Microbial communities of forest soils contaminated by PAHs: a new quality index to evaluate their functional and structural stability

Understanding and assessing changes in microbial community structure and function can predict ecosystem stability under anthropogenic stresses and help to deepen the knowledge of the risks related to ecosystem perturbations, from anthropogenic contamination to environmental changes, a critical issue in ecology. The microbial community succession on forest soils recently contaminated by PAHs was nvestigated during one year in a mesocosm trial. To this end, soils under holm oak, black pine and beech were spiked with three PAHs (phenanthrene, pyrene, benzo[a]pyrene) characterized by different molecular weights and the effects on the tructural and functional stability were studied. The PAH short-term effects on soil microbial community structure were estimated by PLFA pattern and ergosterol content, and those on soil microbial community activity by fungal and bacterial growth. The derived information was summarized with synthetic soil quality indices useful to quantify the ecological impact of soil perturbations. To investigate the microbial community stability, an index proposed in first formulation (MAI) as a soil quality index, was applied to these results. Actinomycetes, fungi and Gram+ bacteria were initially resistant to PAH contamination in all the forest systems. PAH treatments seemed to have an overall negative effect on Gram- groups in all the soils at the beginning of incubation. Considering the MAI values, soil under beech showed the highest resistance to contamination. Overall, microbial community under holm oak and beech selected different groups capable of quick recovering from the new soil condition