Effects of biochar on the transformation and earthworm bioaccumulation of organic pollutants in soil.

Little is known about the effects of biochar on the fate and behavior of micropollutants in soil, especially in the presence of soil macrofauna. Using a 14C-tracer, we studied the fate of 2,4-dichlorophenol and phenanthrene, after 30 days in soil in the presence of a biochar (0-5%, dry weight) produced from China fir at 400 °C and/or the earthworm Metaphire guillelmi. Application of the biochar significantly reduced the degradation and mineralization of both pollutants and strongly increased the accumulation of their metabolites in soil. The earthworm had no significant effects on the degradation of parent molecules of the pollutants but it significantly reduced the mineralization of the pollutants independent of the presence of the biochar. Although at an application rate of <1% the biochar strongly sorbed both pollutants, it did not significantly decrease the bioaccumulation of free dichlorophenol and phenanthrene and their metabolites by the earthworm. Our results demonstrate the complex effects of biochar on the fate, transformation, and earthworm bioaccumulation of organic pollutants in soil. They show that biochar application may not be an appropriate strategy for treating soil contaminated with hydrophobic organic pollutants and underline the importance of soil-feeding earthworms in risk assessments of biochar effects on soil remediation.