Effects of softwood biochar on the status of nitrogen species and elements of potential toxicity in soils

The effects of softwood-derived biochar materials on the chemical behaviour of environmental contaminants in soils were examined in two microcosm scenarios. Addition of the biochar materials into an alkaline sandy soil significantly reduced NH3 volatilization and made it available for conversion into NO3- via nitrification. This process could be enhanced by an increased application rate of biochar produced at a higher pyrolysis temperature. Under the alkaline conditions encountered in the experiment, the biochar surfaces tended to be negatively charged which disfavours the adsorption of NO3-. Therefore, in a fully open system, the addition of biochar materials was likely to contribute to nitrate leaching from the fertilized alkaline sandy soil. The effects of the biochar materials on the immobilization of Fe2+ generated via anaerobic iron reduction in the inundated contaminated soil were not observed, except for the treatment with a higher dose of biochar material produced under pyrolysis temperature at 700 °C after the 240th h of incubation. Arsenic showed similar behaviour to Fe. Zn tended to have a higher affinity to the biochar, as compared to Mn. Immobilization of Pb occurred regardless of whether or not the biochar is present.