Humic acid-bound polycyclic aromatic hydrocarbons (PAHs) in rhizosphere of rice (Oryza sativa L.)

Abstract

To understand the effect of rice root activities on PAHs bound with humic acid in rhizosphere, we conducted a glasshouse study using rhizoboxes to build rhizosphere gradient with 1-5 mm from root surface of rice (Oryza sativa L.) and measured for polycyclic aromatic hydrocarbons (PAHs) bound with humic acid. The concentration of humic acid-bound PAHs was varied with distance from root surface. In soils with high PAH concentrations, the proportion of humic acid-bound PAHs was lower in rhizosphere near root surface, whereas the proportion of humic acid-bound PAHs was higher in rhizosphere near root surface in soils with low PAH concentrations. When evaluating the effect of root on humic acid-bound PAHs with toxicity, the concentration of PAHs were transferred into benzo[a]pyrene concentration with equivalency toxicity. Total toxicity of PAHs in rhizosphere decreased with time. The toxicity of humic acid-bound PAHs in soils with different spiked concentrations was similar. The toxicity concentration of PAHs bound with humic acid was significantly higher near root surface. The results suggest that rice root activities promoted the bound between PAHs and humic acid in low concentration, but inhibited the bound in high concentration. However, the toxicity of PAHs bound with humic acid was higher near root surface.