Annual dynamics of indigenous microbial communities of forest soils after severe PAH contamination

Polycyclic aromatic hydrocarbons (PAHs) are worldwide contaminants that, due to their long-range transport, can accumulate far from the emission sources, e.g. in forest soils. Since changes in soil microbial community can have an impact on ecosystem processes (e.g. nutrient cycling, decomposition, stress stability), studies on the succession of indigenous microbial community following exposure to PAHs are important. Most of these studies are limited to soils aged contaminated with PAHs, since our aim was to fill the knowledge gap on recent PAH contamination. To this aim, soils from three forest systems (holm oak, black pine and beech) were spiked with 3 PAHs (phenanthrene, pyrene and benzo[a]pyrene) and incubated under controlled conditions in mesocosm. During the first year after incubation, changes in the activity and biomass of the soil microbial community were estimated by analysing phospholipid fatty acids (PLFA), fungal biomass (ergosterol content), and fungal and bacterial growth. Findings demonstrated that PAHs affected the soil microbial community structure. Actinomycetes, fungi and Gram+-bacteria were initially resistant to PAH contamination in all the forest systems, whereas for fungi a recovery was observed at the end of incubation, such as ergosterol and growth, in holm oak and beech soils. PAH spikes seemed to have an overall negative effect on bacteria and Gram- groups in all the soils at the beginning of incubation. Considering the values of metabolic activity index (MAI), the composition of the microbial community did not seem to be resistant to the addition of PAHs. However, the soil under beech showed a slightly better resistance to contamination. Furthermore, the community structure did not appear to change in relation to the PAH considered. Overall, in the microbial community of holm oak and beech soils, different groups were able to quickly recover from the new soil conditions.