Soil Science Society of America Journal 66:1829-1835 (2002)
© 2002 Soil Science Society of America
DIVISION S-2—SOIL CHEMISTRY
Content, Distribution, and Solubility of Cadmium in Arable and Forest Soils
Martin K. Andersen*,a,
Anne Refsgaardb,
Karsten Raulund-Rasmussenb,
Bjarne W. Strobela and
Hans C. B. Hansena
a Chemistry Dep., The Royal Veterinary and Agricultural Univ., Thorvaldsensvej 40, DK-1871 Frederiksberg C
b Danish Forest and Landscape Research Institute, Hoersholm Kongevej 11, DK-2970 Hoersholm
* Corresponding author (man{at}kvl.dk)
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ABSTRACT
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Afforestation of former farmland decreases soil pH and thus increases the solubility of Cd in the soil, which may cause Cd leaching to streams and groundwater. The Cd concentration in soil and soil solution were determined in 11 pairs of Danish arable and forest soil profiles representing three different texture classes (sand, loamy sand, and sandy loam). The soil pH did not change or decrease with depth through the arable profiles, but did increase with depth in the forest profiles. Significantly higher Cd contents were found in the upper 30 cm of the arable soil compared with that of the forest soil. The total soil Cd concentrations correlated with the effective cation-exchange capacity (ECEC), clay content, and organic matter content, but not the soil pH. The soil solution pH was unchanged or decreasing downwards through the arable profiles, but increasing with depth in the forest profiles. The soil solution concentration of Cd was significantly higher in the forest soils than in the arable soils. The Cd concentration in the soil solution decreased as pH increased. Both total soil and soil solution Cd concentrations were higher in the sandy loam soils compared with the loamy sand and sand soils. It is concluded that afforestation may lead to higher soil solution concentrations of Cd as decreasing pH and ECEC diminish Cd retention and reduces Cd concentrations in the forest topsoils.
Abbreviations: ECEC, effective cation-exchange capacity • ICP-OES, inductively coupled plasma optical emission spectra • ISO, International Organization for Standardization
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INTRODUCTION
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LARGE AREAS OF INTENSIVELY CULTIVATED agricultural land are afforested in northern Europe to reduce both the agricultural production and the release of nutrients to the surrounding aquatic environment. The areas afforested in Denmark are primarily low productivity agricultural soils and areas that directly impact groundwater reservoirs, the latter for protection of groundwater from pesticides and other unwanted compounds used in agriculture. Unfortunately, soil acidification following afforestation may cause mobilization of heavy metals in the soil, especially the toxic Cd that might be leached to streams and groundwater (Egli et al., 1999; Jug et al., 1999). In the afforested areas, lime is no longer applied and in addition an increased production of organic acids and atmospheric acid deposition causes a decline in pH, which increases Cd solubility. Although increased Cd solubility as a consequence of complexation with dissolved organic matter in the afforested soils, seems negligible (Strobel et al., 2001).
Cadmium can be bound in soil by simple electrostatic forces or intimately associated with metal oxides, carbonates, and organic matter. It is also found that the Cd solubility increases as pH decreases (Christensen, 1989; Chlopecka et al., 1996). A number of investigations have shown Cd solubility to be dependent also on the cation-exchange capacity (CEC), clay content, organic matter, and other metal ions present (Christensen, 1989; Wilkens and Loch, 1997; McBride et al., 1997; Ma et al., 1997; Száková et al., 1999). Cadmium release from soil increases substantially when pH drops below 4.5 (Bergkvist et al., 1989; McBride et al., 1997). This is especially true for sandy soils with low CEC, low acid neutralization capacity, and low ability of the subsoil to sorb Cd ions. Such soils may be vulnerable to Cd leaching after afforestation.
Arable land receives Cd from the application of fertilizers, lime, and to some extent from sewage sludge (Bak et al., 1997). Atmospheric deposition is also a substantial source of Cd to the soil. Atmospheric deposition of Cd is higher in forest areas compared with arable land (Wilcke et al., 1999). A considerable reduction of the atmospheric Cd deposition in Denmark has taken place during the last decade (Hovmand and Kemp, 2000), but years of high atmospheric Cd deposition may have increased the total Cd content in the soil. Whether the Cd added from the various sources results in a net loss or accumulation of Cd from the soil profile will depend on the soil parameters important to Cd solubility, for example, pH and the amount of sorption sites available for Cd.
Studies of the relation between Cd concentrations in the soil solid and solution phases of natural (unpolluted) soil systems are scarce. To determine the effects of afforestation on Cd concentrations, solubility and distribution in soil, comparisons between the two land-uses were made with soils of different texture. The total soil and solution Cd concentrations of 11 pairs of Danish arable and afforested soil profiles were determined as well as other soil properties, that is, pH, organic matter content, and exchangeable base cations. The soil properties important to Cd retention and solubility in these soils are discussed.
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MATERIALS AND METHODS
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Sampling
Soil samples were collected from November 1998 to January 2000 from 11 sites representing three different textural classes: sand, loamy sand, and sandy loam (Table 1). The soils vary with respect to acidity, clay content, and CEC. Each site is comprised of adjacent arable and forest profiles developed on the same parent material. The forest plots vary in tree species and stand age but they were all planted on former farmland. All profiles were excavated to a depth of at least 125 cm and classified according to Soil Taxonomy (Soil Survey Staff, 1997).
Soil was sampled from each horizon in all the profiles (a total of four to eight horizons per profile). At seven of the 11 sites soil solutions were isolated from soil samples taken from each horizon in the profiles and from the remaining four sites (Tisted Nørskov, Løvenholm, Christianssæde, and Stenholtsvang) soil solutions were isolated from soil samples collected using a soil auger (Eijkelkamp Agrisearch Equipment, Giesbeek, NL) and only from three depths (0–5, 20–25, and 70–90 cm). The soil samples were stored in polyethylene bags at 5°C. The natural moist soil was placed in a polyethylene cup with holes in the bottom and this cup was placed on top of a polytetrafluorethene (Teflon) cup. By centrifugation for 20 min. at 3200 x g (3500 rpm) (Allegra 6R, Beckman Coulter, Palo Alto, CA), the soil solution was isolated from the natural moist soil samples into the Teflon cup (Davies and Davies, 1963). The soil solutions were filtered through a 0.45-µm cellulose membrane filter (ME 25, Schleicher & Schnell, Dassel, Germany), transferred into polyethylene containers, and stored at 4°C until analysis. The centrifuged soil were transferred into paper bags, dried at 55°C, sieved through a 2-mm screen and stored in polyethylene containers. All laboratory equipment used for experiments were soaked in 6.5% HNO3 (analytical grade) for 1 h and rinsed thoroughly in MilliQ-water.
Soil Solid Analyses
The Cd concentrations in the soil samples were determined by aqua regia extraction [International Organization for Standardization (ISO), 1995]. The soils were ground in an agate mortar and 2.50 g of soil were transferred into borosilicate tubes and 15.0 mL 30% (wt./wt.) HCl (supra pure) was added followed by 5.0 mL concentrated HNO3 (supra pure), then the suspensions were left covered in a heating block for 2 d at room temperature. The temperature was then slowly increased to 90°C and the soils were digested for 2 h. After cooling the solutions were filtered [Whatman 42 (Kent, UK) filter, prewashed in 12.5 mM EDTA and rinsed in MilliQ water] and diluted to 50 mL using MilliQ water. The concentrations of Cd in the digestates were determined using inductively coupled plasma optical emission spectra (ICP-OES; Optima 3000 XL axial view, AS 90, Perkin Elmer, Norwalk, CT) with a detection limit of 1 µg Cd L-1 equal to 20 µg Cd kg-1 soil. The determination of aqua-regia extractable Cd was carried out in duplicates. Determination of soil texture was done using the hydrometer method (Gee and Bauder, 1986). Total contents of C and N in the soil were determined using a CN-analyzer (LECO CNS-2000, St. Joseph, MI). Exchangeable base cations were determined by extraction with 1.0 M NH4NO3 and exchangeable acidity was determined by end-point titration of the NH4NO3 extract to pH 6.00 (Stuanes et al., 1984). The ECEC of the soils was calculated as the sum of charges of the base cations and the exchangeable acidity. The soil pH was determined in 0.01 M CaCl2 with a soil/solution ratio of 1:2.5 using a Metrohm (Herisau, Switzerland) glass electrode (6.0202.110) connected to a Metrohm 691 pH meter. Soil bulk density was determined by drying soil from each horizon sampled in metal rings of a known volume.
Soil Solution Analyses
The Cd concentrations in the soil solutions were determined by graphite furnace atomic absorption spectrometry (GFAAS) [Perkin Elmer (Wellesley, MA) 5100, Zeeman (Wellesley, MA) 5100]. For conservation of the samples, the soil solution for Cd analysis were acidified to 1.0% (wt./wt.) HNO3 (supra pure). The detection limit was 0.05 µg L-1 and maximum allowed RSD value [RSD = (standard deviation)/(mean value) x 100] between the measurements was 5%. Every sixth sample run was a control of known Cd concentration. The concentration of total organic C (TOC) in the soil solutions was determined using Shimadzu (Kyoto, Japan) 500 total C analyzer, and soil solution pH was determined immediately after the centrifugation, using the same equipment as described for soil pH. The concentrations of Al, Ca, K, Mg, and Na were determined by ICP-OES (Optima 3000 XL axial view, AS 90, Perkin Elmer, Norwalk, CT).
Calculations and Statistics
Average profiles of soil pH and soil Cd concentrations as well as soil solution pH and soil solution Cd concentration were generated for each texture class (Fig. 1 and 2)
. Because the horizons in the different profiles were not sampled at the same depths a data point was calculated for each centimeter, as the mean value of all the observations at this depth in all the profiles in the respective texture class. Each of these calculated points were then averaged across the five overlying and five underlying points to smooth the curve. The average solution pH and Cd concentration profiles were generated for every centimeter using the same procedure.
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Fig. 1. Vertical distribution of Cd contents (µg g-1) and pH of (a, b) arable and (c, d) forest soils in three texture classes.
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Fig. 2. Vertical distribution of soil solution concentration of Cd (µg L-1) and pH in solution of (a, b) arable and (c, d) forest soils in three texture classes.
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The volume based Cd concentration, hereafter referred to as Cd content in the soil was calculated for four depths at each profile: 0 to 30, 30 to 60, 60 to 90, and 90 to 120 cm, by multiplying the mass-based soil Cd concentration found in each horizon with the soil bulk density and the relative part that the horizon comprises of the selected soil layer. The average soil solution concentrations were calculated for the same four depths. Differences between the textural classes were analyzed depthwise by a general linear model (SAS Institute, 1999). A similar analysis was made within each combination of land use and texture class to analyze the differences between the depths. In both analyses the Cd content in the soil and the soil solution concentration were log-transformed to normalize and homogenize the variances. Back-transformed mean values of the total soil content and soil solution concentration of Cd are reported.
To test the difference between the forest and arable soils the arable/forest ratios of the total Cd contents and the soil solution concentrations in each soil profile pair at the four depths were calculated. These figures were then tested depth-wise within each texture class to vary significantly from one as an indicator of different content. The test was carried out as a t-test (MEAN-procedure) (SAS Institute, 1999).
Linear correlations between the soil Cd concentrations and the ECEC, organic C, and the clay content were carried out by GLM (SAS Institute, 1999). Cadmium concentrations below the detection limit of 20 µg kg-1 in soil solids or 0.05 µg L-1 in soil solutions were included in the statistical calculations with half the detection limit.
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RESULTS
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Sites
The soils in the present study represent typical Danish soils. The sandy soils are predominately found on out-wash plains in the western part of Denmark and the till soil types are dominated by moraine soils in the eastern part of the country. Norway spruce [Picea abies (L.) H. Karst.] is the dominating forest vegetation in Western Denmark whereas deciduous forest of beech (Fagus sylvatica L.) and oak (Quercus robur L.) is the most common in Eastern Denmark. The soil profiles were grouped in three texture classes based on the texture in the top 0 to 30 cm of the soil. Nørlund, Klosterhede, Baldersbæk, and Kompedal were characterized as sand soils and the clay content in the horizons in the profiles ranged between 2 and 9%. Løvbakke, Tisted Nørskov, Løvenholm, and Strødam were in the loamy sand group with clay contents between 5 and 17% and the sandy loam soils comprised of Jels, Stenholtsvang, and Christianssæde with clay contents between 8 and 28% (Tables 1 and 2). The content of soil organic C in the top horizons of sand soils were in the range from 13 to 62 g kg-1, in the loamy sand soils between 6 and 36 g kg-1 and in the sandy loam soils from 12 to 26 g kg-1 (Table 2). There was not found any significant difference between the organic matter content in forest and arable soil.
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Table 2. Selected mean results from each horizon type (A, B, or C) within each land-use and texture class. The standard deviations are given in parenthesis.
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Soil Solids
The Cd concentrations in the O-horizons were between 115 and 321 µg kg-1, equivalent to a Cd content of 0.8 and 2.3 mg m-2. The contribution of the O-horizons to the total Cd content of the soil profiles was negligible; therefore data from the O-horizons are excluded from all calculations based on soil depth.
The Cd concentration in the top horizon of the mineral soils was in the range from 24 to 293 µg kg-1 (Table 2). In arable soils the Cd concentration in the topsoil were up to five times higher than in the lower horizons and a significantly higher Cd content was found in the topsoil for the sand and loamy sand soils (Table 3). The Cd content did not vary significantly between depths in the arable sandy loam soils (Table 3, Fig. 1a). The forest soils, on the other hand, showed a uniform Cd distribution with depth, even though the sand soils showed a slightly elevated level of the Cd content in the topsoil (Table 3, Fig. 1c).
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Table 3. Cadmium content (back-transformations of the logarithm means) in soils and mean values of Cd concentration in soil solution at four depths of the soil profiles investigated. Different letters indicate significant difference (p < 0.05) of Cd content and Cd concentration between texture classes within each soil depth and land-use. Different numbers indicate significant difference (p < 0.05) of Cd content and Cd concentration between depths within each texture class and land-use.
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All the layers of the arable sandy loam soils and the 60- to 90- and 90- to 120-cm layers of the forest sandy loam soils had significantly higher Cd content than the loamy sand and sand soils (Table 3, Fig. 1a,c). Among the arable soils, the sand soils had a lower Cd content in the topsoil, than the loamy sand and sandy loam soils. This was not the case in the forest soils (Table 3).
The average Cd content in the 0- to 30-cm layer of the loamy sand soil was significantly higher in the arable soils than in the forest soils. The same trends were also found in the 0- to 30-cm layer in the two other texture classes, but they were not significant (Table 3). No clear trends were found for the deeper layers.
In the arable soils, pH was almost constant throughout the profile except for a gradually decrease with depth in the sandy loam soils (Fig. 1b). The upper 20 cm of the forest soils was more acid compared with the subsoil (Fig. 1d).
Soil Solutions
The Cd concentrations in soil solutions were in the range of <0.05 to 4.0 µg L-1 (Fig. 2). In the arable loamy sand soils, a significant higher concentration was found in the 0- to 30-cm layer than in the subsoil and in the loamy sand forest soils significant higher concentrations were found in the 0- to 30- and 30- to 60-cm layers than in the deeper layers (Table 3). The Cd concentration did not differ between the four depths in the sand soils and the sandy loam soils. In the loamy sand soils the Cd concentration was higher in the upper soil layers than in the lower soil layers (Table 3).
The Cd concentrations in the soil solutions of arable soils were low (<0.2 µg L-1) in all texture classes (Fig. 2a), whereas the solution concentrations of Cd in the forest soils were higher (between 0.2 and 3.3 µg L-1) (Fig. 2c). In the 0- to 30-cm layer the Cd concentration in the forest soil solutions were higher than in the arable soils and the same was found for the loamy sand and sandy loam soils in the 30- to 60- and 60- to 90-cm layers (Table 3).
The pH in the soil solutions of arable sandy loam soils were about 7 in the upper layers and about 5.5 in the lower layers. In the soil solution of sand and loamy sand soils, the pH was below 5 (Fig. 2b). There was a tendency of the pH values to decrease with depth in arable soils and increase with depth in forest soils (Fig. 2b, d). The variations of the pH between texture classes in the forest soil solutions were small (Fig. 2d).
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DISCUSSION
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Soil Solids
The concentrations of Cd in the soils studied were within the ordinary range for natural soils in Denmark, which were found to be 0.07 mg kg-1 for forested sand soils and 0.22 mg kg-1 for arable loam soil (Bak et al., 1997). Higher Cd contents were found in the topsoil than in the lower layers of the arable soils, whereas no difference between topsoil and subsoil was found in the forested profiles. This suggests that agriculture tends to enhance the Cd retention in the topsoil, or that the arable soils receive a higher Cd input in the topsoil than the forest soils. A significantly higher Cd content was found in arable soils than in forest soils within the upper 30 cm of the soil (p = 0.046), but not within the upper 120 cm (p = 0.612) when all the texture classes were included in the calculation. An average of 45% of the total Cd in the arable soil profile from 0 to 120 cm was located in the top 30 cm, whereas only an average of 30% was found in the upper 30 cm in forest soils. Higher contents of Cd in arable soils than in forest soils have been reported previously (Bak et al., 1997; Römkens and Salomons, 1998). The higher Cd contents found in the top layers of the arable soils compared with the forest soils are likely to originate from fertilizer used in intensive agriculture (Holmgren et al., 1993) or because of more intensive leaching from the forest profile (Bergkvist et al., 1989).
The forested sand soils also had a significantly higher content of Cd in topsoil than in subsoil, whereas this was not the case in the loamy sand and sandy loam soils (Table 3). The higher Cd content in the upper layers of the forest sand soils was consistent with a high content of soil organic matter. The organic-rich Bh-horizons were also enriched with Cd, suggesting that Cd is retained by organic matter as observed by Wilkens and Loch (1997). For the forested sand soils, the higher content of organic matter seemed to retain the Cd despite the low pH. This corresponds well with the models presented in Table 4, where the soil C content is included in the modeling of the Cd concentration in the forest soils.
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Table 4. Regression models of the logarithm to the total Cd concentration (µg g-1) in soil and the soil parameters effective cation-exchange capacity (ECEC, cmol + kg-1, clay, %, and organic C, g kg-1).
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Significantly higher Cd contents were found in the sandy loam soils than in the sand and loamy sand soils. This was most pronounced in the forest soils (Table 3). Higher clay contents cause a higher buffer capacity towards acidification, and thus forest soils on sandy loam will exhibit a slower pH decrease, than more coarse-textured soils. The higher total Cd content in the sandy loam soil than in the sand and loamy sand soil can arise from a higher capacity to sorb the Cd added to the soil and a higher Cd content in the parent material. This corresponds well with the significant correlations found between the Cd concentration and clay and ECEC (Table 4). The high correlation between the soil Cd concentration and the ECEC in the arable soils and the clay content in the forest soils suggest that a high density of sorption sites is important to Cd sorption (Table 4). In a sequential extraction study of Cd in Danish forest and arable soils, Andersen et al. (2002) found that Cd was bound only in easily mobile fractions of the soil, which eliminates the possibility that the high correlations between Cd and clay content is caused by Cd being occluded in the clay minerals. Other authors have also found that Cd concentration in soil correlates with ECEC and clay content (Sánchez-Martín and Sánchez-Camazano, 1993; Springob and Böttcher, 1998; Sterckeman et al., 2000). The soil pH was not found to be significant when included in the statistical analyses, which leads to the conclusion that the Cd concentration is less dependent on the soil pH (Egli et al., 1999).
Soil Solution
The majority of the measured Cd concentrations in soil solutions were below 1.0 µg L-1. Similar levels of Cd concentration in soil solutions were found by Bergkvist (1987) in Swedish soils and by Gooddy et al. (1995) in investigations of an acid sandy forest soil in southern England. Solution concentrations above 1.0 µg L-1 were found in the forest soils from Jels, Løvbakke, and Strødam that are all sandy loam or loamy sand soils. The concentrations in these soils rose with depth as illustrated in Fig. 2c. In the sand soils, the general pattern of the Cd concentration was a decrease with soil depth (Fig. 2c). One single horizon in Baldersbæk had a high concentration of Cd (4.0 µg L-1).
Variations in Cd concentrations among texture classes were negligible in arable soils but pronounced in forest soils (Fig. 2a, c). The Cd concentrations in the subsurface horizons of the forest soils were higher in the sandy loam soils compared with the sand soils, but no significant difference between the texture classes were found, presumably because of the few observations (Table 3). Thus, the concentration of mobile Cd in soil solution seems to increase with the content of clay in soil.
The low Cd concentrations in soil solutions in all the arable profiles are attributed to the relatively high pH (Fig. 2). The soil texture as well as the pH in soil solution seemed to have an influence on the Cd concentration. Lower pH values limit the accumulation of Cd in soil, and a significant correlation (p < 0.0001, R2 = 0.39) was found between the log Cd concentration and pH in soil solution (Fig. 3)
. The pH was found not to have any influence on the total Cd content in the soil, but a significant influence on the Cd in solution, and thus on the solubility of Cd in soil. The general understanding has been that Cd solubility decreases at high pH and with high clay content that both favors a high sorption capacity (Sánchez-Martín and Sánchez-Camazano, 1993; Boekhold et al., 1993; Egli et al., 1999; Sterckeman et al., 2000). The Cd solubility in this study decreased with increasing pH, but in contrast to the above findings an increasing Cd concentration in the soil solution was found as the clay percentage increased. This may be explained by the higher Cd content found in the loamy sand and sandy loam soils.
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Fig. 3. The concentration of Cd (µg L-1) versus pH in the soil solutions from arable and forest soils. Log Cd = 0.81 - 0.30pH, R2 = 0.39.
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The present study reports actual field concentrations of Cd in unpolluted soils and soil solutions from various soil types and horizons. The variable origin of Cd in soil makes the correlation with soil parameters more complex (Sanchez-Camazano et al., 1998; Sauvé et al., 2000). The significance of other soil variables than pH on the Cd solubility has been reported with ECEC and organic matter content being the most important ones (Buchter et al., 1989; Sauvé et al., 2000).
Cadmium mobility is increased by afforestation. Decreasing soil solution pH increase Cd in solution, but high ECEC and content of clay and organic matter in the soil can counteract this by binding Cd in the soil. Leaching of soil solution with the concentration levels of Cd found in this investigation of soil with background levels of Cd, does not pose a risk to the surrounding environment.
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ACKNOWLEDGMENTS
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This research was partially supported by a grant from the European Commission (FAIR3-CT96-1983) and a grant from the Danish Forest and Nature Agency.
Received for publication June 20, 2001.
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TOP
ABSTRACT
INTRODUCTION
