Corn (Zea mays L.) root exudates and their impact on 14C-pyrene mineralization

Corn (Zea mays L.) root exudates were flushed from a hydrophobic system that allowed the aseptic separation of soluble exudates from the intact plant root. Plants were grown for 90 d, during which time root exudates flowed from the hydroponic setup directly onto columns containing soil previously contaminated with polycyclic aromatic hydrocarbons (PAHs). Mineralization of the PAH, pyrene, was then determined in soil removed from columns. In addition, exudated samples were directly taken from the hydroponic system for estimation of total organic carbon release and for use in microbial studies. In soil from columns that received root exudates from a planted (versus an unplanted) apparatus, there was a significant increase in 14C-pyrene mineralization. The extent of stimulation was comparable to that measured in rhizosphere soil isolated from plants growing in the same soil. Soil from columns that received solution from apparatuses that were not planted showed no stimulation of 14C-pyrene mineralization. Separate studies confirmed that some members of the soil microbial community were able to utilize these soluble plant compounds. This indicates that root exudates have the potential to increase the degradation of xenobiotics by the growth of soil microorganisms. Separating the chemical impact of the root exudates from any root surface phenomena is an important step in isolating a potential mechanism of phytoremediation. Many studies have speculated on the involvement of root exudates in rhizosphere degradation of organic contaminants, but very few studies go beyond adding simple carbon substrates in short pulses. This study employed a system that exposed the microbial community to real root exudates in the concentrations and over a time period that mimicked actual conditions.