd, Cu, Pb, and Zn to maize (Zea mays L.). An activated wood biochar, pyrolysed at 550°C, was applied at 3 rates (0, 5, and 15 g/kg) in factorial combinations with 3 rates (0, 10, and 50 mg/kg) each of As, Cd, Cu, Pb, and Zn separately to a sandy soil. After 10 weeks of growth, plants were harvested, shoot dry matter yield was measured, and concentration of trace elements in shoots was analysed. The soil in pots was analysed for extractable trace elements. The results showed that the addition of wood biochar to soil did not have any significant effect on the dry matter yield of maize plants, even at the highest rate of application. However, trace element application significantly reduced the dry matter yield from 10 to 93% depending on the type of trace element. Biochar application decreased the concentration of As, Cd, and Cu in maize shoots, especially at the highest rate of trace element application, whereas the effects were inconsistent on Pb and Zn concentrations in the shoots. The concentrations of extractable As and Zn in soil increased with biochar application, whereas extractable Cu did not change, Pb decreased, and Cd showed an inconsistent trend. Sorption of trace elements on biochar with initial loadings up to 200 µmol at pH 7 occurred in the order: Pb > Cu > Cd > Zn > As. The results show that biochar application can significantly reduce the availability of trace elements to plants and suggest that biochar application may have potential for the management of soils contaminated by trace elements.