A study of the structure and metal tolerance of the soil microbial community six years after cessation of sewage sludge applications

Changes in soil microbial community structure and development of metal tole rance as a result of past applications of unamended sewage sludge and metal -amended sewage sludge were found in soils of a long-term field experiment at Braunschweig, Germany. Both the rate of sewage sludge application and me tal amendment affected the size and activity of the microbial biomass and h ad caused changes in microbial community structure as seen by differences i n phospholipid fatty acid (PLFA) profiles. Past sewage sludge additions and metal amendment had an effect on the microbial respiratory response to 15 different C substrates, but both the magnitude and the direction of this re sponse were substrate dependent. Differences between the soils in the respi ratory response to the substrates were therefore probably largely determine d by differences in the composition of the microbial consortia utilizing th e substrates. The level of metal tolerance of the soil bacterial community determined by the thymidine incorporation technique and that of the microbi al consortium growing on glucose in situ (determined from respiration measu rements) increased with the level of metal contamination of the soil. Metal tolerance measurements could identify the metal with the largest toxicity effect in this experiment with multiple metal-polluted sewage sludge.