Changes in soil microbial communities over time resulting from one time application of zinc: a laboratory microcosm study

Soil microcosms each consisting of approximately 20 kg soil were set up in the laboratory. Three microcosms were amended with 6000 mg zinc kg−1, a concentration chosen to approximate the quantities of total zinc found at a contaminated field site. Three control microcosms received no zinc amendment. Three pH control microcosms received H2SO4, adequate to result in the same pH decrease as occurred in the zinc amended soil. Mean soluble zinc contents were 4660 mg kg−1 (amended soils), 1.08 mg kg−1 (unamended soils), and 2.10 mg kg−1 (pH adjusted soils). Biological community assessments included culturable bacteria (plate counts), microbial biomass (chloroform fumigation), dehydrogenase activity, proportion of zinc resistant bacteria (plate counts on zinc amended agar), and changes in community structure (BIOLOG and phospholipid fatty acid (PLFA) analysis). After 15 d of incubation, zinc amendment resulted in an 87% decrease in viable counts and a 47% decrease in microbial biomass. Dehydrogenase activity decreased by 95%, and the proportion of zinc resistant bacteria increased from 0.08% to 0.75%. Microbial community structure alteration resulting from zinc amendment was indicated by shifts in both BIOLOG metabolic profiles and PLFA profiles. Effects of zinc on PLFA profiles included relative decreases in indicator fatty acids for arbuscular mycorrhizal fungi and actinomycetes. Some adaptation of the microbial communities to the zinc amendment was suggested by the fact that after 420 d incubation there were no significant differences in culturable bacterial populations or microbial biomass of the control and zinc amended systems. However, other microbial properties measured in this study showed that zinc still affected microbial community structure and activity after 420 d. Dehydrogenase activity of the zinc treated communities remained 93% lower than the controls. The percentage of zinc resistant populations was still significantly higher for the zinc treated communities. Similarly, the BIOLOG and PLFA profiles continued to show differences in the structure of the zinc treated and control microbial communities. Effects of zinc on PLFA profiles at day 420 included a relative decrease in an indicator fatty acid for arbuscular mycorrhizal fungi and a relative increase in an indicator fatty acid for fungi. The comparison of PLFA profiles also showed mixed results for several fatty acid indicators for actinomycetes. The pH control treatment showed similar effects on population size and activity as the zinc treatment, although in most cases there was a much stronger effect with the zinc treatment. In addition, the pH treatment caused changes in BIOLOG and PLFA profiles which were different than the changes seen with zinc treatment.