Soil Science Society of America Journal 64:138-143 (2000)
© 2000 Soil Science Society of America
DIVISION S-2-SOIL CHEMISTRY
Small-Scale Variability of Metal Concentrations in Soil Leachates
Wolfgang Wilckea
a Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
wolfgang.wilcke{at}uni-bayreuth.de
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ABSTRACT
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Soil tests often use composite soil samples to assess metal bioavailability. Composite soil samples cannot address small-scale soil heterogeneity. In this study, the concentrations of Al, Cd, Cu, Mn, Ni, Pb, Zn, dissolved organic C (DOC), and pH in soil leachates were examined as an index of small-scale soil heterogeneity. Ten undisturbed soil cores (0–4 cm, 100 cm3) from a 1-m2 area of a Lithic Haplumbrept (pH 5.2, 3 g CaCO3 kg-1) and a Typic Hapludoll (pH 4.3) under forest canopy were equilibrated with deionized water. The soil cores were then leached with a mock soil solution (pH 4.0, 6.8–11.4 mg L-1 DOC, 0.001 M CaCl2). In the Haplumbrept, the pH of the first 50-mL fraction of the leachates (deionized water extract) was 4.2 to 7.4, DOC concentrations were 11.4 to 38.9 mg L-1. Aluminum, Cd, Mn, and Ni concentrations were significantly correlated with pH (r = 0.88, 0.93, 0.69, 0.78, respectively; P < 0.05). In the Hapludoll, the pH (4.1–4.6) varied little in the first 50-mL fractions; Cr, Cu, and Pb concentrations were correlated with DOC concentrations (9.6–36.3 mg L-1). The variability in metal concentrations of the first 50-mL fractions (coefficients of variation, CV = 25–91%) was comparable in both soils and did not change with increasing leachate volume (mock soil solution) except for Zn in the Haplumbrept (CV up to 174%). In all leachate fractions, variability was markedly higher than those reported for salt extracts of composite soil samples (CV = 1–18%). Thus, the analysis of composite samples may be insufficient to address metal bioavailability in soils.
Abbreviations: CV, coefficient of variation • DOC, dissolved organic C • DOM, dissolved organic matter • ECEC, effective cation-exchange capacity • Fed, dithionite–citrate-extractable Fe • Ks, saturated water conductivity
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INTRODUCTION
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A NUMBER OF STUDIES have shown that soil chemical properties may vary over distances of a few millimeters (Hildebrand, 1991; Santos et al., 1997; Wilcke and Kaupenjohann, 1997). As the chemical composition of the soil solution is dependent on the chemical properties of the soil matrix, the small-scale variation in bulk soil properties probably leads to small-scale variability in soil solution composition. To evaluate the risk for phytotoxicity and leaching of Al and heavy metals, an assessment of metal concentrations in soil solution is crucial. However, conventional methods used to assess this risk in soils often are based on composite soil samples that are further homogenized by sieving to <2 mm (Herms and Brümmer, 1984; Shuman, 1985; Sims, 1986). Conventional methods for processing soil samples eliminate or minimize the small-scale spatial variability that is present in the in situ soil matrix (Hildebrand, 1991).
The most important factor controlling metal concentrations in soil solution is pH (Brümmer et al., 1986). When a carbonate-containing soil is homogenized, the measured soil pH will be influenced and possibly controlled by the presence of carbonate. As the carbonate will tend to increase the measured soil pH, toxic metals will be assumed to be retained by the soil solid phase (Herms and Brümmer, 1984). However, such a conclusion would be in error particularly in low carbonate-bearing soils where the presence of lime particles is interdispersed with areas of more acid soil without lime particles within the same soil horizon. In southern Germany such soils may frequently be found in landscapes built up by jurassic limestones and covered by pleistocene loess, for example, the Franconian and Swabian Alps, which cover 
20% of the German federal states of Baden-Württemberg and Bavaria (Bundesanstalt für Geowissenschaften und Rohstoffe, 1995).
Besides soil solution pH, the concentration of dissolved organic matter (DOM) in the soil solution will influence metal solubility (König et al., 1986; Guggenberger et al., 1994). The concentration of DOM in soil solution may also vary considerably over small distances in soils because of the uneven distribution of soil organic matter (Amelung and Zech, 1996). Small-scale variability in soil pH will also increase the variation in the concentration of DOM (Kaiser et al., 1996).
The objective of this study was to assess the importance of small-scale soil variability when evaluating metal bioavailability in soils. The variation in soil solution metal concentrations, pH, and DOC obtained from multiple soil cores removed from a relatively small area (1 m2) was used as an index of small-scale variability. The soil solution extracts were obtained using the percolation technique of Hantschel et al. (1988), which has been shown to yield extracts that approximate the chemical composition of the natural soil solution. The DOC concentrations are used as an estimate of DOM. The study soils were selected as representative of the acid German forest soils and with the intention to approximate the expected end members of possible soil solution chemical variability.
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Materials and methods
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Study Sites and Soils
Soil cores were obtained from the A horizons of a fine-silty, calcareous, Lithic Haplumbrept and a fine-silty, serpentinitic, Typic Hapludoll (Soil Survey Staff, 1997). The calcareous soil is located near Nördlingen in the northern Swabian Alb (474 m above sea level), W-Bavaria, Germany. The parent material is limestone covered by loess. The serpentinitic soil is located near Zell (660 m above sea level), Fichtelgebirge, NE-Bavaria. Both sites are forested [100-yr-old Norway Spruce, Picea abies (L.) Karst.]. The organic horizons were removed from an area of 1 m2, and 20 undisturbed soil cores per site were taken from the A horizon in a random fashion using stainless-steel rings (height 4 cm, inner diameter 5.64 cm, volume 100 cm3). Ten soil cores were used to extract soil leachates, the remaining 10 were used to determine the saturated water conductivity. In addition, a representative bulk soil sample was taken from each of the A horizons from an adjacent soil pit. The bulk samples were used for the general characterization of the soil horizon (Table 1)
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Physical Properties
Soil texture was determined by the pipette method (Schlichting et al., 1995) after removal of oxides by dithionite-citrate extraction (Mehra and Jackson, 1960) and destruction of organic matter with hydrogen peroxide (H2O2). To determine the saturated water conductivity (Ks) a second stainless-steel ring was placed on each of 10 rings per horizon containing the undisturbed soil sample. The Ks was calculated from the height of the sample (
), the time needed for the percolation (T) of 100 mL of deionized water, and the height of the water column at the beginning (
) and at time T (0 < H1 < 4 cm) according to Eq. [1]:
 | (1) |
Density and pore volume were determined gravimetrically using saturated and oven-dry (105°C) soil cores.
Soil Leachates
The undisturbed soil cores were saturated with deionized water and allowed to equilibrate for 24 h at 25°C. The saturated soil cores were placed on a filter paper prerinsed with deionized water and inserted at the bottom of the stainless-steel cylinders of the percolation apparatus of Hantschel et al. (1988). Approximately 200 mL of a mock soil solution were filled into the cylinders at the top of the soil cores. A piston was then lowered, forcing the mock soil solution through the soil cores at a constant rate of 340 mm d-1 for the Nördlingen soil and of 400 mm d-1 for the Zell soil, which corresponds to the respective mean saturated water conductivity (Table 1). Consequently, the deionized water was displaced by the mock soil solution and collected as the first 50-mL fraction. Furthermore, a second 50-mL and a third 100-mL fraction were collected. The volume steps of 50 mL were chosen because they approximately correspond with one pore volume of the soil cores.
The mock soil solution was prepared from composite Oe/Oa material of a mixed beech (Fagus sylvatica L.) and Norway spruce stand on an acid Inceptisol by leaching a glass column filled with 200 g of field-fresh organic horizon material. The extract was adjusted to pH 4 and CaCl2 was added to give a final 0.001 M salt solution. Mock soil solutions were prepared separately for each soil prior to the extraction and contained differing DOC and metal concentrations (Table 2)
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Dissolved organic C concentrations (filtered <45 µm) were determined with a dissolved C analyzer (Dohrmann DC 90, Dohrmann, Santa Clara, CA). Aluminum, Ca, Cd, Cr, Cu, K, Na, Mg, Mn, Ni, Pb, and Zn concentrations were measured by atomic absorption spectroscopy using both graphite tube and flame techniques (SpectrAA 400 Z, SpectrAA 10, or SpectrAA 400, Varian, Mulgrave, Australia).
Bulk Soil Properties
The bulk soil samples (<2 mm) from each horizon were characterized by conventional procedures. Effective cation-exchange capacity (ECEC) was determined with unbuffered 1 M NH4OAc (Page et al., 1982). Carbonate was determined using the Scheibler-apparatus (soil carbonates were dissolved with 3 M HCl and the volume of released CO2 measured). Crystalline Fe oxides were extracted with dithionite–citrate (Fed, Mehra and Jackson, 1960). Total C in the soil samples was determined with a CHNS-Analyzer (Elementar Vario EL, Elementar Analysensysteme GmbH, Hanau, Germany) .
The sequential extraction procedure of Zeien and Brümmer (1989) was used to characterize the metal concentrations of each soil horizon. This procedure results in the following fractions: readily soluble and exchangeable metals extracted by unbuffered 1 M NH4NO3 ("exchangeable"), specifically adsorbed and other weakly bound metals (1 M NH4OAc, pH 6.0, "sorbed"), metals bound to Mn oxides (0.1 M NH2OH x HCl + 1 M NH4OAc, pH 6.0, "Mn oxides"), metals bound to organic matter (0.025 M NH4EDTA, pH 4.6, "organic matter"), metals incorporated in amorphous and poorly crystalline Fe oxides (0.2 M NH4Oxalate, pH 3.25, "amorphous Fe oxides"), metals incorporated in crystalline Fe oxides (0.1 M ascorbic acid in 0.2 M NH4Oxalate, pH 3.25, at 95°C, "crystalline Fe oxides"), and residual metals (4 parts concentrated HNO3 and 1 part concentrated HClO4, "residual")
Statistical Analysis
Correlations and regressions were calculated using STATISTICA for Windows (StatSoft of Europe, Hamburg, Germany).
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Results and discussion
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Metal concentrations in both soil horizons are at or below background concentrations for most soils in central Europe, except for Cr and Ni in the Zell soil as determined by comparison of the data in Table 3
with the so-called "S" values established for soil remediation in the Netherlands. The S value is considered to represent the upper limit of background metal concentrations in central European soils (Rosenkranz et al., 1995; Table 3). The serpentinite-derived Zell soil is high in Cr and Ni because of the presence of serpentine minerals (Dixon, 1989).
Metal concentrations in soil solution are a function of the total metal concentration in a soil and the partitioning of metal ions between the various soil solids. Metal partitioning among the seven fractions of the sequential extraction scheme used in this study is element-specific and similar to that reported by Zeien and Brümmer (1989) and Wilcke and Kaupenjohann (1997). This supports the assumption that the soils used in this study can be considered representative for soils in central Europe. The residual fraction is quantitatively the most important fraction for Al, Cr, and Zn in both soils and for Cu and Ni in the Zell soil. In contrast, more than 50% of the extracted Cd is present in the first three fractions, illustrating its potentially high mobility in soils (Herms and Brümmer, 1984). Copper and particularly Pb have high affinities for organic matter in soil (König et al., 1986), which is reflected by high percentages of these elements found in the EDTA-extractable fraction (organic matter). Manganese is found primarily associated with the easily reducible phase (Mn oxides) and in poorly crystalline Fe oxides (amorphous Fe oxides). Because of its lower pH, the Zell soil has higher percentages of exchangeable metals, except for Mn, than the Nördlingen soil.
The first 50 mL of the soil leachate is meant to approximate the equilibrium soil solution of the undisturbed soil core, although some mixing with the added mock soil solution may not be completely ruled out. In this first soil leachate fraction, metal concentrations in solution are assumed to be controlled by desorption, dissolution of salts, and dissolution of organic metal complexes. The pH varies between 4.2 and 7.4 in the leachates of the 10 soil cores of the Nördlingen soil and between 4.1 and 4.6 in those of the Zell soil (Fig. 1)
. The range in observed pH values for the leachates from the Nördlingen soil reflects the presence of differing solid surfaces acting to buffer the soil solution. Using the scheme proposed by Ulrich (1981), the pH range of 6.2 to 7.4 is controlled by the buffering of carbonates and that of 4.2 to 5.0 by the buffering of Al oxides. The remaining pH values can be assumed to be in equilibrium with buffering by silicates and exchangeable cations. Thus, the most important buffer systems of temperate soils as defined by Ulrich (1981) are active in close proximity to each other within the same soil horizon. In contrast, the pH in the extracts of the Zell soil reflect buffering by Al oxides in all 10 soil cores.
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Fig. 1 Solution pH in leachates with deionized water (first 50-mL fractions) from the 10 individual soil cores of the Nördlingen and Zell A horizons
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Dissolved organic C concentrations in the first 50 mL of soil leachates vary between 16.3 and 38.9 mg L-1 for the Nördlingen soil and between 8.3 and 31.6 mg L-1 for the Zell soil (Fig. 2) . Although the Zell soil is higher in soil organic C (Table 1), less DOC is released than from the Nördlingen soil. The lower solubility of organic substances in the Zell soil is due both to lower net negative charge of DOC at the given acid pH and increased DOC sorption by Fe oxides (Kaiser et al., 1996). The variability of the DOC concentrations in the extracts of the 10 soil cores is much higher in the Zell soil than in the Nördlingen soil, as reflected by the coefficients of variation (Table 4)
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Fig. 2 Mean metal and dissolved organic C (DOC) concentrations in the leachates with deionized water (first 50-mL fractions) of the Nördlingen and Zell A horizons (
). Error bars represent the total range of concentrations (i.e., minimum and maximum values)
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Table 4 Mean cumulative metal release and corresponding coefficient of variation of the 0–50, 50–100, 100–200 fractions (50 mL of deionized water and 150 mL of mock soil solution)
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The first 50 mL of the soil leachate of the Zell soil have higher metal concentrations than those of the Nördlingen soil, except for Mn and Zn, which can be attributed to the more pronounced acidity of the Zell soil. This is reflected by the higher metal release from the Zell than from the Nördlingen soil, being particularly true for the lithogenically high Cr and Ni (Table 4). The high variability of the pH in the leachates of the Nördlingen soil is closely related to the high variability in leachate metal concentrations. In the first 50-mL fractions of the leachate, significant correlations exist between pH and Al (
), Cd (
), Mn (
), and Ni (
, Fig. 3)
. In contrast, no significant correlation between metal concentrations and pH was found in the Zell soil. In the Zell soil, a relationship between DOC and Cr, Cu, and Pb (Fig. 4)
was observed. However, the dislocation of the data points did not allow for a valid correlation analysis.
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Fig. 3 Relationship between solution pH and Al, Cd, Ni, and Mn concentrations in the leachates with deionized water (first 50-mL fractions) of the Nördlingen A horizon
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Fig. 4 Relationship between dissolved organic C (DOC) and Cr, Cu, and Pb concentrations in the leachates with deionized water (first 50-mL fractions) of the Zell A horizon
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Extraction with a mock soil solution was used to examine the variability produced by the reaction of the soil to a change in ionic strength and an increase in the concentrations of DOC. Additionally, the leaching potential of the mock soil solution for metals was assessed. In the Nördlingen soil, the variability of Al, Cd, Cr, Cu, Mn, and Ni concentrations did not change between the three extract fractions. Coefficients of variation were 26 to 63, 26 to 65, and 20 to 67% in the first (0–50 mL of leachate, deionized water extract), second (50–100 mL, mock soil solution), and third fractions (100–200 mL, mock soil solution), respectively. The variability in Pb concentrations of the first 50-mL fractions was higher than that of the above metals
. However, Pb was not detected in all 10 first fractions and only in few of the second and third fractions of the leachate. The variability in Zn concentrations increased considerably (
). In the Zell soil, the range of CV values of the metal concentrations in the leachates of the 10 soil cores except for Pb and Zn was comparable to the range of CV values in those of the Nördlingen soil, and the range in CV values did not differ much between the three leachate fractions (25–63, 23–72, 23–71% in the first, second, and third fractions, respectively). The variability measured in the leachates is considerably higher than when metals are extracted with a salt solution using a batch technique. Zeien (1995) reported CVs between 2 and 18% for repeated extracts (
) of Cd, Cu, Mn, Pb, and Zn from three soil samples (pH range 4.7–7.7) with 1 M NH4NO3 and between 1 and 15% for repeated (
) extracts of the same soils with 1 M NH4OAc (pH 6.0) performed sequentially following the 1 M NH4NO3 extract.
The mean cumulative metal release curve steadily increases for most metals in both soils from Leachate Fraction 1 to 3, except for Pb and Zn in the leachates of the Nördlingen soil and of Cu in the leachates of the Zell soil (Table 4). The result illustrates that the extractable pool of most metals is still not exhausted after the displacement of 50 mL of deionized water and the leaching of 150 mL of mock soil solution together being equivalent to 80 mm of rainfall. In contrast, part of the Pb and Zn added with the mock soil solution is, on average, retained by the Nördlingen soil and part of the Cu added is retained by the Zell soil.
When the shape of the release curve is interpreted as a measure for metal mobility in the undisturbed soil cores, three metal groups may be distinguished. The first group consists of Cd, Mn, and Ni in both soils; Al and Cr in the Nördlingen soil; and Zn in the Zell soil, all showing linear release. The result indicates that there are no kinetic constraints for the release of these metals into the leachate. This is expected because, except for Cr (22%), <6% of the metals extractable with 1 M NH4NO3 from the bulk soil sample have been cumulatively released into the leachate.
The second group consists of Cu in the Nördlingen soil and of Al and Cr in the Zell soil, showing a decreasing release rate with increasing leachate volume. This is unexpected because only about 5, <1, and 6%, respectively, of the 1 M NH4NO3–extractable metals in bulk soil have been cumulatively released into the leachate. The result indicates that the extraction of these metals may be limited by kinetic constraints such as the diffusion rate from binding sites that are not in direct contact with the percolating extractant. Diffusion away from exchange sites would be expected to be less for threefold charged Al and Cr cations as observed in this study. Another explanation would be the presence of specific binding sites for these metals in the study soils, which may also explain why the third group of ions (Pb and Zn in the Nördlingen soil and Cu in the Zell soil) are sorbed from the mock soil solution. As Al, Cr, Pb, and Cu are known for their high affinity for organic matter, there may be specific organic binding sites present in the soils. The sorption to these sites may compensate for the release of metals from cation-exchange sites and thus reduce the net release into the leachates. In contrast, Cd, Mn, and Ni are known to have a low affinity for organic matter (König et al., 1986; Dietze and König, 1988; Zelazny and Jardine, 1989). However, the assumption of specific organic binding sites is doubtful for Zn, which is considered to have only a low affinity for organic matter (König et al., 1986).
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Conclusions
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The variability in extracted metal concentrations found in this study for undisturbed soil cores is about three times higher than that for repeated salt extractions of sieved bulk soil samples reported in the literature. The interpretation of salt-extracted metals with respect to bioavailability may be improved when the larger variability in undisturbed soil is considered. The assumption of low metal bioavailability in calcareous soils may not be valid when more acidified soil regions adjacent to still lime-containing ones exist within the same soil horizon. In such soils, metals are possibly transported through pores with strongly acidified surfaces serving as "channels" for enhanced heavy metal transport. Furthermore, plant roots growing along these acidified surfaces are exposed to elevated metal concentrations. As low-carbonaceous soils are common on the large jurassic limestone plateaus in southern Germany, the appropriate risk assessment should include extractions of undisturbed soil cores, particularly because many of these soils are shallow and occur over highly water-conductive rocks, allowing for a rapid transport of metals into groundwater.
Metal release into soil solution in undisturbed soils may be limited by kinetic constraints as shown by the cumulative release for select metal ions (Table 4). This may result in a retardation of metal transport in soils. Soil structure will be important in determining the extent to which metal retardation will occur. However, more research is necessary to elucidate the processes that affect metal transport in undisturbed soils.
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ACKNOWLEDGMENTS
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I thank Martin Kaupenjohann for his support.
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ABSTRACT
INTRODUCTION
