Phytoavailability and Rhizospheric Gradient Distribution of Bound‐Polycyclic Aromatic Hydrocarbon Residues in Soils

Understanding the phytoavailability and rhizospheric distribution of bound residues (reference to parent compounds) of polycyclic aromatic hydrocarbons (PAHs) in soils is of crucial importance for risk assessment of PAH‐contaminated sites. This study investigated the phytoavailability and rhizospheric gradient distribution of bound residues of phenanthrene and pyrene as representative PAHs in soils. Although the available fractions of phenanthrene and pyrene in soils were removed, the clear uptake, accumulation, and translocation of phenanthrene and pyrene by ryegrass (Lolium multiflorum Lam.) indicated a significant phytoavailability of bound‐PAH residues in soils. Rhizosphere soil from the root surface (0–9 mm) was divided into three fractions: the rhizoplane, and strongly and loosely adhering soil. The concentrations of bound‐PAH residues increased, and the rhizosphere effect (R) in the soil weakened with distance away from the root surface after plants were cultivated for 30 to 75 d. In general, the concentrations of bound‐PAH residues decreased over time in all three components of rhizosphere soils; 24 to 40% and 0 to 36% of the bound residues of phenanthrene and pyrene were dissipated in the three rhizosphere soil fractions within 30 to 75 d, respectively. Dissipation ratios decreased in rhizosphere soils with distance from the root surface. The phytoavailability and reduced concentrations of the bound‐PAH residues in the rhizosphere were indicative of their significant release and subsequent uptake by plants and/or degradation in the rhizosphere, which may be related to the reduced soil organic matter and enhanced microbial activity from the input of root exudates in the rhizosphere zone. The results of this work provide useful information on the fate and risk of PAHs in the soil environment.