Atrazine and phenanthrene degradation in grass rhizosphere soil

Organic contaminants often disappear more quickly from planted than from non-planted soils. Five grass species (Sudan grass, ryegrass, tall fescue, crested wheatgrass and switch grass) were grown in soils without (Phase I) or with (Phase II) prior atrazine (ATR) and phenanthrene (PHE) amendment to study the degradation of these compounds by rhizosphere microorganisms. In suspensions of soil without prior chemical exposure, no significant loss of ATR was observed after 16 days incubation. The most probable number (MPN) of ATR-degrading bacteria in the soils was below detection. Phenanthrene degradation was observed in suspensions inoculated with all soils, but the rates of degradation were not significantly different among them. The number of PHE-degrading bacteria was similar in planted and non-planted soils (10⁵ cells g⁻¹ soil) except the number in tall fescue soil was significantly higher than in non-planted soil. In the Phase II study, both compounds were mineralized whether or not soils had been conditioned with ATR or PHE. Prior amendment with either ATR or PHE significantly reduced the acclimation period preceding the onset of mineralization. However, enumeration procedures detected ATR-degrading bacteria only in ATR-amended soils. Prior exposure to PHE did not alter the number of PHE-degrading bacteria significantly.