Impacts of 2,4-D application on soil microbial community structure and on populations associated with 2,4-D degradation.

The effect of 2,4-dichlorophenoxyacetic acid (2,4-D) application rate on microbial community structure and on the diversity of dominant 2,4-D degrading bacteria in an agricultural soil was examined using cultivation-independent molecular techniques coupled with traditional isolation and enumeration methods. Fingerprints of microbial communities established under increasing concentrations of 2,4-D (0-500 mg kg-1) in batch soil microcosms were obtained using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene segments. While a 2,4-D concentration of at least 100 mg kg-1 was required to obtain an apparent change in the community structure as visualized by DGGE, the greatest impact of 2,4-D concentration occurred in the 500 mg kg-1 treatment, resulting in significantly reduced diversity of the dominant populations and enrichment by Burkholderia-like populations. The greatest diversity of 2,4-D degrading isolates was cultivated from the 10 mg kg-1 treatment, indicating that under these conditions, cultivation was more sensitive than DGGE for detecting changes in community structure. Most of these isolates harbored homologs of Ralstonia eutrophus JMP134 and Burkholderia cepacia tfdA catabolic genes. Results from this study revealed that agriculturally relevant application rates of 2,4-D may provide a temporary selective advantage for organisms capable of utilizing 2,4-D as a carbon and energy source.