Influence of soil microbial activity level on the determination of contaminated soil toxicity using Lumistox and MetPlate bioassays

The influence of soil microbial activity level on toxicity bioassay measurements was tested on soils taken from two areas of a site contaminated with metals and hydrocarbons. The level of soil microbial activity was measured by dehydrogenase activity. Microbial activity was artificially increased by spiking the soil samples with a glucose solution and incubating at 18°C, then subsequently depressed by incubating at 4°C for 2 weeks. Lumistox and MetPlate were used as toxicity bioassays. Results showed that the level of soil toxicity as measured by Lumistox and MetPlate bioassays was dependent on the microbial activity level. MetPlate bioassay appeared to be the more sensitive to the variations of soil characteristics. The toxicity determinations varied with the duration of incubation following glucose addition. The toxicity level as measured by both bioassays was increased by glucose addition, probably as a result of a change in the pollutant’s bioavailability due to increases in the soil humidity and the microbial activity. The greater sensitivity of bioassay results to soil microbial activity was observed on the less polluted soil. In such a case of relatively low pollution, great care must therefore be given to the microbial activity of the soil which is related to the period of the year the sampling is done and the humidity of the soil (weather conditions).