Influence of biochar application methods on the phytostabilization of a hydrophobic soil contaminated with lead and acid tar.

A hardwood biochar was examined for its potential use as an amendment to aid in the phytostabilization of a severely-contaminated soil at a former sulfuric acid recycling factory site. The soil, which has remained unvegetated for nearly a century, contained high pseudo-total concentrations of lead, arsenic and antimony and was both highly acidic and hydrophobic due to the presence of petroleum-based acid tar. Three application approaches were tested with 10 and 20% (vol/vol) biochar: Incorporation into soil, top-dressing on the surface, and layering within the soil. The results suggest that the homogeneous mixing of the hardwood biochar into soil would not promote the long-term restoration at this site due to its inherently low alkalinity relative to the very high net acidity of the existing soil. In contrast, surface application of biochar resulted in the most successful growth of Canada wild-rye grass by exploiting the properties inherent to biochar alone.