CHANGES IN MICROBIAL COMMUNITY STRUCTURE DURING LONG-TERM INCUBATION IN 2 SOILS EXPERIMENTALLY CONTAMINATED WITH METALS

The effects of Zn contamination on the microbial community structure o f a forest humus and an arable soil, as estimated by phospholipid fatt y acid (PLFA) analysis, were followed during 18 months. The soils were contaminated at 10 different metal concentrations and incubated in pl astic jars at 22 degrees C. In both soils effects of heavy metal conta mination could be detected after 2 weeks. Qualitatively similar change s in the PLFA pattern were found at the later sampling occasions, alth ough 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 sa mples of this soil type. In the arable soil 18:2 omega 6 showed a stro ng increase in response to the Zn pollution. As in the forest soil, in cubation decreased the mol% of 18:2 omega 6, although the effect was l ess pronounced than in the forest soil. The proportions of several ind ividual bacterial PLFAs changed in both soils due to the treatments, i ndicating shifts within the bacterial community in the soils, but thes e shifts could not be interpreted in terms of changes in the proportio nal abundance of specific taxonomic groups of bacteria. The ratio of 1 6:1 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 h igh-metal samples this ratio decreased during incubation, while it rem ained unchanged in the uncontaminated control. In the arable soil no c lear 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 PLFA s or on the lipid phosphate content, except after 18 months when these biomass measurements decreased.