Changes in microbial community structure during long-term incubation in two soils experimentally contaminated with metals

The effects of Zn contamination on the microbial community structure of a forest humus and an arable soil, as estimated by phospholipid fatty acid (PLFA) analysis, were followed during 18 months. The soils were contaminated at 10 different metal concentrations and incubated in plastic jars at 22 degrees C. In both soils effects of heavy metal contamination could be detected after 2 weeks. Qualitatively similar changes in the PLFA pattern were found at the later sampling occasions, although the changes became more pronounced with prolonged incubation. In the forest soil the double-unsaturated 18:2 omega 6, indicating fungi, increased proportionally due to the metal amendment, while there was a strong negative effect of incubation on the fungal biomass in all samples of this soil type. In the arable soil 18:2 omega 6 showed a strong increase in response to the Zn pollution. As in the forest soil, incubation decreased the mol% of 18:2 omega 6, although the effect was less pronounced than in the forest soil. The proportions of several individual bacterial PLFAs changed in both soils due to the treatments, indicating shifts within the bacterial community in the soils, but these shifts could not be interpreted in terms of changes in the proportional abundance of specific taxonomic groups of bacteria. The ratio of 16:1 omega 7t-to-16:1 omega 7c, which has been proposed as a starvation index, increased in the forest soil due to Zn contamination. In the high-metal samples this ratio decreased during incubation, while it remained unchanged in the uncontaminated control. In the arable soil no clear effect was found on the trans-to-cis ratio either in response to metal contamination or to incubation. The ATP content decreased during incubation. Little or no effect was found on the total amount of PLFAs or on the lipid phosphate content, except after 18 months when these biomass measurements decreased.