Microbial response to repeated applications of low concentrations of pentachlorophenol in an alfisol under pasture.

Columns of an Alfisol under permanent pasture were polluted by repeated additions of pentachlorophenol (PCP) (7 mg l-1) to levels of 102 and 510 mg Kg-1, to simulate a dynamic diffuse pollution. PCP was rapidly sorbed to the soil organic matter, and was only slightly degraded. Measurements of soil microbial biomass-C revealed a 25% decrease in total biomass-C caused by both leaching and PCP toxicity. Microbial biomass-C measurements performed on soil fractions showed that only microorganisms located in the outer compartment of the aggregates were affected. Microorganisms protected by soil micro-aggregates were not affected, suggesting that they were not in contact with PCP, which was thus unavailable for biodegradation. Three gram negative bacterial strains (Si, C3 and C2), able to use PCP as a sole carbon and energy source, were isolated after 0, 1 and 3 months of PCP enrichment respectively, and were identified as Pseudomonas (Si) and Acinetobacter (C3 and C2). In liquid degradation tests, the strains C2 and C3 degraded 60% of PCP within 26 days whereas the Pseudomonas degraded only 25%. A specific immuno-labeling of the three strains permitted to show that repeated PCP additions to soil had a positive, negative or absence of effect on the populations C2, C3 and Si respectively.