Soil and Biota of Serpentine: A World View 2009 Northeastern Naturalist 16(Special Issue 5):193–214 Heavy Metals and Biological Properties of Subalpine Soils on Ophiolites in the Italian Western Alps Michele E. D’Amico1,*, Francesca Calabrese1, Andrea Rossetti1, and Franco Previtali1 Abstract - Soils on ultramafic rocks are usually colonized by plant species and communities adapted to high heavy-metal content and low Ca/Mg ratio. However, the effects of metal speciation on microbial activity and arthropodal communities have scarcely been studied, especially under coniferous forests in boreal or subalpine areas. Six typical subalpine soils, in the ophiolitic area of Mont Avic Natural Park, located in the Western Italian Alps, were studied in order to verify the chemical speciation of Ni, Co, Mn, and Cr and their effects on soil biological properties and microbial activity. Five soils, developed from till composed of mafic and ultramafic materials, showed strong signs of podzolization, while the sixth was polluted by mine spoil. All the samples had high metal content, high acidity, and high metal mobility and bioavailability. These edaphic properties deeply influenced both arthropodal communities and microbial activity, all of which were strictly correlated with parent material and bioavailable Ni, Co, and Mn. Introduction Soils over ultramafic materials are characterized by the “serpentine factor:” low exchangeable Ca, low Ca/Mg ratio, low nutrients, and high Ni, Cr, Co, and Mn content. Such soils are of great ecological interest due to their unique chemical, physical, and biological characteristics (Brooks 1987). The “serpentine factor” makes these soils toxic for organisms that are not adapted to harsh conditions. For example, plant communities on serpentine are usually strikingly different from those on nearby areas on different substrata and are rich in endemic species (e.g., Carex fimbriata Schkuhr, Thlaspi sylvium Gaudin, and Cardamine plumieri Vill. in the European Western Alps) and in taxa tolerant to heavy metals (e.g., Thlaspi caerulescens J. and C., T. sylvium, Cardamine plumieri L., and Biscutella laevigata L. in the study area). Often, microbial communities are also different in these soils, displaying adaptations to edaphic stress caused by heavy metals (Amir and Pineau 2003, Mengoni et al. 2001); they are usually composed of a smaller number of taxa compared to nearby soils on other substrates (Oline 2006). However, very few studies have been undertaken to determine the biological impacts of chemical speciation and the availability of metals, particularly in subalpine or boreal environments (Bulmer and Lavkulich 1994, Gasser et al. 1994, Roberts 1980). Many studies show how biological activity is broadly affected by heavy metals in recently contaminated soils. 1Department of Environmental Sciences, University of Milano Bicocca, 20126, Milano, Italy. *Corresponding author - ecomike77@gmail.com. 194 Northeastern Naturalist Vol. 16, Special Issue 5 In particular, microbial biomass and respiration are normally reduced and the stress symptoms enhanced when metals are added to soils (Giller et al. 1998). This effect is often the result of heavy metal toxicity on microbial communities. The same result has been demonstrated on microarthropodal communities (Edwards 2002). Heavy metals added to the soil have effects at the population level (changes in the life cycle), community level (changes in species diversity and relationships among plants, between plants and microbial communities, and between plants and soil fauna) and ecosystem level (effects on primary and secondary productivity and rate of decomposition of organic matter). The effects of heavy metals on biological activity are not clear in naturally metal-rich soils. Some studies show that on metal-rich, serpentinite-derived soils, microbial activity (biomass and respiration) is not reduced compared to nearby soils with a “typical” chemistry (Schipper and Lee 2004). This maintenance of microbial activity is due to specific adaptations of the microbes to edaphic stress. In contrast, other studies show that long-term metal contamination can lead to a permanently reduced microbial biomass (Giller et al. 1998). The microarthropodal ecology and the microbial activity of soils formed from ophiolitic materials under subalpine or boreal vegetation are poorly studied, particularly on the Alps. In addition, only a few investigations deal with soils developed from ultramafic rocks in temperate climates (Mengoni et al. 2001, Schipper and Lee 2004). Nevertheless, this is an interesting subject, as subalpine environmental conditions (high acidity, humidity, and leaching) should enhance metal mobility and, perhaps, bioavailability and toxicity. The goals of this study were to investigate heavy-metal speciation and bioavailability and assess whether heavy metals and their chemical forms can be considered toxic on microbial communities and on soil-dwelling arthropods. Biological soil quality index (BSQ; Parisi 2001) based on microarthopod communities, microbial activity (biomass and respiration), and stress indicators (i.e., metabolic quotient) were used as edaphic quality indices. The Study Area The six investigated soil pits were located in Mont Avic Natural Park, Valle d’Aosta, northwestern Italy. They were developed under subalpine Pinus uncinata Miller forest and ericaceous shrubs on glacial till composed of mafic and ultramafic rocks in different proportions (Table 1). The soil pits were representative of the typical subalpine soils formed in the study area. They were selected among 67 previously observed and analyzed soil pits. One of the soils (P8) was polluted by mine debris (serpentinite with large amounts of magnetite and other metallic minerals). It was sampled in order to assess the differences in metal speciation and in biological activity between naturally metal-rich and polluted sites. Environmental properties of the sites are summarized in Table 1. The average yearly temperature is 2 °C and the precipitation is 1200 mm/y. The 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 195 precipitation maxima are in autumn and spring, while winter has the lowest value; water is never a limiting factor. Differences in vegetation cover are not important: hyperacidophilous shrubs, sedges, and grasses are the most common plants (Rhodoreto-vaccinietum). The strictly serpentiniculous sedge, Carex fimbriata Schkuhr, and some Brassicaceae grow only where serpentinite is the main lithology in the substratum (D’Amico 2006). The whole valley is carved in ophiolitic rocks, with serpentinite (of antigorite type) as the main lithology. Additionally, there are large outcrops of meta-gabbros and amphibolites. Chlorite-schists are often associated with serpentinitic bodies (Occhipinti 1997). The metal content of the most common rocks in the study area was shown in D’Amico et al. 2008. Materials and Methods Sampling and chemical analysis Soil sampling took place in late August 2006. Around 500 g of soil from each horizon were collected in order to make chemical analyses. Rock fragments were separated, cleaned (using a Calgon solution), observed, and divided according to the different lithologies. The fragments were weighed, in order to distinguish the proportions of the different rocks in the morainic parent material. The parent material was also characterized with the help of X-Ray diffractometry (XRD) analysis of the sandy textural fraction (D’Amico et al. 2008). The pedons were classified according to IUSS Working Group (2006). The following chemical analyses were performed twice on each sample, after drying at air temperature and sieving at 2 mm: pH (in water and KCl, 1:2.5 solution), exchangeable bases extracted with BaCl2 - TEA and ascertained using flame atomic absorption spectroscopy (FAAS), and total Table 1. General information about the pedons and composition (%) of the parent material, according to the lithology of the coarse fraction (BC, CB, or C horizons). Tree Pedon Altitude Aspect Slope Soil type (WRB) Main tree species cover (%) P2 1830 340° 12° Ortsteinic Podzol Pinus uncinata 80 P3 1795 5° 1° Epistagnic Podzol P. uncinata 50 P4 1975 350° 14° Haplic Podzol P. uncinata, Larix decidua 50 P6 2117 30° 10° Haplic Podzol P. uncinata, L. decidua 50 P7 1865 0° 12° Ortsteinic Podzol P. uncinata 70 P8 1685 80° 5° Spolic Technosol P. uncinata 60 Pedon Serpentinite Metagabbro Prasinite Amphibolite Chlorite schist Mine debris P2 20 70.0 7 2 1.0 0 P3 50 38.0 4 4 4.0 0 P4 90 0.5 1 3 5.5 0 P6 95 0.0 4 0 1.0 0 P7 48 37.0 3 3 6.0 0 P8 30 5.0 0 0 0.0 65 196 Northeastern Naturalist Vol. 16, Special Issue 5 exchangeable acidity. Cation exchange capacity (CEC) was calculated with sum of exchangeable bases and acidity. Total organic carbon (TOC) and N were analyzed by CN elemental analyzer (Thermo Electron, NC Soil), and particle size distributions were determined by the pipette method and sieving. Spodic properties were determined through oxalate extractable Fe and Al. Total elemental composition was determined by X-ray fluorimetry (XRF). “Available” Ni, Co, Cr, and Mn (Niav, Coav, Crav, Mnav) were extracted via ammonium acetate-EDTA (0.1M) (2.5 g of soil in 25 ml of solution, shaken for 30 min). Cr(VI) was measured using the diphenyl-carbazide method (Bartlett and James 1996) after 0.1M KH2PO4 extraction on field-humid samples. “Available” Cr(III) was extracted with 1M KCl. The operationally defined fractionation of metals among the compartments of the soil solid phase was investigated by selective sequential extraction (SSE) methods. The SSE was performed on one g of soil. Ni, Co, Cr, Mn, and Fe were determined with FAAS. After each extraction, the samples were washed with 10 ml demineralised water, centrifuged, and dried at 60 °C. A six-step scheme, modified from Tessier et al. (1979), was implemented (Table 2). Biological soil quality Soil quality is defined as the ability of soils to support healthy living communities and maintain biological productivity (Doran and Safley 1997). In particular, biological soil quality (BSQ) is related to the activity of soildwelling communities, their biodiversity, the organic matter content, and its turnover rate. Table 2. Reagents and methods used in the sequential extraction. MW = microwave. Extractant Volume Time Temperature Fraction extracted BaCl2 (1M) 20 ml 15 min. 25 °C Exchangeable. (Niex Coex, Crex, Mnex) NH2OH-HCl (0.1M) 20 ml 30 min. 25 °C Bound to Mn oxides and highly amorphous Fe oxi-hydroxides. (NiMn, CoMn, CrMn, MnMn) Oxalate (1M) 20 ml 4 hours 25 °C Bound to amorphous Fe oxi-hydroxides. (Nox, Cox, Crox, Mnox, Feox) H2O2 (30%) 20 ml 10 hours 65 °C Bound to organic matter. Ammonium-acetate (0.1M)* 20 ml 30 min. 25 °C (Niorg, Coorg, Crorg, Mnorg, Feorg) DCB** 40 ml 14 hours 25 °C Associated with crystalline pedogenic Fe oxides. (Nid, Cod, Crd, Mnd, Fed) Aqua regia 10 ml 90 min. MW oven Residual *H2O2 -solubilized metals are easily readsorbed on the residual cation exchange capacity of the soil (Filgueiras et al. 2002). After drying at 60 °C, the metals were re-dissolved with a solution of ammonium acetate. **DCB is Na-dithionite-citrate bicarbonate. 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 197 Soil microorganisms and microarthropodes are strictly associated with the soil matrix throughout their life cycles and are sensitive to its chemical properties. We chose microbial parameters (biomass, respiration, and stress indexes) and microarthropodal communities (BSQ index; Parisi 2001) as indicators of biological quality. Microbial properties change widely in space and time, and no threshold values for the identification of stress symptoms have been defined yet; the measured values must be compared in every study between soils with different properties (Nielsen and Winding 2002). Microbial indicators alone can give incongruous results. For example, basal respiration can be reduced or increased in the presence of metals (Giller et al. 1998). They are better interpreted when related to each other as stress indices. BSQ index. The BSQ index is based on the number of taxa of microarthropodal communities, the presence of sensitive genera, and the presence of taxonomic groups with soil-dwelling adaptations (Parisi 2001). The assumption is that the higher the soil quality, the greater the number of well-adapted microarthropod groups. Euedaphic biological forms (i.e., forms adapted to soil dwelling) are also known to be sensitive to metal toxicity (Fountain and Hopkin 2004). Microarthropodes were collected from fresh soil samples (500 g, three samples for each soil pit at a depth of 0–10 cm) using Berlese-Tullgren selectors. This method was based on soil fauna vertical tropism in relation to light and humidity (desiccation, due to an incandescence lamp lying overhead, pushes the soil organisms to escape to deeper horizons, thus falling into an underlying box, where they are fixed by a preservative solution [2 parts 75% ethanol and one part glycerol]). This extraction continued for seven days. After the extraction, the soil microarthropodes were observed with a binocular microscope (40X) and classed. Each taxon belonged to a biological form (BF), or ecotype. BFs are taxonomic groups (usually at the genera level) with similar body shapes, showing adaptive convergence to different degrees of soil dwelling (i.e., reduction or absence of visual apparatus, loss of pigmentation, reduced appendices, loss or reduction of flying organs). The BFs were classified according to an eco-morphological index (EMI; Parisi 2001), which gave scores to the different forms. Highest scores were given to the best-adapted, or euedaphic, forms. The BSQ score of the sample was the sum of its EMI scores. This score was then simplified according to an index (Parisi 2001), which produced the final seven quality classes (0–6) according to the complexity of soil arthropodal populations and soil dwelling adaptations. Microbial activity and stress indicators. Microbial biomass (Cmic) and base respiration (Resp) were used as the main indicators of microbial activity. Two samples (500 g) were collected for every pedogenetic horizon (A, AE, E, Bs). Microbial properties were analyzed on fresh, field-moist, mixed samples. Biomass was analyzed with the chloroform fumigation-extraction method (Vance 1987). Base respiration was measured according to the 198 Northeastern Naturalist Vol. 16, Special Issue 5 alkaline fixation method (Farini and Gigliotti 1989): the sample (equivalent to 20 g dry weight) was placed in a hermetically sealed box and stored at 25 °C for three days, the CO2 produced by the soil microorganisms reacted with 10 ml of 0.5 N NaOH solution, the reaction was stopped by adding 10 ml of 1M BaCl2, and the solution was titrated with 0.5 N HCl in order to quantify the amount of CO2 reacted. Cmic was measured after CHCl3 fumigation (two days) and subsequent dissolution in K2SO4 (0.5 M, soil:solution ratio of 1:5) with the chemical oxygen demand method (COD; IRSA 1981). Labile carbon (Clab), which was the readily available substrate for microbial growth, was extracted with a weak solubilizer (K2SO4; Hofman et al. 2002) on a non-fumigated sample treated as above. The results of the previous analysis were used for the calculation of stress indices, such as the specific respiration rate (Resp/ Cmic, also called metabolic quotient or qCO2), the Cmic/TOC ratio and the Clab/Cmic ratio. Resp/ Cmic (expressed in μg C-CO2 d-1mg-1C-biomass) showed the presence of stress factors that increased the metabolic activity of the living microbial community without a growth of the dimension of the population. The qCO2 is known to be a useful indicator of metal stress in soils (Giller et al. 1998); Cmic/TOC and Clab/Cmic showed the existence of limiting factors (stress factors) to microbial growth in the presence of ample and immediately available growing substrate. Statistical analyses A correlation analysis and a principal component analysis (PCA) were performed on the log-transformed soil properties (pH was not transformed) and lithology of the parent material in order to recognize the main factors involved in the variability of the edaphic properties. The relationships with biological activity data were interpreted through correlation analysis and PCA. Some soil properties were excluded due to their high inter-correlation. The program R 3.0 for Windows was used for all statistical analyses. Results and Discussion Soil chemistry Profiles P2, P3, P4, P6, and P7 were characterized by the podzolization process (D’Amico et al. 2008). The P8 profile was less developed and had an important presence of technogenic materials (up to 65% of mine debris) in the top horizons. The content in available metals (Tables 3, 4, 5, 6) in the BC horizon of P8, which had no mining materials in the skeletal fraction, was much higher than in other similar horizons in the study area. This finding could be due to illuvial accumulation related to an incipient podzolization process. Ecologically meaningful pH, exchangeable cations, and organic C and N varied widely between the six soils (Table 7). For example, serpentinite soils typically have a Ca/Mg ratio lower than one. Profiles P3, P4, and P6 fit this serpentinite characteristic, but the P8 profile had a Ca/Mg ratio well 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 199 above one. This ratio was highest in some surface, organic matter-rich A and AE horizons, since Ca is readily utilized by plants and Mg is easily leached away from the profile (Rabenhorst et al. 1982). Heavy metals Generalities. The total content and the mobile and pedogenic forms of metals were most concentrated in the organic matter-rich A and AE Table 3. Chemical speciation, or extractability, of chromium in the six soils. Cr(III) and Crex always occur below the detection limit (bdl). CrMn Crox Crorg Crd Cr tot Crav Cr(VI) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) P2E 0.02 2.90 15.31 Bdl 900 1.94 2.21 P2Bs 0.27 31.10 13.33 4.12 1000 2.24 1.52 P3A 0.00 12.06 38.31 3.54 1900 0.95 2.12 P3E 0.06 11.40 24.04 3.56 1600 1.29 1.94 P3Bsg 0.00 17.46 16.44 4.75 1800 0.55 9.98 P4A 0.02 14.10 36.31 10.08 2800 2.25 3.28 P4E 0.14 20.89 38.12 3.28 2500 3.14 3.57 P4Bs1 0.29 47.54 46.92 22.79 2700 4.09 12.08 P6AE 0.02 11.18 38.68 4.05 1401 1.74 3.46 P6E 0.17 10.19 29.72 2.99 1400 2.24 1.12 P6Bs 0.23 35.60 32.47 11.19 1901 2.85 2.83 P7A 0.53 14.33 19.61 2.52 868 2.83 1.49 P7E 0.33 6.49 16.55 3.99 660 1.84 0.18 P7Bs 0.52 23.60 21.83 6.60 660 2.62 1.76 P8Ah1 0.08 13.00 62.92 11.94 12000 1.93 1.07 P8Ah2 0.81 29.61 36.40 7.63 4020 1.30 1.73 P8BC 0.15 16.12 26.21 7.30 1900 0.60 Bdl Table 4. Chemical speciation, or extractability, of nickel in the six soils. Niex is omitted (<0.01 mg kg-1). NiMn Niox Niorg Nid Nitot Niav (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) P2E 6.32 6.89 9.95 6.46 347 1.09 P2Bs 0.95 8.00 11.05 14.40 395 2.14 P3A 49.71 22.40 38.31 10.62 526 28.02 P3E 2.88 6.15 24.81 15.64 590 4.75 P3Bsg 8.70 14.70 28.05 26.51 597 10.44 P4A 37.49 13.74 26.50 117.75 1071 21.03 P4E 8.99 2.35 15.44 28.69 612 8.26 P4Bs1 5.08 10.17 17.79 56.15 838 7.13 P6AE 25.28 6.79 15.47 6.48 342 17.49 P6E 8.82 2.30 12.08 4.48 344 10.00 P6Bs 9.93 6.88 18.34 0.80 597 9.19 P7A 0.00 1.57 10.78 0.42 845 2.59 P7E 0.00 0.00 9.54 0.00 901 1.04 P7Bs 0.00 15.08 23.02 7.02 835 4.99 P8A1 231.08 668.83 79.12 293.13 6535 100.51 P8A2 86.75 155.31 27.70 110.13 996 30.40 P8BC 40.53 35.34 43.88 25.56 790 20.28 200 Northeastern Naturalist Vol. 16, Special Issue 5 horizons, and, to a lesser extent, in the spodic B horizon. This finding was due to the stabilizing effect of organic molecules. The high correlation values between these forms and TOC (Table 8) point to the same effect. The value of the metalMn+ox+d+org+ex/metaltot ratio was lowest in the E horizons because the pedogenic fraction is easily removed by the leaching related to the podzolization process. All the pedogenic forms of Ni, Co, and Mn were significantly correlated, both among forms and among elements (Table 8), in agreement with the Table 5. Chemical speciation, or extractability, of cobalt in the six soils. Coex CoMn Coox Coorg Cod Cotot Coav (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) P2E 0.00 0.00 0.48 0.48 0.4 26 0.10 P2Bs 0.01 0.01 0.10 1.05 5.76 33 0.05 P3A 0.02 0.43 0.69 5.30 10.18 27 3.40 P3E 0.00 0.00 0.10 3.37 3.56 29 0.05 P3Bsg 0.05 0.05 0.10 3.00 11.08 35 0.25 P4A 0.11 1.05 0.49 3.43 11.00 37 10.79 P4E 0.05 0.16 0.10 3.13 4.51 34 1.64 P4Bs1 0.06 0.20 0.00 4.01 8.95 38 1.10 P6AE 0.03 0.28 0.19 4.06 4.46 30 2.39 P6E 0.01 0.07 0.00 2.68 4.48 30 0.68 P6Bs 0.02 0.04 0.00 3.15 6.79 36 0.35 P7A 0.01 0.03 0.00 6.86 7.97 36 0.20 P7E 0.00 0.01 0.00 5.06 6.79 43 0.05 P7Bs 0.01 0.03 0.00 5.75 7.43 64 0.15 P8A1 0.03 1.53 6.39 4.19 16.12 191 19.05 P8A2 0.03 0.67 3.84 6.07 6.67 87 0.40 P8BC 0.01 1.45 1.75 6.21 7.30 80 4.30 Table 6. Chemical speciation, or extractability, of manganese in the six soils. MnMn Mnox Mnorg Mnd Mntot Mnav (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) P2E 3.44 5.74 0.57 6.87 468 0.70 P2Bs 2.86 2.29 0.76 9.88 487 0.80 P3A 38.51 16.90 6.88 22.57 552 11.49 P3E 2.12 3.46 0.00 4.98 572 0.35 P3Bsg 4.26 7.74 0.97 17.01 595 1.04 P4A 243.77 53.39 3.53 25.66 1459 22.68 P4E 26.98 22.29 0.00 11.48 771 11.20 P4Bs1 32.26 19.94 2.35 48.42 938 7.63 P6AE 36.79 17.36 3.96 21.88 757 12.06 P6E 6.33 2.30 0.00 16.43 730 3.56 P6Bs 4.20 0.38 0.57 15.98 830 1.90 P7A 1.37 0.78 0.00 7.56 521 0.25 P7E 2.53 0.00 0.00 4.79 507 0.94 P7Bs 1.79 1.19 0.00 12.38 572 0.44 P8A1 358.63 177.69 22.69 155.22 3257 39.61 P8A2 125.78 118.50 10.92 49.11 1371 4.04 P8BC 272.82 186.80 6.80 34.08 1482 23.03 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 201 Table 7. Selected chemical properties of the six soils. Pedon Depth pH pH TOC N Ca Mg Na K Exch. acidity (cm) Horizon H2O KCl (g kg-1) (g kg-1) (Cmol kg-1) (Cmol kg-1) (Cmol kg-1) (Cmol kg-1) (Ca/Mg) (Cmol kg-1) P2 0–5 OF-OH 3.7 2.9 191 4.36 1.33 0.48 0.37 3.28 - 5–15 E 4.4 3.7 14 0.8 0.58 0.36 0.41 0.11 1.61 9.07 15–27 Bs 4.7 4.2 52 1.2 0.62 0.38 0.43 0.11 1.63 14.99 27–34 Bsm 4.7 4.3 17 0.42 0.24 0.41 0.20 1.75 19.38 34–42+ Cm 4.8 4.5 8 0.41 0.29 0.45 0.16 1.41 8.11 P3 0–10 Ah 4.8 4.2 70 2.2 1.30 1.44 0.50 0.14 0.90 6.87 10–20 Eg 5.0 4.2 7 0.6 0.46 0.65 0.37 0.16 0.71 5.61 20–31 Bsg 5.4 4.4 9 0.5 0.68 0.92 0.40 0.09 0.74 5.94 31–50+ Cg 5.3 4.7 4 0.58 0.48 0.49 0.15 1.21 4.06 P4 4–13 A 4.9 4.0 23 2.4 1.18 1.91 0.32 0.19 0.62 11.88 13–27 E 4.6 3.5 23 0.8 1.43 2.21 0.38 0.10 0.65 3.75 27–37 Bs1 4.9 3.9 9 0.8 1.19 1.86 0.40 0.10 0.64 9.70 27–53 Bs2 5.3 4.2 4 0.89 1.25 0.41 0.10 0.71 6.56 P6 3–10 AE 4.7 3.4 31 1.1 1.24 1.85 0.43 0.35 0.67 9.06 10–22 E 4.7 3.3 17 0.8 0.66 1.15 0.39 0.15 0.57 9.07 22–42 Bs 4.8 3.7 11 0.8 0.18 1.36 0.44 0.10 0.13 9.05 42–51 Bshg 4.8 3.8 18 1.26 1.54 0.01 0.03 0.82 10.92 P7 3–10 AE1-AE2 3.9 3.9 13 0.8 1.20 0.74 0.64 0.29 1.62 2.50 10–24 E 3.8 4.0 7 0.5 1.34 0.54 0.75 0.29 2.48 2.19 24–48 Bs 4.7 4.7 5 0.5 1.01 0.74 0.62 0.11 1.36 1.87 48–70+ Cm 5.2 5.0 P8 1–10 Ah1 5.4 5.3 51 3.0 5.63 1.54 0.57 0.22 3.50 3.30 10–21 Ah2 5.5 5.5 50 3.0 5.63 1.54 0.57 0.22 3.60 3.44 21–42 BC 5.6 5.5 8.1 1.0 3.24 1.54 0.68 0.12 2.10 1.87 202 Northeastern Naturalist Vol. 16, Special Issue 5 high geochemical affinity between these metals (Gasser et al. 1994, Jarvis 1984). The correlations with the respective Fe fractions were slightly lower. The correlation between these metals and Cr was low: only Crorg was related with all the forms of Ni, Co, and Mn because of the affinity between organic matter and all metals. Niav, Coav, and Mnav were strictly correlated with each other. Their highest correlations were with the respective Mn-associated form, followed by the oxalate-extractable fraction, as shown for Mn by Gambrell (1996). Organic forms and the ones associated with crystalline Fe oxides are less related with the so-called “bioavailable” fraction. Iron. Mn oxide-associated Fe usually decreased with depth (Table 9), while all the other forms showed a strong podzolic depth trend, with lowest values in the bleached E horizons and higher ones in TOC-rich A and AE and in spodic B horizons. Pedon P8 did not show a similar trend, but the values in the non-polluted BC horizon, which were much higher than in other similar horizons in the study area, suggested illuviation from the upper horizons. Chromium. Total Cr was highest on serpentinite (Table 3), but in soils formed from mainly mafic materials (P2, P7), Cr was much higher than usual (on mafic rocks it is usually around 200–300 mg kg-1 [Brooks 1987]), maybe because of the presence of Cr-rich chlorite in the parent material (D’Amico et al. 2008). In all the pedons, except P8, total Cr was lowest in the E and highest in the A, AE, and B horizons. This finding suggested unusual mobility of this element. In fact, chromium is usually residually accumulated in the most weathered pedogenic horizons in subalpine and alpine soils because of the high stability of Cr-bearing spinels (chromite and magnetite; Bulmer and Lavkulich 1994, Roberts 1980). Contrarily, Gasser et al. (1994) reported an increasing trend with depth due to aeolian inputs of Cr-poor, felsic minerals on the soil surface, that are here unlikely from P2 through P7 (D’Amico et al. 2008). The high Cr mobility is probably related to its high concentration in easily weatherable chlorites, and to subalpine climate and vegetation. The low available Cr(III) values verified its low mobility. This value should increase at pH values below 3.5 (Oze et al. 2004), or below 4.5 (Cooper 2002). Crorg, Crd, and Crox values were significantly correlated with the respective Fe values, showing both the association of Cr with the respective Fe pedogenic oxi-hydroxides and a similar susceptibility to the cheluviation and illuviation processes of podzolization. The extremely low CrMn values show the impossibility of Cr association with Mn: Cr is easily oxidized and solubilised in presence of Mn oxides (Fendorf 1995). The toxic Cr(VI) was high in some B horizons, despite the low pH and the high organic matter content. This form should be readily reduced to Cr(III) in acidic conditions in the presence of Fe2+ and organic matter (Fendorf 1995), but the high levels of Mn oxidize Cr to the toxic forms. Temporary waterlogging at snowmelt, causing moderate reducing conditions, followed by drying, can favour the temporary reduction of Mn oxides and the simultaneous oxidation of Cr(III). During drying, Mn oxides can form again (Cooper 2002). 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 203 Table 8. Correlation analysis between the main chemical properties and metal speciation of the six soils. Significant results (P-value < 0.05) are above 0.49. Cr(VI), Crav, Cro, Crmn, Crd, Coorg, Mgex, and drainage are omitted, as they are never significantly correlated with any other. Mnav Mnmn Mnox Mnorg Mnd Coav Comn Coox Cod Nimn Nio Niorg Nid Niav pH CO Caex Mnav 1.00 Mnmn 0.92 1.00 Mnox 0.79 0.91 1.00 Mnorg 0.80 0.81 0.82 1.00 Mnd 0.80 0.77 0.74 0.94 1.00 Coav 0.94 0.88 0.66 0.81 0.84 1.00 Comn 0.91 0.99 0.93 0.81 0.73 0.82 1.00 Coox 0.70 0.78 0.84 0.96 0.90 0.73 0.76 1.00 Cod 0.64 0.61 0.47 0.65 0.70 0.72 0.59 0.54 1.00 Nimn 0.80 0.80 0.77 0.98 0.95 0.86 0.77 0.96 0.66 1.00 Nio 0.73 0.71 0.68 0.92 0.96 0.82 0.65 0.93 0.64 0.97 1.00 Niorg 0.83 0.82 0.80 0.90 0.86 0.83 0.82 0.84 0.74 0.89 0.85 1.00 Nid 0.79 0.82 0.71 0.89 0.93 0.89 0.75 0.89 0.70 0.93 0.92 0.81 1.00 Niav 0.84 0.80 0.74 0.97 0.95 0.89 0.77 0.92 0.71 0.99 0.96 0.92 0.92 1.00 Crorg 0.75 0.60 0.51 0.73 0.79 0.71 0.59 0.62 0.55 0.73 0.67 0.65 0.73 0.77 pH 0.44 0.66 0.81 0.68 0.56 0.40 0.68 0.73 0.48 0.61 0.56 0.72 0.55 0.56 1.00 CO 0.52 0.50 0.51 0.82 0.70 0.59 0.50 0.78 0.52 0.81 0.75 0.64 0.68 0.78 0.50 1.00 Ca.ex 0.62 0.74 0.87 0.88 0.78 0.58 0.75 0.93 0.48 0.84 0.77 0.71 0.77 0.78 0.78 0.71 1.00 Ca/Mg 0.42 0.58 0.72 0.74 0.64 0.43 0.57 0.82 0.42 0.71 0.69 0.58 0.63 0.63 0.77 0.62 0.91 204 Northeastern Naturalist Vol. 16, Special Issue 5 Nickel. The absolute values of Ni (Table 4) on serpentinite were similar to the normal contents in acidified “serpentine soils” formed in temperate climatic conditions (Chardot et al. 2007, Hseu 2006). “Available” nickel (Niav) was not as high as in many temperate or boreal, well-developed soils formed on ultramafic materials (Lombini et al. 1998, Slingsby and Brown 1977); weathering released Ni from parent minerals, but high acidity and temporary reductive conditions at snowmelt probably caused leaching of Niav away from the soil profile. Smaller concentrations of Niav were found in poorly developed soils (Roberts 1980, Sanchez-Marañon et al. 1999). The Niav depth trend mainly followed TOC, a trend contrary to that of pH. This result is different from what was reported by Gasser et al. (1994) for subalpine soils on serpentinite in the Swiss Alps, where Niav values increased with depth because of higher pH causing the adsorption of metals onto phyllosilicates. Worldwide, Niav is negatively correlated with pH (Echevarria et al. 2006), but in the six soils assayed no significant relationship between these two properties was found. The Niav toxicity level (around 6 mg kg-1; Gasser et al. 1994, Proctor and Woodell 1975) was always reached in the investigated soils formed from a parent material containing more than 60–70% serpentinite (P3, P4, P6) and in the polluted P8 soil. Cobalt. Total Co was similar on all the lithologies. It was quite low in comparison to other ultramafic soils (Roberts 1980). This finding may be due to the high leaching related with extreme acidity and podzolization. The Coav values were nevertheless highest on ultramafic parent materials (Table 5). Cod and Coox (Table 5) were in the same order of magnitude as in the subalpine soils shown by Bulmer and Lavkulich (1994). The high weatherability of Co-bearing minerals was shown by the particularly high fraction of Co associated with pedogenic materials (CoMn+ox+org+d/Cotot). Weatherability of Table 9. Chemical speciation, or extractability, of Iron. FeMn is hydroxylamine-extractable Fe. FeMn Feox Feorg Fed (mg kg-1) (mg kg-1) (mg kg-1) (mg kg-1) P2E 355 5217 4027 10,291 P2Bs 260 10,175 4611 42,202 P3A 458 9273 10,774 37,699 P3E 95 2638 4233 24,224 P3Bsg 167 8650 9310 42,758 P4A 568 8334 8418 45,549 P4E 272 6029 4506 30,381 P4Bs1 200 8090 9587 46,734 P6AE 342 6581 7119 33,998 P6E 231 3931 4777 24,183 P6Bs 169 7782 6989 42,481 P7A 436 6933 6100 32,298 P7E 125 2251 2806 26,072 P7Bs 118 7694 3528 40,887 P8A1 203 10,490 18,932 58,973 P8A2 785 11,895 7537 54,665 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 205 primary minerals and leaching to the B horizons suggest high potential bioavailability of this metal in subalpine environments. Manganese. Total Mn depended on serpentinite content in the parent material. It was highest in the polluted soil P8 (Table 6). The detected values were normal for serpentinite soils (Oze et al. 2004). Mnd and Mno usually accumulated in the B horizons (due to the podzolization process). This accumulation was not the case with P6, where temporary waterlogging probably caused leaching (Table 6). These values were lower than in other subalpine, less acidic soils (Bulmer and Lavkulich 1994) because of the podzolization process, high acidity, and seasonal waterlogging, which favored strong leaching. The high values found in the BC horizon of P8 are presumably due to illuviation. Biological soil quality The highest BSQ value was two (Table 10), which corresponded to a low absolute value. Considering the high acidity and the low nutrient content of these soils, the poor biological activity of the mor or moder humus forms, and the harsh climatic condition, this value is actually relatively quite high. It is related to the profiles with the lowest metal content (P2, P7). Table 10. Microarthopodal forms and BSQ values in the samples of the six pedons. Avg. P is the average value for the profile. COr = Collembola Orchesella, CF = Collembola Folsomia, CN = Collembola Neanura, CE = Collembola Euedaphic (others), COn = Collembola Onichiurides. Total QBS COr CF CN CE COn Protura Araneida score index P2a 1 2 39.0 1.0 P2b 4 10 58.0 1.0 P2c 2 12 16 1 70.0 4.0 Avg. P2 56.7 2.0 P3a 1 1 40.0 1.0 P3b 1 4 2 48.0 1.0 P3c 2 6 6 2 1 51.0 3.0 Avg. P3 46.3 1.6 P4a 7 8 38.0 1.0 P4b 1 25 30 10 40.0 1.0 P4c 9 36.0 1.0 Avg. P4 38.0 1.0 P6a 2 14 15 1 41.0 1.0 P6b 2 17 6 38.0 1.0 P6c 12 4 1 51.0 1.0 Avg. P6 1.0 P7a 2 1 56.0 4.0 P7b 2 15 46.0 1.0 P7c 2 1 38.0 1.0 Avg. P7 2.0 P8a 20.0 0.0 P8b 1 8 1 35.0 1.0 P8c 7 36.0 1.0 Avg. P8 0.6 206 Northeastern Naturalist Vol. 16, Special Issue 5 The BSQ value was slightly lower (1.6) in the A horizon of P3, where the parent material had a higher serpentinite content and the available metals were higher (particularly Co and Mn). Here, TOC and N were much higher, and the pH value was slightly higher, thus representing a better habitat for soil dwelling fauna. High metal concentrations can cause stress, which reduces the diversity and the adaptation of micro-arthropodal communities. Despite the higher organic matter content of these soils, high Mn, Co, and Ni contents seemed to be correlated with the low (1) BSQ index in serpentinite soils P4 and P6, and even more in the contaminated P8 (BSQ = 0.6). P8 also had the highest nutrient content and the highest pH value, but these positive properties were not able to counterbalance the extremely high available metals. In pedon P8, the arthropodal community was extremely poor, dominated by Acari (Table 10). The accumulation of organic matter on the top of this profile could be related with the low biological activity due to metal stress (as shown by Giller et al. 1998). Statistical analysis (PCA) supports the hypothesis of an existing link between low soil arthropodal diversity and available and total metal content (Fig. 1). The first factor of the PCA biplot explains the highest variance of the data (60.58%), and it is strongly related to metal content, Ca/Mg ratio, N, and BSQ. Ca/Mg and N are, however, of little interest because they were Figure 1. Biplot of the PCA analysis between the main edaphic factors and the BSQ. 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 207 highly intercorrelated with TOC and metals (P8 had the highest metal content, Ca/Mg, TOC, and N values). It was expected that biological activity and arthropodal diversity would be increased by these properties (N is a nutrient, and a high Ca/Mg value is optimal for plant life and for primary productivity of the habitat), but here the correlation was negative. “Available” Co, Mn, and Ni had a strongly significant negative correlation with BSQ. Other edaphic and environmental properties (exchangeable bases, Cr, TOC, drainage, altitude, aspect, stoniness) were less important. The “available” Crav and the toxic Cr(VI) were scarce in the organic-matter rich surface horizons, and were not related with the composition and the soil-dwelling adaptation of micro-arthropodal communities. Differences in vegetation did not contribute to the explanation of BSQ values. In the literature, there are many examples regarding all metals tested. For example, Co (Lock et al. 2004) and Mn (Phillips et al. 2002) toxicities were demonstrated for Collembola (Folsomia candida), and for animals in general (Lison et al. 2001). Kuperman et al. (2003) demonstrated growth inhibition for Collembola (Folsomia candida) and Oligochaeta due to Mn and Co. Ni toxicity was documented for a species of Folsomia (Scott Fordmands et al. 1999). Microbial properties Both Resp and Cmic followed more or less the TOC depth trend, but with significant differences among the different pedons (Table 11). Sometimes Cmic increased in B horizons, contrary to the results of Fritze et al. (2000). “Available” metals were not significantly related to Cmic or Resp (Table 12). The depth trend of stress indices is not easily interpretable. It seems that in most cases the B horizons had high Cmic/TOC values, but the qCO2 and Clab/ Cmic did not show clear trends. Cmic was correlated with Clab, expressing adaptation of microbial communities to edaphic factors (Table 12). The highest correlation values were between stress parameters and Mnav, Coav, and Niav (Table 12); Crav and Cr(VI) were not correlated with any microbial parameter. A graphical representation of the relationships between Coav and qCO2 and between Coav and Cmic/TOC is shown in Figure 2 for A and B horizons. In the A horizons, the R2 value was less significant than it was in the B horizons, probably because of the hidden effect of the different nutrient content: the positive effect of a high available substrate for growth (TOC, Clab, N) neutralized the stress caused by metals. In B horizons, the nutrient content did not significantly change between the six soils, and the metal stress was clearer. A similar trend was evident for Niav (data not shown). The statistical analyses confirmed the relationship between microbial activity and metals, even if the number of samples was too small to warrant statistical significance (Table 12). Omitting the polluted P8, the results were still confirmed. Some statistically significant results (P-value < 0.05) were found for qCO2 and Niav, Mnav, Coav, and for Cmic/TOC and the same “available” metals. The Clab/Cmic ratio was less strictly related with metals, but was significantly related with the other stress indices. The 208 Northeastern Naturalist Vol. 16, Special Issue 5 positive correlation between pH values and qCO2 was not significant from an ecological point of view because higher soil pH should have positive effects on microbial activity (Pennanen 2001). According to the PCA analysis (Fig. 3), nutrients are the best contributors to the second principal axis, which accounts for the 28.46% of the total variability. Resp and Cmic were similarly related with both axes. The first factor (which explained 57.20% of the total variability) was influenced by stress indices, available Co, Ni, Mn, and the autocorrelated pH and Ca/Mg. Conclusions In the six studied pedons, the total and “available” heavy metal content (Ni, Co, Mn, and Cr) were highly correlated with the serpentinite content in the parent material, as expected in soils on ophiolitic rocks (Brooks 1987). The chemical fractionation of the metals showed that weathering efficiently releases metal from the parent material, and the released forms are mobilized and redistributed in the profile by the podzolization process. This high mobility enhances bioavailability and toxicity: EDTA extractable (“bioavailable”) Ni, Table 11. Microbial characterization of the six soils. Results from the B horizon of profile P7 and from the A horizon of profile P3 are omitted because of analytical problems. TOC1 Clab 2 Resp3 Cmic 4 qCO2 5 Cmic/TOC Clab/Cmic BSQ P2 E 17.2 0.066 46.5 2356.52 10.1 0.137 0.028 2.0 BS1 21.1 0.083 36.0 2479.28 7.4 0.118 0.033 P3 A 70.0 nd nd nd nd nd nd 1.6 E 4.1 0.030 18.0 977.96 9.4 0.239 0.031 Bsg 4.6 0.031 13.5 768.40 9.0 0.167 0.041 P4 A 37.4 0.116 82.5 1390.44 29.7 0.037 0.083 1.0 E 16.8 0.062 24.0 1308.54 9.4 0.078 0.048 Bs1 6.7 0.067 30.0 922.68 16.7 0.138 0.073 P6 AE 26.1 0.087 87.0 2523.63 17.2 0.097 0.035 1.0 E 12.7 0.055 28.5 872.63 16.7 0.069 0.063 Bs 7.9 0.048 19.5 1532.36 6.5 0.194 0.032 P7 AE 13.2 0.061 43.5 760.08 29.3 0.058 0.080 2.0 E 7.0 0.070 46.5 833.82 27.8 0.119 0.084 P8 Ah1 49.8 0.060 67.5 880.39 38.3 0.018 0.068 0.6 Ah2 49.8 0.117 163.5 2705.47 30.2 0.054 0.043 0.6 BC 7.0 0.048 54.0 532.22 50.7 0.076 0.090 1 (g/kg). 2(g C-CO2/kg dry soil). 3(μg C-CO2/g dry soil). 4(μg Cmic/g dry soil). 5 (μg C-CO2 d-1mg-1Cmic). 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 209 Table 12. Correlations between microbial properties and the main soil parameters. Clab/ Crav Mnav Coav Niav Cr(VI) N TOC pH Resp Cmic qCO2 Clab Cmic/TOC Cmic Crav 1 -0.08 -0.05 -0.16 0.28 -0.11 -0.08 -0.54 -0.24 -0.01 -0.30 0.15 -0.07 0.11 Mnav 1 0.94 0.84 -0.14 0.64 0.57 0.49 0.27 -0.22 0.65 0.17 -0.61 0.45 Coav 1 0.89 -0.17 0.70 0.65 0.43 0.25 -0.20 0.54 0.19 -0.58 0.38 Niav 1 -0.18 0.75 0.72 0.60 0.38 -0.09 0.51 0.11 -0.53 0.19 Cr(VI) 1 -0.26 -0.33 -0.16 -0.30 -0.21 -0.33 -0.19 0.32 -0.03 N 1 0.97 0.49 0.81 0.34 0.46 0.68 -0.68 0.12 TOC 1 0.44 0.80 0.47 0.38 0.72 -0.69 0.02 pH 1 0.48 -0.09 0.69 0.08 -0.29 0.26 Resp 1 0.54 0.49 0.80 -0.56 0.08 Cmic 1 -0.33 0.63 -0.02 -0.64 qCO2 1 0.23 -0.6 0.78 Clab 1 -0.57 0.12 Cmic/TOC 1 -0.56 210 Northeastern Naturalist Vol. 16, Special Issue 5 Co, and Mn were significantly correlated with the redistribution of organic matter and iron oxides within the soil profiles, typical of podzolic soils. Toxic levels are reached on substrates dominated by serpentinite. Toxic Cr(VI) could have been a significant problem only in some B horizons. In the six pedons, the micro-arthropodal communities were affected by metals as shown by the BSQ index and its strong, negative correlation with Niav, Coav, and Mnav. Where available metals were higher, the communities were simplified and the number of soil-dwelling adapted forms decreased. Cr had no visible effects within the surface horizons, probably because of the low content of the toxic Cr(VI) due to the high TOC amount. In fact, organic matter and ferrous iron (Fe2+) are the main reducing factors for Cr(VI) in soils (Fendorf 1995). The proposed strong effects of natural loads of heavy metals on arthropods was supported: the simplified communities in metal-rich soils were probably the result of toxicity at the organism level (changes in life cycle and reproductive efficiency), at the population level, and at the community level (different reactions of different species to the same ecological factors) (Edwards 2002). However, it was not clear if all the considered metals had effects on microarthropodal ecology or just one or few of them; the metals are highly intercorrelated. Figure 2. Relationship between qCO2, Cmic/TOC, and available Co in A and B horizons. The correlation is particularly evident in the B horizons. 2009 M.E. D’Amico, F. Calabrese, A. Rossetti, and F. Previtali 211 Microbial activity indices (base respiration and biomass) were not clearly related to the soil metal contents because of the masking effect of nutrients; stress indices (qCO2, TOC/Cmic, Clab/Cmic) were related to “available” Ni, Co, Mn, pH values, Ca/Mg, TOC, and Clab. Metals were, however, the only significant parameters from an ecological point of view. In fact, TOC and Clab are nutrients, and high nutrient content should decrease the stress in the soil living communities. Higher pH values usually support microbial activity, while the Ca/Mg ratio is normally strongly related with the primary productivity of the habitat and higher values should have positive effects on the soil dwelling communities. These properties are positively related with enhanced stress symptoms in microbial communities because of the strong intercorrelation with all the forms of metals. Metal stress on microbial communities was evident in the additional amount of energy required by the microorganisms to support metal tolerance mechanisms. In turn, this energy could not be used for growth. Contrary to common belief (Schipper and Lee 2004), there are limits to the evolution of metal-resistant microbial strains. The thick organic layer on the top of the heavily polluted P8 could be related to the slow decomposition rate caused by low arthropodal and Figure 3. Biplot of the PCA analysis performed on the microbial MDS on available metals and on the main soil properties (B horizons). 212 Northeastern Naturalist Vol. 16, Special Issue 5 microbial activity, which was probably an effect of the extremely high metal content of anthropic input. Further studies on the different composition of the microbial communities in soils with different metal content could reveal more information regarding their adaptive mechanisms. 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