Toxicity to soildwelling organisms and to higher plants

In spite of a considerable amount of effort by scientists and regulators in many different countries, it is not an easy task to define clear and unequivocal soil quality guidelines for trace metals. The various ways of expressing limits for metals in soil, either as total metal in bulk soil, as metal dissolved in the soil solution or as free metal ion activity, can all be subjected to criticism, because for one reason or another, they are not equivalent to the bioavailable fraction of the metal in question. Moreover, "the bioavailable fraction" is also a variable, since it might not be the same for a higher plant, a soil invertebrate or a soil microorganism. In stead of giving numerical values specifying a kind of global limit for each trace metal in the soil, some authors suggest that it is necessary to consider the regional natural background and formulate the permissible level of metal contamination in terms of "critical" enhancement, i.e. the permissible number of times metal concentrations can be increased relative to the background level. But again, freshly added metals have a different behaviour (bioavailability and toxicity) in the soil than metals gradually introduced over a long period of time; the problem of metal aging in the soil has to be taken into account.

In order to overcome the difficulty of defining threshold values for metals in soils, is has been proposed to use, in stead, "critical" concentration in the tissues of plants growing on the soils to be assessed. For Cu, the critical plant tissue concentrations appears to be in the range 10-30 mg Cu/kg dry tissue, and for Zn it might be in the range 200-300 mg/kg. However, metal "hyperaccumulators" may exhibit much higher metal concentrations in their tissues.

Comprehensive studies of metal speciation, bioavailability and toxicity in urban soils with a long history of heavy contamination with mixed metals are rare. Metals in the soils at an old railway yard in Montreal were recently found to show a low solubility, and those occurring in the dissolved phase were largely bound in metal-fulvic acid complexes. Wild plants growing in the yard accumulated much less metals than the same plant species used in short-term pot experiments. Soil respiration was not affected, despite the very high metal concentrations in the bulk soil (1,000-1,600 mg/kg for Cu, Pb and Zn), but soil nitrification was inhibited at the most contaminated sites, confirming the generally high metal sensitivity of this function.

Another major study was conducted with 15 uncontaminated topsoils with total Zn levels ranging from 7 to 191 mg/kg dry matter, which - after spiking with Zn - were used in 6 different bioassays to assess the bioavailability and toxicity of Zn in these different matrixes. The ranges in no-effect concentration obtained in the 6 assays were generally larger among soils than among tests, confirming the major role of soil properties for the toxicity of Zn. Based on the relationships observed between toxicity and various soil parameters, it would be possible to normalise the toxicity data with regard to soil type, and thereby derive "soil sensitivity factors" that would explain the differences in toxicity.

A second part of the above referred study consisted of a comparison between the effects of Zn originating from gradual field contamination of soils (from galvanized transmission towers) and those of Zn in freshly spiked soils. Zinc toxicity was consistently lower in the field contaminated soils than in the corresponding spiked soils, as demonstrated in three microbial assays and one plant growth assay. This confirms again the importance of aging for the expression of soil toxicity and perhaps also that microbial communities may adapt to long-term exposure to a trace metal in the soil, which may reduce their sensitivity.

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