Toxicity of trace metals in contaminated urban soils

A recent study was conducted in Montreal, Qu├ębec, Canada, with the aim of estimating chemical speciation of metals in urban sites and to quantify the relative metal bioavailability using uptake in plants and microbial assays (Ge et al., 2002). Therefore, comprehensive investigations of the speciation and behaviour of cadmium copper, lead, nickel and zinc were made at a metal-enriched railway yard. Soil total concentrations of cadmium varied up to about 30 mg/kg DM, of copper up to about 1000 mg/kg DM, of nickel up to 125 mg/kg DM, of lead up to almost 1500 mg/kg DM, and of zinc up to 1600 mg/kg DM, while pH varied between 5.8 and 8.6, and soil organic carbon between 1 and 18%. In soil solutions, free nickel and zinc ions represented as much as 72% and 62%, respectively, of the total dissolved metals. Free metal ions were present especially in soil samples with pH <6.5, while trace metals were strongly bound by fulvic acid when pH was >8.2. Copper was strongly bound by dissolved organic matter, and metal-fulvic acid complexes represented as much as 99% of the total dissolved copper. The chemical analyses showed that, overall, much of the soil metals had a very low solubility, and in the soil solution, most dissolved copper (and lead) were complexed, suggesting a low bioavailability.

The authors noted that the metal contamination levels in the investigated soils in the railway yard were high enough to prevent reuse of the site, even for industrial purposes, according to local regulations. However, the research team did not observe any drastic biological effects. For example, they were unable to find a metal pool, such as free ions or dissolved or total soil metals, that could consistently predict metal uptake by the test plant species (potted chicory) used in the study. Although some metal accumulation was found in either leaves or roots of wild plants growing in the yard, it was clearly shown that chicory grown in the pot experiments had significantly higher bioconcentration factors for the metals than wild chicory. Data from the microbial assays showed that the soil respiration was not affected by the level of metal contamination in the yard, but soil nitrification was inhibited for the most contaminated soils (enhanced levels of copper, nickel and zinc). The higher sensitivity of nitrification over microbial respiration (of glucose) reflects the larger number of microbial species able to decompose glucose. In contrast, nitrification is much more specific and has less redundancy, so if any species involved is affected by metal contamination, the result on the ecosystem is more readily observed and has larger repercussions on proper soil functioning (Andren and Balandreau, 1999).

As a general conclusion, Ge et al. (2002) resume that the metals present in the soil of the railway yard in Montreal seem to have a low bioavailability, because most of the bioassay results suggested a lack of dramatic effects, despite high portions of dissolved free Ni2+ and Zn2+ found in soil solution. However, results also suggested that nitrogen cycling might be affected. It was not possible to sort out which metal was primarily causing this impact on nitrification, but maximum values of total soil metal (1000 mg/kg for Cu, 1500 mg/kg for Pb and 1600 mg/kg for Zn) were indeed so high that it is not at all unexpected that some biological effect would appear in these most contaminated soils.

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