Effect of soil structure on the bioavailability of polycyclic aromatic hydrocarbons within aggregates of a contaminated soil

Biodegradation of polycyclic aromatic hydrocarbon (PAH) was investigated in the whole matrix and in the different aggregate size fractions of a sandy soil contaminated by a mixture of 8 PAHs and incubated at water holding cap acity. The distribution of PAHs and of phenanthrene-degrading bacteria were determined in the bulk soil and in 4 size aggregate fractions correspondin g to sand, coarse silt, fine silt and clay. The microbial communities able to degrade phenanthrene were detected at a similar level in the different a ggregate fractions of the soil before contamination. After soil contaminati on and incubation, a significant growth of bacteria was observed and their distribution within aggregates was modified. Bacterial communities of phena nthrene-degraders were present in a higher density in the aggregates corres ponding to sand (2000-50 mum) and clay (< 2 mum). Chemical analysis show th at remaining PAHs (low and high molecular weight) were much more concentrat ed in the fine soil fractions (fine silt and clay) and were present at a ve ry low content in the larger aggregate size fractions. The interactions of well defined aggregates with PAHs and bacteria were also studied using phen anthrene as PAH model substrate and individual aggregates corresponding to sand and clay size fractions. Incubation of sand and clay aggregate fractio ns enriched with phenanthrene in the presence of a bacterial isolate NAH1 l ed to the simultaneous solubilization and biodegradation of phenanthrene. D ifferences in amounts of solubilized phenanthrene between sand and clay agg regate size fractions would be related to difference in adsorption capaciti es of phenanthrene by clay and sand aggregates. (C) 2001 Elsevier Science L td. All rights reserved.