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Content archived on 2024-04-16

Heavy metal toxicity to soil microbes


The objective is to establish which metals, and at which concentrations, can result in the negative effects observed in symbiotic nitrogen fixation, functioning of VA mycorrhiza and the turnover of carbon and nitrogen through the microbial biomass in soils contaminated with metals. Also to evaluate the need for revision of the maximum permitted metal concentrations in agricultural soils, as stipulated in CEC directive 86/278/CEC, on the basis of the observed metal toxicity effects on soil microorganisms and soil microbial processes.
Standardization and evaluation of methods have been carried out to examine the relative toxicity to microorganisms of a range of contained soils collected from throughout Europe. Metal salt spiked soil has been used to determine the relative toxicity of metals in soils. Chemical characterization of soils from different regions of Europe has been undertaken. Solution systems have been developed for the testing of metal toxicity to isolates of soil microorganisms. The numbers of indigenous rhizobium in a number of metal contaminated soils was determined. There was some indication of a reduction in numbers with increasing soil metal content, but no large effects. Numbers of introduced rhizobium decreased with increasing soil zinc content after 18 months incubation in a gradient of metal contaminated heavy clay soils. MPN determination of rhizobium numbers indicated survival in uncontaminated soil. Methodology for the examination of rhizobium diversity has been finalized and isolations of Rhizobium made. Determination of metal tolerance of a number of species and strains of rhizobium using a water based method proved unsatisfactory. A system involving the growth of zea mays in acid washed sand was used to examine the effect of zinc, cadmium and copper on the formation of arbuscular mycorrhiza. Mycorrhizal formation was inhibited by 500 um copper; 1000 um zinc and 30 um cadmium in nutrient solution and was more sensitive than plant growth for cadmium and copper. Spore germination was examined in a soil spiked with up to 100 mg per kg{-1} of zinc. There was no inhibition of spore germination with increasing soil metal content. Sampling of respired carbon and biomass carbon indicated that any effects of environmental stress are most evident in the long term. However due to the limited buffering capacity a method of evaluating relative metal tolerance employing an ion exchanged resin was developed and evaluated. Preliminary results using the technique identified differences in the growth rate of sensitive and tolerant rhizobium strains at free cadmium concentrations as low as 1.4 ug/l. Results so far suggest that toxic effect on soil microorganisms occur in the longer term and cannot be assessed in the short term and current research addresses this.
For each of the biological systems to be examined, namely Rhizobium, mycorrhiza and the functioning of the microbial biomass, there are 4 main tasks:
to develop and standardize methods for evaluating toxicity to the microorganisms in metal contaminated soils;
to develop and standardize methods for examining the relative toxicity of different metals and what concentrations are toxic (essentially laboratory based studies);
to use the methods developed in the first 2 tasks to examine toxicity to microorganisms in a range of contaminated soils in Europe;
and to evaluate the extent to which populations of microorganisms in contaminated soil have become resistant to the elevated concentrations of metals.

These main tasks will be accompanied by the following supporting activities:
provide chemical characterization of the contaminated soils;
develop solution systems for the testing of metal toxicity to isolates of soils microorganisms;
and collect contaminated soils from different regions of Europe.


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University of London
TN25 5AH Ashford
United Kingdom

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