Gradient Distribution of Root Exudates and Polycyclic Aromatic Hydrocarbons in Rhizosphere Soil

Abstract

The gradients of root exudates and of persistent organic pollutants, including polycyclic aromatic hydrocarbons (PAHs), in rhizosphere soil in proximity to the root surface are still not well elucidated. In this work, a greenhouse experiment was conducted to investigate the distribution gradients of root exudates and phenanthrene and pyrene, as selected PAHs, in rhizosphere soil close to (0-8 mm) the root surface of ryegrass [Ldium multiflorum Lam.) as the host plant. Rhizosphere soil from the root surface (0-8 mm) was divided into three layers: the rhizoplane and strongly and loosely adhering soil. Root exudates were characterized as soluble organic C, organic acids, and total soluble sugars. In PAH-spiked rhizosphere soils, the concentration of root exudates decreased with distance from the roots. In a sterilized treatment, the amounts of root exudates in the three rhizosphere layers were higher than when the soil was not sterilized, indicating that microbial consumption contributed significantly to the loss of root exudates in the rhizosphere. Tlie residual concentrations of phenanthrene and pyrene clearly increased in die order of rhizoplane to loosely adhering soil after 40 to 50 d, which was significantly and negatively correlated with the amount of root exudates in the rhizosphere. In total, 87 to 97% of phenanthrene and 69 to 79% of pyrene dissipated in all three layers of rhizosphere soil after 40 d, and >99% of phenanthrene and 93% of pyrene dissipated after 50 d. The degradation ratio of PAHs decreased in rhizosphere soils with distance from the roots.Abbreviations: OA, organic acid; PAH, polycyclic aromatic hydrocarbon: SOC, soluble organic carbon: TSS,) total soluble sugar.

Root Exudation: The Ecological Driver of Hydrocarbon Rhizoremediation

“…Even if plants can also produce several enzymes to degrade organic compounds (such as peroxidases and phenol oxidases), they are generally considered as a minor contributor to the dissipation of organic contaminants in soil [67]. Phytoremediation of hydrocarbons depends primarily on rhizoremediation (Figure 1) which involves the breakdown of contaminants in soil as a result of microbial activity at the roots [66,68,69].…”

“…The quantity and quality of root exudates are determined by the cultivar, plant species, developmental stage, various environmental factors (soil type, pH, temperature, nutrient availability) and the presence of microorganisms [49, [144][145][146][147][148][149]. The greatest concentration of exudates occurs mostly at the root tips and at sites of lateral branching, decreasing with increasing distance from the root surface [54, 67,144,150]. Gao et al [67] proposed that decreases in concentrations of root exudates depend mainly on two aspects: diffusion and degradation due to chemical (sorption or desorption) [151,152] or biological process (e.g., microbial consumption).…”

“…The greatest concentration of exudates occurs mostly at the root tips and at sites of lateral branching, decreasing with increasing distance from the root surface [54, 67,144,150]. Gao et al [67] proposed that decreases in concentrations of root exudates depend mainly on two aspects: diffusion and degradation due to chemical (sorption or desorption) [151,152] or biological process (e.g., microbial consumption). Microbial consumption contributes to the dissipation of root exudates because it provides the nutrition and energy necessary for survival and reproduction of rhizosphere microorganisms [140,141].…”

“…Root exudation is now considered to be the most important factor in the mediation of hydrocarbon biodegradation in the rhizosphere [54, 67,104,141,[227][228][229][230][231][232]. Root exudates serve as a carbon source and energy for microorganisms, and also improve the hydrocarbon degradation in the rhizosphere by stimulating hydrocarbon-degrader populations [67].…”

“…Root exudates serve as a carbon source and energy for microorganisms, and also improve the hydrocarbon degradation in the rhizosphere by stimulating hydrocarbon-degrader populations [67].…”