Plant heavy metal concentrations and soil biological properties in agricultural serpentine soils

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Abstract
Soils developed on serpentinitic rocks have serious limitations for agriculture. They have high levels of magnesium (Mg) and heavy metals [copper (Cu), manganese (Mn), nickel (Ni), and chromium (Cr)] and are deficient in some macronutrients. In parts of Northwestern Spain, serpentine soils have been subjected to intensive management, based on the use of manure and harvesting residues. Although these practices have allowed the growth of crops, plants may have accumulated high amounts of metals. This study was carried out to assess the effect of the management practices on the uptake of heavy metals by crops, and to analyze the relationship between the concentrations of these metals in plants, and soil properties. Moderate levels of Ni and Mn and low levels of Cr and Cu were found in soil extractable fractions of these metals. In spite of this, analysis of plant tissues revealed high levels of Cr and Ni and moderate contents of Mn. Concentrations of Mn and Ni in foliage were correlated to soil extractable contents, whereas simple linear regression between concentration of Cr in plants and the soil‐extractable Cr showed a poor relationship, possibly because the availability of this metal, as Cr(VI), is determined by temporal environmental conditions. To assess the effects of the management on the uptake of heavy metals by plants, a complementary bioassay experiment was carried out in the laboratory in which Festuca rubra and Agrostis stolonifera were sown on serpentine soil with low organic matter content, and amended with peat and/or lime. This experiment confirmed that there is a reduction in heavy metal concentration in plants after organic amendment and suggested that the lower metal availability is partly due to the higher soil microbial activity, produced as a consequence of addition of organic matter.