Polycyclic aromatic hydrocarbons in biochar amended soils: Long-term experiments in Brazilian tropical areas

The interest in charcoal for agricultural use (biochar) has sharply increased in recent years. However, biochar can contain groups of compounds such as polycyclic aromatic hydrocarbons (PAHs) that are considered persistent pollutants, and are formed concomitantly with biochar during its production by pyrolysis. Soil samples were collected in three experimental areas at different intervals (1, 3, 5 or 6 years) after the application of 16 Mg ha^-1 of biochar. The total concentrations of PAHs; benzo[a]pyrene; and the estimated total cancer risk in biochar treated plots were larger than found in the control ones, but they decreased over time, equaling the control values after three years in the high C content soil (11.2 g C kg^-1 soil), or after six years in the low C content soil (6.8 g C kg^-1 soil). Nevertheless, the sum of PAH concentrations found in the biochar amended plots, in the range of 15.80-39.40 ng g^-1, were around two orders of magnitude below the limits of prevention established by Brazilian legislation (8100 ng g^-1) and some European regulations for soils (3000 ng g^-1) and also lower than the observed in previous studies about biochar amended soils. Our results indicate that, under the conditions evaluated, the application of biochar to soil in the studied proportion (16 Mg ha^-1, every six years) is safe concerning soil contamination by PAHs.