Heavy metals and their effects on biological activity in subalpjne soils on serpentinite (Western Italian Alps)

Soils over ophiolitic materials are known to be toxic for non-adapted organisms, due to high heavy metal content (Ni, Cr, Co, Mn), combined with a low Ca/Mg ratio and a high total Mg. Subalpine soils developed from ultramafic materials are strongly acidic, thus increasing metal mobility and bioavailability. The aim of this study was to assess wether BSQ (Biological Soil Quality), based on microarthropodes communities, and microbial activity would be lower on metal-rich ultramafic soils, and wether microbial stress indicators, (i.e.: metabolic quotient) would be higher. Six soils were studied (chosen out of 83), under subalpine Pinus uncinata forest and ericaceous shrubs, on till composed of mafic and ultramafic rocks in different amounts. One pedon is polluted by mine debris (serpentine with great quantities of magnetite and other metallic minerals). All the pedons are strongly podzolized. Standard chemical analysis were performed, together with labile organic carbon, total elemental composition, available forms of metals. Microbial biomass, respiration and metabolic quotient were analyzed by chloroform fumigation –extraction method. Biological Soil Quality (BSQ), based on microarthopodal communities (Parisi 2001), was calculated in the surface mineral horizons. The results show that soils on mafic parent materials had medium to high metal contents, a higher one was on ultramafic parent materials, while in the polluted site the values are extreme (i.e.: Ni is between 360, 900 and 3500 ppm, respectively). The differences between soils and horizons on the same parent material are due to leaching and/or accumulation. Metal speciation showed that most metals are associated with pedogenic FE oxides and organic matter. Similarly, EDTA extractable metals show significant differences between the soils on the different parent materials (i.e.: EDTA extractable Ni is between 5, 14, 80 respectively). BSQ (expressed on a scale frm 0 to 5) shows a significant correlation with substrate and available and total metal content: the quality is very low (0.6) in the polluted soil, though it is rich in organic matter; it is low (1) on serpentinite, medium on mafic rocks (2). Microbial parameters (in similar horizons in the different soils)seem only weakly related with wither substrate and total or available metal content. The statistical significance is very low. Microbial respiration and biomass usually decrease from the A to the E horizons, and then increase again in the Bs horizons, reflecting a relation with available fine particled organic matter. Stress indicators are slightly higher in metal-rich serpentine soils. The present study shows that the high metal toxicity is particularly visible for microarthropode communities, which are impoverished in biodiversity and in exigent euedaphic forms, while microbial activity doesn’t seem to be much influenced: this is different from what happens in contaminated soils in non-ophiolitic areas, where pollution reduces microbial respiration and increases stress indicators (Giller et al., 1998). This difference can be due to the presence of communities adapted to high meial content, as shown by Shipper and Lee (2001) and Amir and Pineau (2003).