Large areas of intensively cultivated agricultural

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					         Content, Distribution, and Solubility of Cadmium in Arable and Forest Soils
Martin K. Andersen,* Anne Refsgaard, Karsten Raulund-Rasmussen, Bjarne W. Strobel, and Hans C. B. Hansen

                           ABSTRACT                                                exchange capacity (CEC), clay content, organic matter,
   Afforestation of former farmland decreases soil pH and thus in-                 and other metal ions present (Christensen, 1989; Wil-
creases the solubility of Cd in the soil, which may cause Cd leaching              kens and Loch, 1997; McBride et al., 1997; Ma et al.,
to streams and groundwater. The Cd concentration in soil and soil so-              1997; Sza      ´
                                                                                             ´kova et al., 1999). Cadmium release from soil in-
lution were determined in 11 pairs of Danish arable and forest soil                creases substantially when pH drops below 4.5 (Berg-
profiles representing three different texture classes (sand, loamy sand,           kvist et al., 1989; McBride et al., 1997). This is especially
and sandy loam). The soil pH did not change or decrease with depth                 true for sandy soils with low CEC, low acid neutral-
through the arable profiles, but did increase with depth in the forest
                                                                                   ization capacity, and low ability of the subsoil to sorb
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
                                                                                   Cd ions. Such soils may be vulnerable to Cd leaching
soil Cd concentrations correlated with the effective cation-exchange               after afforestation.
capacity (ECEC), clay content, and organic matter content, but not the                Arable land receives Cd from the application of ferti-
soil pH. The soil solution pH was unchanged or decreasing downwards                lizers, lime, and to some extent from sewage sludge (Bak
through the arable profiles, but increasing with depth in the forest               et al., 1997). Atmospheric deposition is also a substan-
profiles. The soil solution concentration of Cd was significantly higher           tial source of Cd to the soil. Atmospheric deposition of
in the forest soils than in the arable soils. The Cd concentration in              Cd is higher in forest areas compared with arable land
the soil solution decreased as pH increased. Both total soil and soil              (Wilcke et al., 1999). A considerable reduction of the
solution Cd concentrations were higher in the sandy loam soils com-                atmospheric Cd deposition in Denmark has taken place
pared with the loamy sand and sand soils. It is concluded that afforesta-          during the last decade (Hovmand and Kemp, 2000),
tion may lead to higher soil solution concentrations of Cd as decreasing
pH and ECEC diminish Cd retention and reduces Cd concentrations
                                                                                   but years of high atmospheric Cd deposition may have
in the forest topsoils.                                                            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
L  arge areas of intensively cultivated agricultural
     land are afforested in northern Europe to reduce
both the agricultural production and the release of nutri-
                                                                                   example, pH and the amount of sorption sites available
                                                                                   for Cd.
                                                                                      Studies of the relation between Cd concentrations in
ents to the surrounding aquatic environment. The areas
                                                                                   the soil solid and solution phases of natural (unpolluted)
afforested in Denmark are primarily low productivity
                                                                                   soil systems are scarce. To determine the effects of af-
agricultural soils and areas that directly impact ground-
                                                                                   forestation on Cd concentrations, solubility and distri-
water reservoirs, the latter for protection of ground-
                                                                                   bution in soil, comparisons between the two land-uses
water from pesticides and other unwanted compounds
                                                                                   were made with soils of different texture. The total soil
used in agriculture. Unfortunately, soil acidification fol-
                                                                                   and solution Cd concentrations of 11 pairs of Danish ar-
lowing afforestation may cause mobilization of heavy
                                                                                   able and afforested soil profiles were determined as
metals in the soil, especially the toxic Cd that might be
                                                                                   well as other soil properties, that is, pH, organic matter
leached to streams and groundwater (Egli et al., 1999;
                                                                                   content, and exchangeable base cations. The soil proper-
Jug et al., 1999). In the afforested areas, lime is no longer
                                                                                   ties important to Cd retention and solubility in these
applied and in addition an increased production of or-
                                                                                   soils are discussed.
ganic acids and atmospheric acid deposition causes a de-
cline in pH, which increases Cd solubility. Although in-
creased Cd solubility as a consequence of complexation                                         MATERIALS AND METHODS
with dissolved organic matter in the afforested soils,                                                        Sampling
seems negligible (Strobel et al., 2001).
                                                                                      Soil samples were collected from November 1998 to January
   Cadmium can be bound in soil by simple electrostatic
                                                                                   2000 from 11 sites representing three different textural classes:
forces or intimately associated with metal oxides, car-                            sand, loamy sand, and sandy loam (Table 1). The soils vary with
bonates, and organic matter. It is also found that the Cd                          respect to acidity, clay content, and CEC. Each site is com-
solubility increases as pH decreases (Christensen, 1989;                           prised of adjacent arable and forest profiles developed on the
Chlopecka et al., 1996). A number of investigations have                           same parent material. The forest plots vary in tree species and
shown Cd solubility to be dependent also on the cation-                            stand age but they were all planted on former farmland. All
                                                                                   profiles were excavated to a depth of at least 125 cm and clas-
                                                                                   sified according to Soil Taxonomy (Soil Survey Staff, 1997).
M.K. Andersen, B.W. Strobel, and H.C.B. Hansen, Chemistry Dep.,
                                                                                      Soil was sampled from each horizon in all the profiles (a
The Royal Veterinary and Agricultural Univ., Thorvaldsensvej 40,
DK-1871 Frederiksberg C; A. Refsgaard and K. Raulund-Rasmussen,                    total of four to eight horizons per profile). At seven of the
Danish Forest and Landscape Research Institute, Hoersholm                          11 sites soil solutions were isolated from soil samples taken
Kongevej 11, DK-2970 Hoersholm. Received 20 June 2001.*Corre-
sponding author (                                                      Abbreviations: ECEC, effective cation-exchange capacity; ICP-OES;
                                                                                   inductively coupled plasma optical emission spectra; ISO, Interna-
Published in Soil Sci. Soc. Am. J. 66:1829–1835 (2002).                            tional Organization for Standardization.

1830                                  SOIL SCI. SOC. AM. J., VOL. 66, NOVEMBER–DECEMBER 2002

Table 1. Site description and classification according to soil taxonomy (1997).
Site                    Land use           UTM coordinates                 Classification               Texture class          Sediment
Nørland                  Arable             32VNH114144              Typic Durorthod                     Sand                Fluvial   sand
                         Forest                                      Oxyaquic Haplorthod                 Sand                Fluvial   sand
Klosterhede              Arable             32VMH681589              Typic Haplorthod                    Sand                Fluvial   sand
                         Forest                                      Typic Haplorthod                    Loamy sand          Fluvial   sand
Baldersbæk               Arable             32UMG947668              Psammentic Haplumbrept              Sand                Fluvial   sand
                         Forest                                      Typic Durorthod                     Sand                Fluvial   sand
Kompedal                 Arable             32VNH187294              Psammentic Haplumbrept              Sand                Fluvial   sand
                         Forest                                      Typic Durorthod                     Sand                Fluvial   sand
Løvbakke                 Arable             32UNG997668              Oxyaquic Haplumbrept                Loamy sand          Till
                         Forest                                      Typic Haplorthod                    Loamy sand          Till
Tisted Nørskov           Arable             32VHN622944              Entic Haplumbrept                   Loamy sand          Till
                         Forest                                      Entic Haplumbrept                   Loamy sand          Till
Løvenholm                Arable             32VHN944581              Oxyaquic Glossudalf                 Loamy sand          Till
                         Forest                                      Entic Haplumbrept                   Loamy sand          Till
Strødam                  Arable             33UUC30153               Psammentic Haplumbrept              Loamy sand          Till
                         Forest                                      Typic Udipsamment                   Loamy sand          Till
Jels                     Arable             32UNG141372              Typic Paleudult                     Sandy loam          Till
                         Forest                                      Typic Paleudult                     Loamy sand          Till
Stenholtsvang            Arable             33UUC348045              Oxyaquic Hapludalf                  Sandy loam          Till
                         Forest                                      Oxyaquic Hapludalf                  Sandy loam          Till
Christianssæde           Arable             32UPF522726              Typic Hapludalf                     Sandy loam          Till
                         Forest                                      Mollic Hapludalf                    Sandy loam          Till

from each horizon in the profiles and from the remaining               NH4NO3 and exchangeable acidity was determined by end-
four sites (Tisted Nørskov, Løvenholm, Christianssæde, and             point titration of the NH4NO3 extract to pH 6.00 (Stuanes et
Stenholtsvang) soil solutions were isolated from soil samples          al., 1984). The ECEC of the soils was calculated as the sum
collected using a soil auger (Eijkelkamp Agrisearch Equip-             of charges of the base cations and the exchangeable acidity.
ment, Giesbeek, NL) and only from three depths (0–5, 20–25,            The soil pH was determined in 0.01 M CaCl2 with a soil/
and 70–90 cm). The soil samples were stored in polyethylene            solution ratio of 1:2.5 using a Metrohm (Herisau, Switzerland)
bags at 5 C. The natural moist soil was placed in a polyethylene       glass electrode (6.0202.110) connected to a Metrohm 691 pH
cup with holes in the bottom and this cup was placed on top            meter. Soil bulk density was determined by drying soil from
of a polytetrafluorethene (Teflon) cup. By centrifugation for          each horizon sampled in metal rings of a known volume.
20 min. at 3200 g (3500 rpm) (Allegra 6R, Beckman Coulter,
Palo Alto, CA), the soil solution was isolated from the natural
                                                                                             Soil Solution Analyses
moist soil samples into the Teflon cup (Davies and Davies,
1963). The soil solutions were filtered through a 0.45- m cellu-          The Cd concentrations in the soil solutions were determined
lose membrane filter (ME 25, Schleicher & Schnell, Dassel,             by graphite furnace atomic absorption spectrometry (GFAAS)
Germany), transferred into polyethylene containers, and stored         [Perkin Elmer (Wellesley, MA) 5100, Zeeman (Wellesley,
at 4 C until analysis. The centrifuged soil were transferred           MA) 5100]. For conservation of the samples, the soil solution
into paper bags, dried at 55 C, sieved through a 2-mm screen           for Cd analysis were acidified to 1.0% (wt./wt.) HNO3 (supra
and stored in polyethylene containers. All laboratory equip-           pure). The detection limit was 0.05 g L 1 and maximum al-
ment used for experiments were soaked in 6.5% HNO3 (ana-               lowed RSD value [RSD (standard deviation)/(mean value)
lytical grade) for 1 h and rinsed thoroughly in MilliQ-water.          100] between the measurements was 5%. Every sixth sample
                                                                       run was a control of known Cd concentration. The concentra-
                     Soil Solid Analyses                               tion of total organic C (TOC) in the soil solutions was deter-
                                                                       mined using Shimadzu (Kyoto, Japan) 500 total C analyzer,
    The Cd concentrations in the soil samples were determined          and soil solution pH was determined immediately after the cen-
by aqua regia extraction [International Organization for Stan-         trifugation, using the same equipment as described for soil
dardization (ISO), 1995]. The soils were ground in an agate            pH. The concentrations of Al, Ca, K, Mg, and Na were deter-
mortar and 2.50 g of soil were transferred into borosilicate           mined by ICP-OES (Optima 3000 XL axial view, AS 90, Perkin
tubes and 15.0 mL 30% (wt./wt.) HCl (supra pure) was added             Elmer, Norwalk, CT).
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 in-
                                                                                            Calculations and Statistics
creased to 90 C and the soils were digested for 2 h. After                Average profiles of soil pH and soil Cd concentrations as
cooling the solutions were filtered [Whatman 42 (Kent, UK)             well as soil solution pH and soil solution Cd concentration
filter, prewashed in 12.5 mM EDTA and rinsed in MilliQ                 were generated for each texture class (Fig. 1 and 2). Because
water] and diluted to 50 mL using MilliQ water. The concen-            the horizons in the different profiles were not sampled at the
trations of Cd in the digestates were determined using induc-          same depths a data point was calculated for each centimeter,
tively coupled plasma optical emission spectra (ICP-OES;               as the mean value of all the observations at this depth in all the
Optima 3000 XL axial view, AS 90, Perkin Elmer, Norwalk,               profiles in the respective texture class. Each of these calculated
CT) with a detection limit of 1 g Cd L 1 equal to 20 g Cd              points were then averaged across the five overlying and five
kg 1 soil. The determination of aqua-regia extractable Cd was          underlying points to smooth the curve. The average solution
carried out in duplicates. Determination of soil texture was           pH and Cd concentration profiles were generated for every
done using the hydrometer method (Gee and Bauder, 1986).               centimeter using the same procedure.
Total contents of C and N in the soil were determined using               The volume based Cd concentration, hereafter referred to
a CN-analyzer (LECO CNS-2000, St. Joseph, MI). Exchange-               as Cd content in the soil was calculated for four depths at
able base cations were determined by extraction with 1.0 M             each profile: 0 to 30, 30 to 60, 60 to 90, and 90 to 120 cm, by
                               ANDERSEN ET AL.: CONTENT, DISTRIBUTION, AND SOLUBILITY OF CADMIUM                                  1831

Fig. 1. Vertical distribution of Cd contents ( g g 1) and pH of (a, b) arable and (c, d) forest soils in three texture classes.

multiplying the mass-based soil Cd concentration found in                    wash plains in the western part of Denmark and the till
each horizon with the soil bulk density and the relative part                soil types are dominated by moraine soils in the eastern
that the horizon comprises of the selected soil layer. The aver-             part of the country. Norway spruce [Picea abies (L.)
age soil solution concentrations were calculated for the same
                                                                             H. Karst.] is the dominating forest vegetation in Western
four depths. Differences between the textural classes were
analyzed depthwise by a general linear model (SAS Institute,                 Denmark whereas deciduous forest of beech (Fagus
1999). A similar analysis was made within each combination of                sylvatica L.) and oak (Quercus robur L.) is the most
land use and texture class to analyze the differences between                common in Eastern Denmark. The soil profiles were
the depths. In both analyses the Cd content in the soil and the              grouped in three texture classes based on the texture
soil solution concentration were log-transformed to normalize                in the top 0 to 30 cm of the soil. Nørlund, Klosterhede,
and homogenize the variances. Back-transformed mean values                   Baldersbæk, and Kompedal were characterized as sand
of the total soil content and soil solution concentration of Cd              soils and the clay content in the horizons in the profiles
are reported.
   To test the difference between the forest and arable soils
                                                                             ranged between 2 and 9%. Løvbakke, Tisted Nørskov,
the arable/forest ratios of the total Cd contents and the soil               Løvenholm, and Strødam were in the loamy sand group
solution concentrations in each soil profile pair at the four                with clay contents between 5 and 17% and the sandy
depths were calculated. These figures were then tested depth-                loam soils comprised of Jels, Stenholtsvang, and Chris-
wise within each texture class to vary significantly from one                tianssæde with clay contents between 8 and 28%
as an indicator of different content. The test was carried out               (Tables 1 and 2). The content of soil organic C in the
as a t-test (MEAN-procedure) (SAS Institute, 1999).                          top horizons of sand soils were in the range from 13 to
   Linear correlations between the soil Cd concentrations and                62 g kg 1, in the loamy sand soils between 6 and 36 g
the ECEC, organic C, and the clay content were carried out
by GLM (SAS Institute, 1999). Cadmium concentrations be-                     kg 1 and in the sandy loam soils from 12 to 26 g kg 1
low the detection limit of 20 g kg 1 in soil solids or 0.05 g                (Table 2). There was not found any significant differ-
L 1 in soil solutions were included in the statistical calculations          ence between the organic matter content in forest and
with half the detection limit.                                               arable soil.

                           RESULTS                                                                       Soil Solids
                             Sites                                             The Cd concentrations in the O-horizons were be-
  The soils in the present study represent typical Danish                    tween 115 and 321 g kg 1, equivalent to a Cd content
soils. The sandy soils are predominately found on out-                       of 0.8 and 2.3 mg m 2. The contribution of the O-hori-
1832                                        SOIL SCI. SOC. AM. J., VOL. 66, NOVEMBER–DECEMBER 2002

Fig. 2. Vertical distribution of soil solution concentration of Cd ( g L 1) and pH in solution of (a, b) arable and (c, d) forest soils in three
   texture classes.

zons to the total Cd content of the soil profiles was                               ble 2). In arable soils the Cd concentration in the topsoil
negligible; therefore data from the O-horizons are ex-                              were up to five times higher than in the lower horizons
cluded from all calculations based on soil depth.                                   and a significantly higher Cd content was found in the
   The Cd concentration in the top horizon of the min-                              topsoil for the sand and loamy sand soils (Table 3). The
eral soils was in the range from 24 to 293 g kg 1 (Ta-                              Cd content did not vary significantly between depths in
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.
Textural class,                    Number of                                                      ECEC          Cd (AR)†           Ca‡
                                                                             1                             1
Land-use               Horizon     horizons n      pH soil        C g kg          Clay %          molc g           gg 1           molc g   1
                                                                                                                                                Density Mg m    3

Sand, arable               A            6         5.28   (0.55)   23.9   (15.6)    3.9   (1.3)   516   (354)   0.090   (0.071)   473   (327)   1.28   (0.28)
                           B           14         4.82   (0.33)    4.6   (7.6)     3.0   (1.3)    80   (122)   0.021   (0.014)    64   (109)   1.50   (0.08, n 13)
                           C            4         4.88   (0.15)    0.5   (0.3)     1.8   (0.5)    18   (3)     0.012   (0.013)     9   (3)     1.50   (0.09)
Sand, forest               A            6         3.05   (0.17)   26.2   (23.4)    2.6   (1.1)    51   (44)    0.064   (0.055)    13   (14)    1.23   (0.19)
                           B           13         3.96   (0.59)   19.6   (22.1)    4.6   (2.4)    39   (40)    0.069   (0.063)     4   (5)     1.26   (0.28)
                           C            5         4.56   (0.07)    1.2   (0.6)     2.3   (0.9)     7   (1)     0.024   (0.011)     1   (0)     1.49   (0.12)
Loamy sand, arable         A            5         5.53   (0.62)   21.6   (5.6)     7.4   (2.1)   519   (529)   0.223   (0.119)   468   (486)   1.32   (0.08)
                           B           10         5.36   (0.66)    6.0   (7.2)     9.4   (4.0)   177   (200)   0.039   (0.015)   153   (186)   1.44   (0.19)
                           C            6         4.86   (0.58)    0.6   (0.4)     6.8   (4.5)    40   (77)    0.019   (0.013)    35   (70)    1.59   (0.15, n 4)
Loamy sand, forest         A            4         3.62   (0.23)   19.3   (12.5)    6.1   (1.2)    74   (117)   0.051   (0.019)    42   (70)    1.16   (0.31)
                           B           10         4.36   (0.48)    6.4   (4.5)     7.0   (3.0)    33   (23)    0.052   (0.023)     8   (7)     1.38   (0.16)
                           C            7         4.63   (0.37)    1.2   (1.0)     7.6   (5.2)    44   (48)    0.040   (0.031)    17   (23)    1.57   (0.09, n 5)
Sandy loam, arable         A            3         6.17   (0.47)   16.2   (5.1)    13.2   (6.1)   373   (480)   0.195   (0.051)   327   (422)   1.54   ( , n 2)
                           B            7         6.21   (0.47)    2.9   (1.9)    14.6   (5.1)   266   (367)   0.085   (0.060)   222   (303)   1.61   (0.13, n 4)
                           C            3         6.07   (1.54)    1.6   (1.0)    17.5   (9.2)   336   (429)   0.121   (0.137)   265   (323)   1.80   ( , n 2)
Sandy loam, forest         A            4         4.46   (0.67)   15.5   (8.6)    14.1   (4.3)    85   (30)    0.127   (0.090)    52   (27)    1.24   (0.37)
                           B            7         5.23   (0.99)    3.1   (2.6)    21.6   (5.4)   118   (36)    0.094   (0.063)    71   (50)    1.63   (0.09)
                           C            3         5.57   (1.99)    1.8   (1.2)    16.8   (6.6)   138   (64)    0.134   (0.128)    67   (48)    1.77   ( , n 2)
† Half the detection limit is used when concentration is below the detection limit.
‡ Exchangeable Ca2 .
                               ANDERSEN ET AL.: CONTENT, DISTRIBUTION, AND SOLUBILITY OF CADMIUM                                         1833

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 con-
  centration 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.
                                                      Arable                                      Forest
                                       Cd in soil        Cd in soil solution       Cd in soil        Cd in soil solution
Depth             Texture class         mg m 2                  gL 1                mg m 2                  gL 1           Solid      Solution
0–30 cm:          Sand                    141 1                 0.14                  92                      0.59         1.55        0.23*
                  Loamy sand              206 1                 0.20 1                68                      0.90 12      3.17*       0.21*
                  Sandy loam              330                   0.06                 150                      1.19         1.47        0.11**
30–60 cm:         Sand                     32 a
                                              2                 0.09 a                35                      0.34         0.70        0.21
                  Loamy sand               67 ab
                                              2                 0.08 a
                                                                     2                43                      1.72 1       1.94        0.05*
                  Sandy loam              164 b                 0.02 b               167                      2.69         0.92        0.01†
60–90 cm:         Sand                     24 a
                                              2                 0.15                  31 a                    0.47         0.63        0.13
                  Loamy sand               66 2                 0.08 2                44 ab                   0.43 2       1.14        0.17*
                  Sandy loam              136                   0.08                 167 b                    3.46         0.82        0.03**
90–120 cm:        Sand                     29 a
                                              2                 0.14                  36 a                    0.31         0.92        0.39
                  Loamy sand               61 ab
                                              2                 0.06 2                47 ab                   0.312        1.03        0.21
                  Sandy loam              134                   0.30                 174 b                    3.46         0.89        0.08†
* and ** indicate p 0.05 and p 0.01 respectively, for the t-test of the hypotheses that Arable/Forest-ratio      1.
† No statistical difference due to missing values.

the arable sandy loam soils (Table 3, Fig. 1a). The forest                     (Fig. 2c). In the 0- to 30-cm layer the Cd concentration in
soils, on the other hand, showed a uniform Cd distribu-                        the forest soil solutions were higher than in the arable
tion with depth, even though the sand soils showed a                           soils and the same was found for the loamy sand and
slightly elevated level of the Cd content in the topsoil                       sandy loam soils in the 30- to 60- and 60- to 90-cm layers
(Table 3, Fig. 1c).                                                            (Table 3).
   All the layers of the arable sandy loam soils and the                          The pH in the soil solutions of arable sandy loam
60- to 90- and 90- to 120-cm layers of the forest sandy                        soils were about 7 in the upper layers and about 5.5 in
loam soils had significantly higher Cd content than the                        the lower layers. In the soil solution of sand and loamy
loamy sand and sand soils (Table 3, Fig. 1a,c). Among                          sand soils, the pH was below 5 (Fig. 2b). There was a
the arable soils, the sand soils had a lower Cd content                        tendency of the pH values to decrease with depth in
in the topsoil, than the loamy sand and sandy loam soils.                      arable soils and increase with depth in forest soils
This was not the case in the forest soils (Table 3).                           (Fig. 2b, d). The variations of the pH between texture
   The average Cd content in the 0- to 30-cm layer of                          classes in the forest soil solutions were small (Fig. 2d).
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                                               DISCUSSION
classes, but they were not significant (Table 3). No clear                                             Soil Solids
trends were found for the deeper layers.
   In the arable soils, pH was almost constant throughout                        The concentrations of Cd in the soils studied were
the profile except for a gradually decrease with depth in                      within the ordinary range for natural soils in Denmark,
the sandy loam soils (Fig. 1b). The upper 20 cm of the                         which were found to be 0.07 mg kg 1 for forested sand
forest soils was more acid compared with the subsoil                           soils and 0.22 mg kg 1 for arable loam soil (Bak et al.,
(Fig. 1d).                                                                     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
                         Soil Solutions                                        forested profiles. This suggests that agriculture tends to
   The Cd concentrations in soil solutions were in the                         enhance the Cd retention in the topsoil, or that the
range of 0.05 to 4.0 g L 1 (Fig. 2). In the arable                             arable soils receive a higher Cd input in the topsoil than
loamy sand soils, a significant higher concentration was                       the forest soils. A significantly higher Cd content was
found in the 0- to 30-cm layer than in the subsoil and                         found in arable soils than in forest soils within the upper
in the loamy sand forest soils significant higher concen-                      30 cm of the soil (p 0.046), but not within the upper
trations were found in the 0- to 30- and 30- to 60-cm                          120 cm (p       0.612) when all the texture classes were
layers than in the deeper layers (Table 3). The Cd con-                        included in the calculation. An average of 45% of the
centration did not differ between the four depths in the                       total Cd in the arable soil profile from 0 to 120 cm was
sand soils and the sandy loam soils. In the loamy sand                         located in the top 30 cm, whereas only an average of
soils the Cd concentration was higher in the upper soil                        30% was found in the upper 30 cm in forest soils. Higher
layers than in the lower soil layers (Table 3).                                contents of Cd in arable soils than in forest soils have
   The Cd concentrations in the soil solutions of ar-                          been reported previously (Bak et al., 1997; Romkens ¨
able soils were low ( 0.2 g L 1 ) in all texture classes                       and Salomons, 1998). The higher Cd contents found in
(Fig. 2a), whereas the solution concentrations of Cd in                        the top layers of the arable soils compared with the
the forest soils were higher (between 0.2 and 3.3 g L 1)                       forest soils are likely to originate from fertilizer used in
1834                                  SOIL SCI. SOC. AM. J., VOL. 66, NOVEMBER–DECEMBER 2002

Table 4. Regression models of the logarithm to the total Cd con-
  centration ( g g 1 ) in soil and the soil parameters effective
  cation-exchange capacity (ECEC, cmol        kg 1, clay, %, and
  organic C, g kg 1).
Model                                          R2          p
Arable and forest soils
  log Cd 1.206 0.008 ECEC 0.011 C
    0.033 Clay 0.001 ECEC Clay                 0.54       0.0001
Arable soils
  log Cd 1.256 0.010 ECEC                      0.61       0.0001
Forest soils
  log Cd 1.319 0.25 Clay 0.012 C               0.41       0.0001

intensive agriculture (Holmgren et al., 1993) or because
                                                                   Fig. 3. The concentration of Cd ( g L 1) versus pH in the soil solu-
of more intensive leaching from the forest profile                    tions from arable and forest soils. Log Cd     0.81     0.30pH,
(Bergkvist et al., 1989).                                             R2    0.39.
   The forested sand soils also had a significantly higher
content of Cd in topsoil than in subsoil, whereas this             concentration in soil solutions were found by Bergkvist
was not the case in the loamy sand and sandy loam soils            (1987) in Swedish soils and by Gooddy et al. (1995) in
(Table 3). The higher Cd content in the upper layers of            investigations of an acid sandy forest soil in southern
the forest sand soils was consistent with a high content           England. Solution concentrations above 1.0 g L 1 were
of soil organic matter. The organic-rich Bh-horizons               found in the forest soils from Jels, Løvbakke, and Strø-
were also enriched with Cd, suggesting that Cd is re-              dam that are all sandy loam or loamy sand soils. The
tained by organic matter as observed by Wilkens and                concentrations in these soils rose with depth as illus-
Loch (1997). For the forested sand soils, the higher               trated in Fig. 2c. In the sand soils, the general pattern
content of organic matter seemed to retain the Cd de-              of the Cd concentration was a decrease with soil depth
spite the low pH. This corresponds well with the models            (Fig. 2c). One single horizon in Baldersbæk had a high
presented in Table 4, where the soil C content is in-              concentration of Cd (4.0 g L 1).
cluded in the modeling of the Cd concentration in the                 Variations in Cd concentrations among texture
forest soils.                                                      classes were negligible in arable soils but pronounced
   Significantly higher Cd contents were found in the              in forest soils (Fig. 2a, c). The Cd concentrations in the
sandy loam soils than in the sand and loamy sand soils.            subsurface horizons of the forest soils were higher in
This was most pronounced in the forest soils (Table 3).            the sandy loam soils compared with the sand soils, but no
Higher clay contents cause a higher buffer capacity to-            significant difference between the texture classes were
wards acidification, and thus forest soils on sandy loam           found, presumably because of the few observations (Ta-
will exhibit a slower pH decrease, than more coarse-               ble 3). Thus, the concentration of mobile Cd in soil so-
textured soils. The higher total Cd content in the sandy           lution seems to increase with the content of clay in soil.
loam soil than in the sand and loamy sand soil can arise              The low Cd concentrations in soil solutions in all the
from a higher capacity to sorb the Cd added to the soil            arable profiles are attributed to the relatively high pH
and a higher Cd content in the parent material. This               (Fig. 2). The soil texture as well as the pH in soil solution
corresponds well with the significant correlations found           seemed to have an influence on the Cd concentration.
between the Cd concentration and clay and ECEC (Ta-                Lower pH values limit the accumulation of Cd in soil,
ble 4). The high correlation between the soil Cd concen-           and a significant correlation (p        0.0001, R 2     0.39)
tration and the ECEC in the arable soils and the clay              was found between the log Cd concentration and pH
content in the forest soils suggest that a high density of         in soil solution (Fig. 3). The pH was found not to have
sorption sites is important to Cd sorption (Table 4). In           any influence on the total Cd content in the soil, but a
a sequential extraction study of Cd in Danish forest and           significant influence on the Cd in solution, and thus on
arable soils, Andersen et al. (2002) found that Cd was             the solubility of Cd in soil. The general understanding
bound only in easily mobile fractions of the soil, which           has been that Cd solubility decreases at high pH and
eliminates the possibility that the high correlations be-          with high clay content that both favors a high sorption
tween Cd and clay content is caused by Cd being oc-                             ´            ´
                                                                   capacity (Sanchez-Martın and Sa    ´nchez-Camazano, 1993;
cluded in the clay minerals. Other authors have also               Boekhold et al., 1993; Egli et al., 1999; Sterckeman et
found that Cd concentration in soil correlates with                al., 2000). The Cd solubility in this study decreased with
                             ´            ´
ECEC and clay content (Sanchez-Martın and Sanchez- ´               increasing pH, but in contrast to the above findings an
Camazano, 1993; Springob and Bottcher, 1998; Stercke-
                                   ¨                               increasing Cd concentration in the soil solution was
man et al., 2000). The soil pH was not found to be sig-            found as the clay percentage increased. This may be
nificant when included in the statistical analyses, which          explained by the higher Cd content found in the loamy
leads to the conclusion that the Cd concentration is less          sand and sandy loam soils.
dependent on the soil pH (Egli et al., 1999).                         The present study reports actual field concentrations
                                                                   of Cd in unpolluted soils and soil solutions from various
                       Soil Solution                               soil types and horizons. The variable origin of Cd in soil
  The majority of the measured Cd concentrations in                makes the correlation with soil parameters more com-
soil solutions were below 1.0 g L 1. Similar levels of Cd                                                        ´
                                                                   plex (Sanchez-Camazano et al., 1998; Sauve et al., 2000).
                                ANDERSEN ET AL.: CONTENT, DISTRIBUTION, AND SOLUBILITY OF CADMIUM                                              1835

The significance of other soil variables than pH on the                     Holmgren, G.G.S., M.W. Meyer, R.L. Chaney, and R.B. Daniels.
                                                                                1993. Cadmium, lead, zinc, copper, and nickel in agricultural soils
Cd solubility has been reported with ECEC and organic
                                                                                of the United States of America. J. Environ. Qual. 22:335–348.
matter content being the most important ones (Buchter                       Hovmand, M., and K. Kemp. 1998. Rep. no. 313. p. 1–28. In Tungmetal
et al., 1989; Sauve et al., 2000).                                              nedfald i Danmark. (In Danish.) Danmarks Miljøundersøgelser,
   Cadmium mobility is increased by afforestation. De-                          Copenhagen.
creasing soil solution pH increase Cd in solution, but                      International Organization for Standardization. 1995. Soil quality—
                                                                                Extraction of trace elements soluble in aqua regia. ISO 11466. ISO,
high ECEC and content of clay and organic matter in                             Geneva, Switzerland.
the soil can counteract this by binding Cd in the soil.                     Jug, A., F. Makeschin, K.E. Rehfuss, and C. Hofmann-Schielle. 1999.
Leaching of soil solution with the concentration levels                         Short-rotation plantations of balsam poplars, aspen and willows
of Cd found in this investigation of soil with background                       on former arable land in the Federal Republic of Germany. III.
levels of Cd, does not pose a risk to the surrounding en-                       Soil ecological effects. For. Ecol. Manage. 121:85–99.
                                                                            Ma, L.Q., F. Tan, and W.G. Harris. 1997. Concentrations and distribu-
vironment.                                                                      tions of eleven metals in Florida soils. J. Environ. Qual. 26:769–775.
                                                                            McBride, M.B., S. Sauve, and W. Hendershot. 1997. Solubility control
                    ACKNOWLEDGMENTS                                             of Cu, Zn, Cd and Pb in contaminated soils. Eur. J. Soil Sci. 48:
  This research was partially supported by a grant from the                 Romkens, P.F.A.M., and W. Salomons. 1998. Cd, Cu, and Zn solubility
European Commission (FAIR3-CT96-1983) and a grant from                          in arable and forest soils: Consequences of land use changes for
the Danish Forest and Nature Agency.                                            metal mobility and risk assessment. Soil Sci. 163:859–871.
                                                                            Sanchez-Camazano, M., M.J. Sanchez-Martin, and L.F. Lorenzo. 1998.
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