Enhanced Mineralization of [U- ¹⁴ C]2,4-Dichlorophenoxyacetic Acid in Soil from the Rhizosphere of Trifolium pratense

Abstract

Enhanced biodegradation in the rhizosphere has been reported for many organic xenobiotic compounds, although the mechanisms are not fully understood. The purpose of this study was to discover whether rhizosphere-enhanced biodegradation is due to selective enrichment of degraders through growth on compounds produced by rhizodeposition. We monitored the mineralization of [U-14 C]2,4-dichlorophenoxyacetic acid (2,4-D) in rhizosphere soil with no history of herbicide application collected over a period of 0 to 116 days after sowing of Lolium perenne and Trifolium pratense. The relationships between the mineralization kinetics, the number of 2,4-D degraders, and the diversity of genes encoding 2,4-D/␣-ketoglutarate dioxygenase (tfdA) were investigated. The rhizosphere effect on [ 14 C]2,4-D mineralization (50 g g ؊1 ) was shown to be plant species and plant age specific. In comparison with nonplanted soil, there were significant (P < 0.05) reductions in the lag phase and enhancements of the maximum mineralization rate for 25-and 60-day T. pratense soil but not for 116-day T. pratense rhizosphere soil or for L. perenne rhizosphere soil of any age. Numbers of 2,4-D degraders in planted and nonplanted soil were low (most probable number, <100 g ؊1 ) and were not related to plant species or age. Single-strand conformational polymorphism analysis showed that plant species had no impact on the diversity of ␣-Proteobacteria tfdA-like genes, although an impact of 2,4-D application was recorded. Our results indicate that enhanced mineralization in T. pratense rhizosphere soil is not due to enrichment of 2,4-D-degrading microorganisms by rhizodeposits. We suggest an alternative mechanism in which one or more components of the rhizodeposits induce the 2,4-D pathway.The rhizosphere is the zone of soil directly influenced by the presence of plant roots. It receives inputs of an array of low (e.g., sugars, organic and amino acids, phenolics, and other secondary metabolites)-and high (e.g., cellulose, lignin, mucilage, proteins)-molecular-mass compounds as a result of rhizodeposition and is a zone of complex plant-microbe interactions (56). Seminal work by Hsu and Bartha (23) demonstrated that mineralization of organophosphate pesticides in the rhizosphere is increased relative to that in nonplanted soils. Since this study, numerous reports of rhizosphere-enhanced biodegradation have been published for several classes of organic pollutants, including polyaromatic (2, 35, 48) and aliphatic (5, 38, 48) hydrocarbons, chlorophenols (3, 13

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Isolation and evaluation of potent Pseudomonas species for bioremediation of phorate in amended soil

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