Vertical Distribution of Trace Metals in Natural Soil Horizons from Japan Part 2: Effects of Organic Components in Soil

VERTICAL DISTRIBUTION OF TRACE METALS IN NATURAL SOILHORIZONS FROM JAPAN

PART 2: EFFECTS OF ORGANIC COMPONENTS IN SOIL

YOKO FUJIKAWA∗ and MASAMI FUKUIResearch Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-04094, Japan

(∗ author for correspondence, e-mail: [email protected]; fax: +81 724512620)

(Received 12 November 1997; accepted 21 December 2000)

Abstract. The association of Al, Mn, Fe, Ba, Zn, Cr, Ni, Co and Pb with soil organic matter (SOM)was investigated in three Japanese soils. Organically bound metals were assayed by elemental ana-lysis of a fraction extracted with acidified hydrogen peroxide (H2O2) and the humic acid extractedwith alkaline reagent, from soil sampled at various depths of solums. A Dystric Cambisol showedhigher extractability with acidified H2O2 for most of the metals than a Humic Andosol and anOrthic Acrisol. A Humic Andosol had more metals associated with humic acid than the other twosoils. Cu showed high extractability with acidified H2O2 and also significant association with humicacid, while Pb showed high extractability with acidified H2O2 but its association with humic acidwas relatively low among the metals investigated. As humic acid is highly resistant to weathering,retention of Cu with SOM may last longer than that of Pb. The binding of metals with SOM probablyhas contributed to the accumulation of some metals in organic-rich shallow horizons of soil. Suchmetals were Zn, Cu, Ni and Pb in a Dystric Cambisol, Cu in a Humic Andosol, and Pb and Cu in anOrthic Arisol.

Keywords: humic acid, selective extraction, soil organic matter, trace metals, vertical distribution

1. Introduction

Soil organic matter (SOM) retains various trace elements by the ion-exchange, pro-ton displacement, and inner or outer-sphere complex formation (Schnitzer, 1986).The association of metals with SOM in soil has been often investigated using se-quential selective extraction method (extensively reviewed by Beckett, 1989; Ross,1994). In this method, the fractionation of metals is operationally defined by theextractants that can or cannot dissolve them (Hickey and Kittrick, 1984). Resultsobtained depend on extracting reagents and method of applying them, character-istics of soil subjected to extraction, and chemical form of metals in solid phase.Inevitably, selective extraction method cannot be specific to a chemical form of themetals investigated, which has been addressed as ‘low selectivity’ (e.g. Emmerichet al., 1982; Miller and McFee, 1983; Hickey and Kittrick, 1984). The ‘low se-lectivity’ is governed by the combination of metals investigated and characteristicsof soil or sediments subjected to extraction. An extraction method, therefore, canbe effective for a group of metals but not for the other group of metals in a certain

Water, Air, and Soil Pollution 131: 305–328, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

306 Y. FUJIKAWA AND M. FUKUI

soil. Despite the limitations, selective extraction applied in sequential scheme hasbeen regarded as a useful tool to infer the chemical species of metals in soil andsediment.

Metal retention in topsoils has often been attributed to its reaction with SOMwhich is abundant in the shallow layers of soil (Marsh and Siccama, 1997). Theobjective of this work is to examine the effect of SOM on the vertical distributionof some metals in soil. To achieve the objective, the amount of metal associatedwith SOM was estimated by conducting extraction tests on soil samples collectedat 3–10 cm intervals to 70–90 cm depth from three solums in Japan.

In this work, selective extraction method using acidified H2O2, which was pro-posed by Gupta and Chen (1975) and Tessier et al. (1979), was applied in se-quential scheme to extract metals associated with SOM. The reagent was selectedbecause it contained less metal impurities compared with reagents such as sodiumpyrophosphate (Miller et al., 1986a) and sodium hypochlorite (Shuman, 1983).Recently the acidified H2O2 extraction method of Tessier et al. (1979) was adop-ted, after minor modification, in the so-called BCR method developed under theauspices of Community Bureau of Reference (Quevauviller et al., 1994).

As there was a possibility that the acidified H2O2 extraction was not selectivefor all of the metals investigated, the selectivity of extraction method were critic-ally evaluated before interpreting the obtained results. In this work, the correlationcoefficients were calculated between the organic carbon (OC) contents of soil andthe amount of metal extracted with the reagent from soil sampled at various depthsat each site. Only metals which exhibited statistically significant correlation at thesignificance level of 0.05 were discussed further regarding their association withSOM.

Amounts of the metal–humic acid complexes in soils were also evaluated in thepresent study by measuring the contents of metals contained in humic acid.

2. Materials and Methods

2.1. SOIL SAMPLES

The soil used were: (1) a brown forest soil (hereafter soil-b), Dystric Cambisols ofFAO/UNESCO soil classification (United Nations, Food and Agriculture Organiz-ation, 1988), sampled at Naka-machi, Ibaragi, Japan; (2) an Ando soil (hereaftersoil-d), meaning a dark-colored soil in Japanese classification, Humic Andosolsof FAO/UNESCO soil classification (1988), sampled at Naka-machi, Ibaragi, Ja-pan; and (3) a red and yellow soil (hereafter soil-KUR), Orthic Acrisols or Dys-tric Cambisols of FAO/UNESCO soil classification (1988), sampled at the site ofthe Kyoto University Reactor in Kumatori-cho, Osaka, Japan. All the soils weresampled at 3–10 cm intervals to a depth of 70–90 cm. A detailed description ofthese has been given in Fujikawa et al. (2000). The organic carbon contents of these

EFFECTS OF ORGANIC COMPONENTS IN SOIL 307

TABLE I

Organic carbon contents of soils

Soil-b Soil-d Soil-KUR

Depth Organic C Depth Organic C Depth Organic C

(cm) (mg kg−1) (cm) (mg kg−1) (cm) (mg kg−1)

0–3 82 0–5 83 0–3 16

3–10 30 5–10 63 3–5 11

10–20 22 10–20 53 5–10 13

20–30 14 20–30 40 10–15 7.8

30–40 7 30–40 39 15–20 7.2

40–50 5 40–50 44 20–30 3.9

50–60 5 50–57 42 30–40 3.5

60–70 4.5 57–63 41 40–50 2.3

63–70 42 50–60 2.3

60–70 3.8

70–80 2.2

80–90 N.D.

soils at each sampling depth estimated by dry combustion of samples (explained inSection 2.5) are shown in Table I.

2.2. REAGENTS

The nitric acid (68%) was for a class-100 environment, AA-100 grade, supplied byTama Chemical Co. (Tokyo, Japan). Hydrofluoric (40%) and hydrochloric (30%)acids were suprapur grade supplied by Merck (New Jersey, U.S.A.). NaOH ofmicroselect grade supplied by Fluka (Buchs, Switzerland) and tetra-methyl am-monium hydroxide (TMAH) of high purity grade supplied by Wako ChemicalsCo. (Osaka, Japan) were used to extract the humic acid. Palladium nitrate 99.999%supplied by Aldich (Wisconsin, U.S.A.) and magnesium nitrate (Suprapur grade)supplied by Merck were the matrix modifiers for electrothermal atomization atomicabsorption spectrometry (hereafter ETAAS). The other reagents used were of re-agent grade.

2.3. APPARATUS AND ANALYTICAL METHOD

Pneumatic tube of Kyoto University Research Reactor (Osaka, Japan) with thermalneutron flux of 2.75 × 1013 (n cm−2 sec−1) and germanium semi-conductivity gammaradioactivity detector manufactured by Canberra Industries (Connecticut, U.S.A.)were used for instrumental neutron activation analysis (INAA). Inductively coupled


plasma–atomic emission spectrometer ICPS-1000 TR manufactured by ShimadzuScientific Instruments, Inc. (Kyoto, Japan), electro-thermal atomization atomic ab-sorption spectrometer Z-9000 with Zeeman background correction manufacturedby Hitachi Instruments Service Co. (Tokyo, Japan), and inductively coupled plasma-mass spectrometer HP-4500 manufactured by Yokogawa Analytical Systems (To-kyo, Japan) were also used for metal analyses. X-ray diffractometer RINT-2000manufactured by Rigaku International Co. (Tokyo, Japan) was used to identifyminerals contained in soil.

INAA was conducted to analyze iron (Fe) and cobalt (Co), inductively coupledplasma–atomic emission spectrometry (hereafter ICP–AES) to analyze Fe, man-ganese (Mn), aluminum (Al), and inductively coupled plasma–mass spectrometry(ICP–MS) to analyze zinc (Zn), copper (Cu), chromium (Cr), nickel (Ni), Co, lead(Pb) and barium (Ba), in the humic acid samples. Metals in leachates were analyzedusing ICP–AES, ETAAS and ICP–MS. Prior to analysis by ICP–MS, the humicacid was decomposed with nitric acid in a sealed Teflon vessel under microwaveirradiation in an MDS-2000 microwave oven manufactured by CEM Co. (NorthCarolina, U.S.A.) at a pressure of 4.2 kgf cm−2 for 30 min.

Trace metals in the soil were analyzed as described in Fujikawa et al. (2000).The total organic carbon content in the humic acid was measured using total

organic carbon analyzer TOC-5000A manufactured by Shimadzu Scientific Instru-ments, Inc. that was equipped with an infrared spectrophotometer to detect the CO2

generated by dry combustion of the samples.

2.4. EXTRACTION OF TRACE METALS BOUND TO SOIL ORGANIC MATTER

Trace metals associated with SOM by mechanisms other than electrostatic interac-tion were extracted with an acidified H2O2 reagent (Gupta and Chen, 1975; Tessieret al., 1979). After sequential extraction of soil with 1 N ammonium acetate (pH7) and 0.2 M NH2OH-HCl in 25% acetic acid to extract metals sorbed to soilelectrostatically and those associated with amorphous iron or manganese oxides(Fujikawa et al., 2000), 1 g of the soil was first extracted with 2 mL of 0.02 MHNO3 combined with 5 mL of 30% H2O2 (pH 2) for 2 hr at 85 ◦C (Gupta andChen, 1975). Three milliliters of 30% H2O2 (pH 2) were then added and extractionwas done for 3 hr at 85 ◦C. Finally 5 mL of 3.2 M ammonium acetate in 20% v/vHNO3 and water was added to dilute the cooled leachate to 20 mL, and extractionwas done for 30 min at ambient temperature.

In the above sequential extraction scheme, extractions with 1 N ammoniumacetate and 0.2 M NH2OH-HCl in 25% acetic acid were optimized by (1) conduct-ing extraction until transient equilibrium was obtained for the metal concentrationsin the extractant (equilibrium was attained for Ca, Mg and Pb after 1 week ex-traction with 1 N ammonium ecetate), and (2) adopting 0.2 M NH2OH-HCl in25% acetic acid to extract amorphous iron and manganese oxides after preliminary


experiment with 0.04, 0.2 and 2 M in NH2OH-HCl concentrations (Fujikawa et al.,2000).

2.5. EXTRACTION OF SOIL HUMIC ACID

Upon extracting humic acid from soil samples, the volume of extractant appliedto unit amount of soil was optimized based on the organic carbon content of soilin order to maximize the extraction efficiency, after ‘Nagoya method’ proposed byKuwatsuka et al. (1992). NaOH concentration was increased to 0.5% from 0.4% inthe original Nagoya method. The Nagoya method is different from that of widelyused IHSS method (International Humic Substances Society, 1981) in that whilethe latter applies 0.1 N NaOH solution to soil with 10 times the volume (mL)of soil weight (g), e.g., 10 mL solution for 1 g of soil, the former uses the samesolution with 300 times (mL) the weight (g) of organic carbon in soil, e.g., 300 mLof solution for soil that contains 1 g of organic carbon. The ratio of extractantvolume to weight of soil in Nagoya method, therefore, varies with the organiccarbon contents of soil. The extraction efficiency of humic substances in Nagoyamethod was reported to tbe higher than that in IHSS method (Kuwatsuka et al.,1992).

Humic acid was extracted by adding an alkaline reagent (0.5% NaOH or 5%TMAH) to a soil sample and heating the whole at 85 ◦C for 30 min under an N2

atmosphere. To maximize the recovery of humic acid, soil sample extracted by30 mL of the extractant was no more then 7.5 g and contained maximum 100 mgof organic carbon.

The supernatant, obtained after centrifuging the extractant at 10,000 rpm(16,000 G) and 4 ◦C for 30 min twice, was filtered through a 0.45 µm pore mem-brane filter made of Teflon to remove soil particles. The filtrate was acidified topH 1 with HCl to separate fulvic acid from humic acid. The precipitated fractionconsisted of humic acid and metals, and fulvic acid that were co-precipitated withhumic acid. The precipitate was further purified by two dissolutions in NaOH andre-precipitations with HCl to separate fulvic acid from humic acid. Then the precip-itate was de-ashed by shaking with 0.1 M HCl + 0.3 M HF for three days, rinsedwith double distilled water and freeze-dried at 30 ◦C, yielding powdered humicacid.

Humic acid was not extracted from soil-KUR and soil-b at depths below 10 cm,at which little organic carbon was present. As for soil-d, humic acid was extractedto a depth of 60 cm because this soil had relatively abundant organic matter. Theaverage organic carbon concentrations of the humic acids extracted from soil-b,soil-d and soil-KUR were 41.3, 25.7 and 20.1%, respectively.

Use of 0.5% NaOH or 5% TMAH to extract humic acid gave virtually identicalresults in terms of the trace metal concentrations in the humic acid, although thelatter reagent contained fewer impurities (and was much more expensive) than the


former. Therefore, in most cases, we used NaOH to extract the humic acid fromthe soil samples.

To obtain an approximate estimate of the amount of organic carbon in the soil,a sample dried at 110 ◦C was dry combusted at 375 ◦C for 16 hr. The amountof organic carbon in the sample was estimated from the empirical equation forJapanese soils (Department of Agricultural Chemistry, Tokyo University, 1988):

y = 0.458x − 0.4 (1)

where y (mg) is the amount of organic carbon in the soil, and x (mg) the decreasein the soil mass after combustion.

3. Results and Discussion

3.1. EVALUATION OF EXTRACTION METHODS

3.1.1. Selective Extraction Method Using Acidified H2O2

The present study applied the acidified H2O2 after sequential extraction of ex-changeable metal fraction and amorphous iron and manganese fraction as proposedby Tessier et al. (1979). This sequential extraction technique has been developedbecause acidified H2O2 not only destroys organic matter and thereby releases tracemetals retained with it, but also dissolves sulfides (Gupta and Chen, 1975; Tessieret al., 1979), some oxides of manganese (Shuman, 1979; Miller et al., 1986b)and carbonates (Miller et al., 1986a). Major modification in this study was thatcarbonate fraction was not extracted beforehand because the acid Japanese soils innatural environment generally did not contain carbonates.

We tried to examine whether the amount of metals extracted with the acidifiedH2O2 reagent reflected the fractions of metals bound to SOM or not. This is be-cause information obtainable from selective extraction was often affected by lowselectivity of extracting reagents (Qiang et al., 1994; Ramos et al., 1994). Hereselectively of extracting reagents to metals can be defined as an ability to dissolvediscrete phase or fraction such as carbonates, exchangeable forms, those sorbed toiron and manganese oxide, and those sorbed to organic phase. Sequential extractionscheme has been invented to overcome low selectivity problem.

Studies using synthetic model soils and sediments, however, have shown thateven under the sequential scheme, an extractant does not always attack an uniquephase. Mechanisms that compromise the fractionation of metals based on their ex-tractability are (1) re-distribution (post-extraction redistribution) of once extractedfraction to other phases (Lo and Yang, 1998; Rendell et al., 1980; Kheboian andBauer, 1987; Raksasataya et al., 1996), (2) insufficient extraction from specificphase (Kheboian and Bauer, 1987; Qiang et al., 1994), (3) extraction in the previ-ous phase (Lo and Yang, 1998), and (4) alteration of chemical forms of metalsthrough extraction (Gruebel et al., 1988). The post-extraction redistribution of


metal has been studied most and shown to be a function of nature of extractionscheme, nature of the metal investigated and characteristics of soil and sedimentextracted. It sometimes depends on time after extraction was started (Litaor andIbrahim, 1996) and the amount of manganese oxide and humic acid in the solidphase (Raksasataya et al., 1996). For example, in the case of Pb, organic matterand Mn and Fe oxides were the principal destination phase of Pb redistribution(Kheboian and Bauer, 1987; Ajayi and Vanloon, 1989). Tessier’s extraction schemereportedly resulted in less redistribution of Pb at iron and manganese oxide extrac-tion step than BCR method, because of the higher temperature applied (96 ◦C) andpresence of acetic acid (Raksasataya et al., 1996). As there are variety of mech-anisms that may afflict the selectivity of an extraction procedure, each procedureshould be examined critically regarding its specificity to a chemical form of metalin the solid phase before proceeding further to interpretation of obtained results.

The amounts (mg kg−1-soil) of metals extracted from soil with acidified H2O2

and ratios of metals extracted with the reagent to total amount in the soil are shownin Table II. Amounts (mg kg−1) and percentages extracted were weighted–averagedover depths where humic acid was extracted (0–10 cm for soil-b and soil-KUR, and0–60 cm for soil-d) and are shown in the table. The percentage were higher in theorder of Co > Mn > Cu > Pb > Ni > Zn > Cr > Al > Fe in soil-b, Cu > Al > Pb >Co > Fe > Zn in soil-d, and Pb > Co > Zn > Cr > Fe > Al in soil-KUR.

In order to evaluate whether metals extracted with acidified H2O2 are associ-ated exclusively with SOM or not, linear correlation coefficients were calculatedbetween the amounts of organic carbon in soil and the amounts (mg kg−1-soil)of metals extracted with acidified H2O2 in each of the three soils (see Table III).Statistical significance for the correlation coefficient (ρ) was tested by the nullhypothesis that ρ = 0. The null hypothesis is rejected at a significance level of0.05 when ρ is higher than 0.71, 0.67 and 0.55 for soil-b, soil-d and soil-KUR,respectively (Brownlee, 1960). In discussing the results of acidified H2O2 extrac-tion regarding association of metals with SOM, we only dealt with metals that hadstatistically significant values for ρ (Table III). Such metals were Al, Mn, Fe, Zn,Cu, Cr, Ni, Co and Pb (soil-b), Al, Fe, Zn, Cu, Co and Pb (soil-d), Al, Fe, Zn, Cr,Co and Pb (soil-KUR).

As for metals with low ρ values in Table III, i.e. Ba (soil-b), Mn, Ba, Cr and Ni(soil-d), and Mn, Ba, Cu and Ni (soil-KUR), the amount extracted with acidifiedH2O2 may not precisely represent the fraction specifically associated with SOM.Probable reasons of low correlation coefficient for those metals are: (1) solubiliza-tion of part of SOM in the earlier step, as have been observed for Pb and Zn by Loand Yang (1998), for Pb by Raksasataya et al. (1996), and for Ni and Zn by Whalleyand Grant (1994); (2) metals left in solid phase by post-extraction redistribution inthe previous step were released at the acidified H2O2 extraction step (Kheboian andBauer, 1987; Ajayi and Vanloon, 1989); and (3) extraction in previous phase, e.g.Fe and Mn oxide extraction with reducing reagents, was insufficient (Kheboian and


TAB

LE

II

Res

ults

ofac

ififi

edH

2O

2ex

trac

tion

.Am

ount

(mg

kg−1

)of

met

alex

trac

ted

wit

hac

idifi

edH

2O2

from

soil

and

perc

enta

geof

the

amou

ntex

trac

ted

wit

hth

ere

agen

tto

tota

lmet

alco

nten

tsof

soil

are

give

n

Dep

thA

lin

soil

-bM

nin

soil

-bF

ein

soil

-bB

ain

soil

-ba

Zn

inso

il-b

(cm

)%

mg

kg−1

%m

gkg

−1%

mg

kg−1

%m

gkg

−1%

mg

kg−1

0–3

8.0

4354

.530

.264

3.9

5.6

1974

.99.

223

.619

.26.

4

3–10

3.4

1863

.324

.439

2.6

0.8

298.

85.

614

.55.

32.

4

10–2

03.

115

53.3

30.5

500.

30.

313

5.0

5.8

14.7

4.1

1.6

20–3

02.

713

89.3

26.7

363.

00.

286

.94.

912

.62.

70.

7

30–4

00.

630

1.5

3.4

33.1

0.0

11.0

1.5

3.9

0.0

0.0

40–5

01.

355

3.8

16.3

144.

40.

129

.69.

725

.21.

00.

2

50–6

00.

966

1.9

24.1

174.

60.

135

.46.

020

.20.

20.

1

60–7

01.

268

3.2

22.3

168.

70.

00.

09.

623

.90.

00.

0

Wei

ghte

dav

erag

e4.

861

0.7

26.1

468.

02.

280

1.6

6.7

17.2

9.5

3.6

over

dept

hof

0–10

cm

aC

orre

lati

onbe

twee

nth

eam

ount

sex

trac

ted

wit

hac

idifi

edH

2O2

and

orga

nic

carb

onco

nten

tsof

soil

sw

asst

atis

tica

lly

insi

gnifi

cant

for

the

met

al.


TAB

LE

II

(con

tinu

ed)

Dep

thC

uin

soil

-bC

rin

soil

-bN

iin

soil

-bC

oin

soil

-bP

bin

soil

-b

(cm

)%

mg

kg−1

%m

gkg

−1%

mg

kg−1

%m

gkg

−1%

mg

kg−1

0–3

33.5

8.1

7.9

3.5

16.8

1.3

38.1

5.7

19.6

7.6

3–10

17.6

3.8

7.6

2.4

11.5

0.5

34.3

5.6

28.8

7.5

10–2

010

.52.

15.

61.

79.

20.

529

.84.

922

.35.

0

20–3

05.

41.

05.

92.

612

.30.

623

.34.

313

.41.

7

30–4

00.

80.

10.

30.

13.

20.

14.

01.

01.

20.

1

40–5

04.

10.

74.

71.

510

.40.

59.

52.

914

.21.

8

50–6

04.

90.

84.

41.

415

.80.

617

.53.

110

.91.

1

60–7

04.

20.

84.

71.

410

.40.

48.

62.

511

.21.

1

Wei

ghte

dav

erag

e22

.45.

17.

72.

713

.10.

835

.45.

626

.07.

5

over

dept

hof

0–10

cm

aC

orre

lati

onbe

twee

nth

eam

ount

sex

trac

ted

wit

hac

idifi

edH

2O2

and

orga

nic

carb

onco

nten

tsof

soil

sw

asst

atis

tica

lly

insi

gnifi

cant

for

the

met

al.


TAB

LE

II

(con

tinu

ed)

Dep

thA

lin

soil

-dM

nin

soil

-da

Fe

inso

il-d

Ba

inso

il-d

aZ

nin

soil

-d

(cm

)%

mg

kg−1

%m

gkg

−1%

mg

kg−1

%m

gkg

−1%

mg

kg−1

0–5

13.0

5530

.230

.424

3.2

1.3

484.

722

.032

.81.

21.

1

5–10

10.3

5324

.219

.728

8.2

0.6

386.

426

.735

.70.

60.

4

10–2

010

.956

58.2

16.0

256.

40.

634

7.7

24.0

40.5

0.6

0.4

20–3

08.

750

10.2

17.8

285.

40.

422

4.6

28.4

48.1

0.0

0.0

30–4

06.

542

48.2

17.5

271.

60.

319

1.4

24.5

47.2

0.0

0.0

40–5

07.

849

60.2

4.0

63.7

0.4

251.

824

.940

.90.

00.

0

50–5

78.

947

68.2

4.3

59.8

0.5

273.

025

.438

.90.

00.

0

57–6

35.

842

18.2

5.7

87.8

0.3

187.

723

.642

.20.

00.

0

63–7

06.

546

46.2

4.9

76.4

0.6

322.

323

.539

.60.

00.

0

Wei

ghte

dav

erag

e8.

849

48.1

13.8

196.

40.

527

8.5

25.1

41.8

0.2

0.2

over

dept

hof

0–63

cm

aC

orre

lati

onbe

twee

nth

eam

ount

sex

trac

ted

wit

hac

idifi

edH

2O2

and

orga

nic

carb

onco

nten

tsof

soil

sw

asst

atis

tica

lly

insi

gnifi

cant

for

the

met

al.


TAB

LE

II

(con

tinu

ed)

Dep

thC

uin

soil

-dC

rin

soil

-da

Nii

nso

il-d

aC

oin

soil

-dP

bin

soil

-d

(cm

)%

mg

kg−1

%m

gkg

−1%

mg

kg−1

%m

gkg

−1%

mg

kg−1

0–5

27.3

9.9

5.0

2.1

5.5

0.5

8.2

1.6

15.9

4.2

5–10

29.0

9.4

7.4

2.8

5.8

0.5

8.6

1.4

15.6

3.2

10–2

023

.47.

97.

02.

54.

90.

59.

21.

69.

71.

8

20–3

017

.55.

07.

92.

45.

90.

55.

61.

18.

70.

8

30–4

015

.94.

38.

92.

53.

10.

25.

00.

95.

30.

5

40–5

04.

41.

37.

02.

53.

10.

21.

10.

23.

80.

3

50–5

74.

31.

36.

72.

03.

20.

21.

00.

22.

70.

2

57–6

34.

31.

26.

92.

22.

20.

21.

10.

21.

60.

1

63–7

04.

71.

27.

12.

43.

20.

21.

60.

37.

30.

7

Wei

ghte

dav

erag

e15

.14.

77.

32.

44.

10.

34.

90.

97.

31.

2

over

dept

hof

0–63

cm

aC

orre

lati

onbe

twee

nth

eam

ount

sex

trac

ted

wit

hac

idifi

edH

2O2

and

orga

nic

carb

onco

nten

tsof

soil

sw

asst

atis

tica

lly

insi

gnifi

cant

for

the

met

al.


TAB

LE

II

(con

tinu

ed)

Dep

thA

lin

soil

-KU

RM

nin

soil

-KU

Ra

Fein

soil

-KU

RB

ain

soil

-KU

Ra

Zn

inso

il-K

UR

(cm

)%

mg

kg−1

%m

gkg

−1%

mg

kg−1

%m

gkg

−1%

mg

kg−1

0–3

0.6

386.

68.

136

.24.

155

8.0

1.9

11.0

11.1

8.7

3–5

0.7

394.

48.

748

.52.

035

4.0

2.0

11.7

9.6

7.6

5–10

0.4

329.

28.

049

.81.

024

3.0

1.7

9.6

3.4

3.6

10–1

50.

425

3.2

23.1

142.

70.

512

0.0

5.8

28.0

2.4

2.1

15–2

00.

324

7.6

13.0

111.

40.

250

.04.

318

.82.

31.

9

20–3

00.

328

7.4

13.3

95.1

0.1

52.0

3.3

14.0

5.2

3.6

30–4

00.

425

8.6

8.6

35.3

0.1

48.0

1.1

5.6

7.9

4.9

40–5

00.

324

1.6

2.9

15.9

0.4

92.0

0.5

3.0

2.2

1.8

50–6

00.

426

4.2

7.7

35.5

0.3

72.0

1.4

6.3

2.2

1.8

60–7

00.

319

0.9

7.1

33.9

0.3

59.6

0.7

4.0

2.0

1.7

70–9

00.

324

1.0

2.7

10.7

0.2

48.2

0.4

2.2

1.8

1.5

80–9

00.

216

4.8

1.4

2.0

0.2

49.0

0.2

1.3

1.2

0.9

Wei

ghte

dav

erag

e0.

535

9.5

8.2

45.4

2.1

359.

71.

810

.57.

05.

9

over

dept

hof

0–10

cm

aC

orre

lati

onbe

twee

nth

eam

ount

sex

trac

ted

wit

hac

idifi

edH

2O2

and

orga

nic

carb

onco

nten

tsof

soil

sw

asst

atis

tica

lly

insi

gnifi

cant

for

the

met

al.


TAB

LE

II

(con

tinu

ed)

Dep

thC

uin

soil-

KU

Ra

Cr

inso

il-K

UR

Nii

nso

il-K

UR

aC

oin

soil

-KU

RPb

inso

il-K

UR

(cm

)%

mg

kg−1

%m

gkg

−1%

mg

kg−1

%m

gkg

−1%

mg

kg−1

0–3

0.5

0.1

6.3

1.4

7.5

0.8

10.9

1.5

69.5

3.7

3–5

0.2

0.0

5.6

1.5

2.0

0.3

18.3

1.4

100.

03.

7

5–10

0.6

0.1

1.9

1.3

0.0

0.0

4.8

1.1

9.1

3.6

10–1

52.

30.

51.

20.

84.

81.

57.

61.

413

.14.

2

15–2

02.

30.

51.

00.

60.

10.

08.

31.

510

.93.

4

20–3

03.

10.

60.

80.

70.

70.

48.

81.

412

.13.

2

30–4

02.

90.

51.

00.

70.

00.

04.

50.

98.

82.

5

40–5

03.

40.

51.

10.

60.

00.

02.

20.

67.

42.

0

50–6

03.

80.

70.

80.

50.

00.

04.

50.

97.

92.

6

60–7

02.

30.

40.

50.

30.

00.

02.

71.

16.

61.

6

70–8

01.

40.

30.

80.

50.

00.

02.

70.

810

.41.

2

80–9

00.

30.

10.

70.

40.

00.

00.

70.

29.

21.

6

Wei

ghte

dav

erag

e0.

50.

154.

01.

42.

70.

39.

31.

345

.43.

6

over

dept

hof

0–10

cm

aC

orre

lati

onbe

twee

nth

eam

ount

sex

trac

ted

wit

hac

idifi

edH

2O

2an

dor

gani

cca

rbon

cont

ents

ofso

ils

was

stat

isti

call

yin

sign

ifica

ntfo

rth

em

etal

.


TABLE III

Linear correlation coefficient ρ between the organiccarbon contents of soil and the amounts of metalsextracted from soil with acidified H2O2

Soil-b Soil-d Soil-KUR

Al 0.99 0.72 0.80

Mn 0.84 0.40∗1 0.22∗1

Fe 0.97 0.92 0.88

Ba 0.22∗1 –0.80∗1 0.40∗1

Zn 1.00 0.98 0.71

Cu 0.95 0.81 –0.76∗1

Cr 0.78 –0.10∗1 0.87

Ni 0.88 0.65∗1 0.34∗1

Co 0.71 0.69 0.65

Pb 0.82 0.96 0.76

∗1 The null hypothesis ρ = 0 was not rejected (i.e. nocorrelation between amount of metals extracted withacidified H2O2 and the organic carbon content of soil)or ρ was negative.

Bauer, 1987; Quiang et al., 1994), leaving some metals to be extracted at the laterextraction step.

Destruction of soil mineral structure caused by consecutive extractions wasconsidered to be minor in the present study although it has been a matter of generalconcern. At least, there were no clear difference among X-ray diffraction spectraof soil-b, -d and -KUR before and after extraction with ammonium acetate, 0.2 MNH2OH-HCl in 25% acetic acid, and acidified H2O2. These treatments were tooweak to affect the structures of clay mineral and other principal minerals.

Table III also suggests that the optimum procedure to extract metals associatedwith SOM probably differs between soils. Under the same extraction scheme, nineout of ten metals showed statistically significant ρ values in soil-b, whereas onlysix out of ten metals had statistically significant ρ values in soil-d and soil-KUR.Obviously the present extraction scheme is more adequate for soil-b than for soil-dand soil-KUR concerning the metals investigated in this work.

Extraction of soil samples from different depths of solum, as conducted inthis study, was useful to evaluate the selectivity of the extraction technique toorganically complexed form of metals. This is because a solum derived from oneparent material and consequently has similar soil mineralogy throughout the depthis expected to contain SOM that has relatively constant concentration of metalsirrespective of their sampling depth. Also, a solum usually contains varying amountof SOM depending on the depth. In such case, whether an extractant is attacking


organic materials specifically or not can be inferred from correlation between SOMcontent and amount of metal extracted.

3.1.2. Extraction of Humic Acid from SoilHumic acid is stable organic matter which is resistant to weathering in oxidative en-vironment. By evaluating the amount of metal contained in humic acid fraction, weintended to identify the long-term fate of metals which were likely to be governedby their speciation as metal-organic complexes.

Multiple mechanisms exist in the association of metals with soil humic acid(Senesi et al., 1989; Warsaw, 1993; Schnitzer, 1995): (1) outer-sphere complexformation, i.e., a relatively weak electrostatic interaction of metals principally withcarboxylic or phenolic groups in humic acid; (2) inner-sphere complex formation,i.e., strong interaction of dehydrolyzed metals with ligands in humic acid; (3) phys-ical adsorption such as inclusion of inorganic particles in three-dimensional pocket-like structure in humic acid and precipitation of metals on humic acid molecules.

For the extraction of humic acid, we used NaOH rather than Na4P2O7 as anextractant in this study, because the latter reagent is known to induce structuralmodification of humic acid through inclusion of pyrophosphate into it, resulting inhumic acid more abundant in metals than that extracted with NaOH (Francioso etal., 1998).

De-ashing of humic acid was of prime interest in this study because extractedhumic acid was considered to contain inorganic impurities which originally werenot chemically associated with it, either by co-precipitation of metals leached frommineral phase of soil by the extractant, or by trapping of small mineral particlessuch as clay within voids of the precipitated humic acid. Under the alkaline con-dition when humic acid is extracted, some of iron and silicate minerals are knownto be solubilized (Tole et al., 1986; Chou and Wollast, 1984), leading to principalmetals such as Al and Fe and trace metals associated with them being leachedwith humic substances. Because dissolved organic carbon co-precipitates with Aland Fe at low pH (Dolfing et al., 1999), metals solubilized by alkaline extractantprobably will be transferred to humic acid fraction in a step of precipitating humicacid at pH 1 to separate it from fulvic acid. Unless these co-precipitated metalsare eliminated from the humic acid fraction, metal contents of humic acid will notproperly reflect the amount of metals chemically associated with it.

In this work we de-ashed humic acid by washing it with HCl and HF (Piccolo,1988). By de-ashing of humic acid, we intended to eliminate metals that werecoprecipitated with it, those that were physically sorbed to it, and those that wereassociated with it by electrostatical interaction as far as possible, and leave onlythose that were associated with humic acid by strong chemical bond, for example,as inner-sphere complex or chelate.

The percentage (R) of a metal contained in the humic acid to the amount of themetals in the soil is calculated from the following equation:


R = 100 mhEh/(msEs) (2)

where mh and ms are the metal concentrations [mg kg−1] in the humic acid and soilrespectively, and Eh is the amount (kg) of humic acid extracted from soil (Es (kg)).The value estimated using Equation (2) can be an underestimate, because recoveryof humic acid from soil samples is not 100%. But as we optimized the extractionprocedure by controlling amount of soil (g): volume of extractant (mL) ratio basedon organic carbon contents of soil samples (refer to the Section Materials andMethods), recovery of humic acid is expected to be high in this work.

The R values of Al, Mn, Fe, Zn, Cu, Cr, Ni, Co, Pb and Ba (calculated fromEquation (2)) are shown in Table IV together with metal contents of humic acid.The ratios of metals present as metal-humic acid complex were higher in the orderof Cu > Ni � Fe � Co � Zn � Cr � Pb � Al � Mn > Ba (not detected) in soil-b,Cu > Ni > Fe � Co � Zn � Cr � Pb � Al � Mn > Ba (not detected) in soil-d, andCu > Co > Ni � Fe � Zn � Cr > Al � Mn � Pb > Ba (not detected) in soil-KUR.

3.2. ASSOCIATION OF METALS WITH SOM

Features of metal association with SOM in soil-b, -d and -KUR clarified fromanalysis of acidified H2O2 extractable fraction and humic acid are summarized inthe following subsections. As for the results of acidified H2O2 extraction, onlymetals that showed statistically significant correlation coefficient in Table III werediscussed.

3.2.1. Difference Between SoilsRatios of the metals extracted with acidified H2O2 to their total amount in soil werehighest in soil-b among the three soils for most of the metals (Table II). Of the threesoils, soil-d has the most SOM being present to a depth of more than 60 cm (seeTable I), but the amounts of metals extracted with acidified H2O2 from soil-d werelower than those from soil-b. Namely, the percentages of metals extracted withacidified H2O2 were higher than 10% for Mn, Pb, Ni, Zn, Cu and Co in soil-b,while they were higher than 10% only for Cu (soil-d) and Co and Pb (soil-KUR)in the other soils. Soil-KUR is the poorest in SOM of the three soils, as shown byorganic carbon contents in Table I. The ratio of Pb extracted with acidified H2O2

from soil-KUR is the highest among these soils, nevertheless, and ratios of Feand Zn extracted with acidified H2O2 from soil-KUR were higher than those fromsoil-d which had higher organic carbon content.

Regarding binding of metal with humic acid, more metals were contained inhumic acid from soil-d than that from soil-b and soil-KUR. The ratios of metalscontained in humic acid to the total amount of metal in soil are highest in soil-d forCu, Cr, Zn and Pb, in soil-b for Fe, Ni and Co, and in soil-KUR for Co.

In our study, comparison among soil-b, soil-d and soil-KUR, of which theformer two had higher OC content than the latter one, has shown that generallysoil rich in organic carbon has higher ratio of metal association with SOM. An


TAB

LE

IV

Res

ults

ofhu

mic

acid

anal

yses

.Met

alco

nten

tof

hum

icac

idan

dth

epe

rcen

tage

ofam

ount

ofm

etal

cont

aine

din

hum

icac

idto

tota

lmet

alco

nten

tsof

soil

are

give

n

Dep

thA

lin

soil-

bM

nin

soil-

bFe

inso

il-b

Ba

inso

il-ba

Zn

inso

il-b

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

(cm

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

(%)

(%)

(%)

(%)

(%)

0–3

0.1

5306

.80.

011

3.4

0.8

3134

0.2

N.D

.aN

.D.

0.2

19.7

3–10

0.0

0.0

0.0

10.2

0.0

3502

.2N

.D.

N.D

.0.

660

.4

Wei

ghte

d0.

015

92.0

0.0

41.2

0.3

1185

3.6

N.D

.N

.D0.

548

.2

aver

age

Dep

thC

uin

soil-

bC

rin

soil-

bN

iin

soil-

bC

oin

soil-

baPb

inso

il-b

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

(cm

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

(%)

(%)

(%)

(%)

(%)

0–3

1.4

39.5

0.2

10.9

0.5

3.1

0.8

13.2

0.1

2.9

3–10

1.1

50.1

0.0

2.3

0.5

5.0

0.2

8.2

0.1

4.3

Wei

ghte

d1.

246

.90.

14.

90.

54.

40.

49.

70.

13.

9

aver

age

aN

.D.=

Not

dete

cted

.


TAB

LE

IV

(con

tinu

ed)

Dep

thA

lin

soil

-dM

nin

soil

-dF

ein

soil

-dB

ain

soil

-da

Zn

inso

il-d

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

(cm

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)(%

)(%

)(%

)(%

)(%

)

0–5

0.0

429.

50.

04.

90.

030

23.3

N.D

.N

.D.

0.0

0.1

5–10

0.0

241.

80.

02.

40.

231

53.8

N.D

.N

.D.

0.4

12.2

10–2

00.

035

2.3

0.0

3.4

0.1

3423

.3N

.D.

N.D

.0.

00.

020

–30

0.0

492.

00.

05.

40.

141

44.9

N.D

.N

.D.

0.0

0.0

30–4

00.

045

3.3

0.0

4.6

0.1

3638

.5N

.D.

N.D

.0.

00.

0

40–5

00.

040

6.0

0.0

4.7

0.1

3804

.5N

.D.

N.D

.0.

523

.4

50–5

70.

026

9.6

0.0

1.7

0.0

2232

.0N

.D.

N.D

.0.

16.

957

–63

0.0

463.

40.

03.

00.

137

86.0

N.D

.N

.D.

0.0

0.0

Wei

ghte

d0.

039

7.8

0.0

3.9

0.1

3481

.6N

.D.

N.D

.0.

15.

5

aver

age

Dep

thC

uin

soil

-dC

rin

soil

-dN

iin

soil

-dC

oin

soil

-da

Pb

inso

il-d

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

(cm

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)w

ith

HA

(mg

kg−1

)

(%)

(%)

(%)

(%)

(%)

0–5

1.2

80.5

0.1

8.6

0.1

1.9

0.1

4.6

0.1

6.7

5–10

6.8

89.2

0.4

6.5

1.2

3.6

0.7

4.6

0.9

7.5

10–2

05.

410

3.3

0.3

5.5

0.3

1.5

0.3

3.1

0.1

1.3

20–3

04.

014

5.3

0.2

8.4

0.3

2.6

0.1

2.4

0.1

1.2

30–4

04.

615

9.4

0.3

9.0

0.6

5.8

0.0

1.0

0.1

1.7

40–5

04.

615

4.3

0.2

6.3

0.4

3.9

0.0

1.1

0.0

0.3

50–5

73.

913

6.6

0.2

6.2

0.2

1.3

0.0

1.3

0.0

0.0

57–6

34.

515

2.2

0.1

5.4

0.1

1.4

0.0

1.0

0.0

0.0

Wei

ghte

d4.

513

2.4

0.2

7.0

0.4

2.9

0.2

2.2

0.1

1.9

aver

age

aN

.D.=

Not

dete

cted

.


TAB

LE

IV

(con

tinu

ed)

Dep

thA

lin

soil-

KU

RM

nin

soil-

KU

RFe

inso

il-K

UR

Ba

inso

il-K

UR

aZ

nin

soil-

KU

R

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

(cm

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

(%)

(%)

(%)

(%)

(%)

0–3

0.0

134.

20.

03.

10.

118

22.6

N.D

.N

.D.

0.1

9.8

5–10

0.0

134.

10.

03.

10.

026

61.8

N.D

.N

.D.

0.0

0.0

Wei

ghte

d0.

013

4.1

0.0

3.1

0.1

2347

.1N

.D.

N.D

.0.

03.

7

aver

age

Dep

thC

uin

soil-

KU

RC

rin

soil-

KU

RN

iin

soil-

KU

RC

oin

soil-

KU

Ra

Pbin

soil-

KU

R

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

Met

alM

etal

cont

ent

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

asso

ciat

edof

HA

(cm

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

with

HA

(mg

kg−1

)w

ithH

A(m

gkg

−1)

(%)

(%)

(%)

(%)

(%)

0–3

3.7

67.0

0.3

6.6

0.2

1.9

0.7

11.1

0.1

2.0

5–10

1.4

89.6

0.0

8.4

0.0

2.5

0.2

14.1

0.0

1.9

Wei

ghte

d2.

281

.10.

17.

70.

12.

30.

413

.00.

02.

0

aver

age

aN

.D.=

Not

dete

cted

.


TABLE V

Comparison among the results of acidifified H2O2 extraction and humic acid extraction in presentstudy and values obtained by selective extraction of organic matter in the other study. Percentages ofmetals extracted with acidified H2O2 and those present in humic acid to total metal contents in soilwere weighted–averaged over depths where humic acid was extracted. Values in parentheses [ ] arethose of metals which showed low correlation coefficient values in Table III

Acidified H2O2 extraction Humic acid Literature value

soil-b soil-d soil-KUR soil-b soil-d soil-KUR

(%)

Al 4.8 8.8 0.5 0.0 0.0 0.0 <1%∗1

Mn 26.1 [13.8] [8.2] 0.0 0.0 0.0 0.1%∗2,∗3,∗4, 0.01–49%∗5

Fe 2.2 0.5 2.1 0.3 0.1 0.1 1–3%∗2, 2%∗4, 0.01–10.2%∗5

Ba [6.7] [25.1] [1.8] N.D. N.D. N.D. No report

Zn 9.5 0.2 7.0 0.5 0.1 0.0 5–9%∗6, 2%∗7, 7%∗5

Cu 22.4 15.1 [0.5] 1.2 4.5 2.2 30%∗8, 24%∗9

Cr 7.7 [7.3] 4.0 0.1 0.2 0.1 5–31%∗4,∗10

Ni 13.1 [4.1] [2.7] 0.5 0.4 0.1 1.4–6.3%∗4,∗11

Co 35.4 4.9 9.3 0.4 0.2 0.4 1–12%∗4,∗10

Pb 26.0 7.3 45.4 0.1 0.1 0.0 14.8%∗4, 41%∗9, 12%∗6

∗1 Only reference found was that of Schultz et al., 1998, less than 1% for marine sediment.∗2 Sing et al., 1988 (soil with low organic carbon content).∗3 Gibbs, 1973 (soil).∗4 Tessier et al., 1979 (sediment).∗5 Shuman, 1985 (soil).∗6 Harrison et al., 1981 (roadside soil and standard soil samples MAG-1, BCR-142, BCR-143).∗7 Iyengar et al., 1981 (soil).∗8 McLaren and Crawford, 1973 (soil).∗9 Miller and McFee, 1983 (soil).∗10 Li et al., (standard soil).∗11 Hickey and Kittrick, 1984 (soil).

exception was high extractability of Pb with acidified H2O2 from soil-KUR whichis low in OC content.

The characteristics of metal association with SOM varied between soils. Whileboth soil-b and soil-d had similarly high OC content at 0–5 cm horizon (ca. 80 mgkg−1 OC as shown in Table I), metals in soil-b showed higher extractability withacidified H2O2 than those in soil-d (Table II). This discrepancy may be partlycaused by different origin, and therefore different characteristics, of SOM in thesesoils. Namely, while the origin of SOM in soil-b is rich and fresh litterfall fromdeciduous forest, SOM in soil-d has been probably derived from grass roots in thepast but present supply of organic materials is from coniferous forest and thereforeis smaller than that for soil-b.


3.2.2. Comparison of Extraction Results Among Different MetalsIn Table V, the ratio (%) of metal extracted with acidified H2O2 to total amount insoil are listed with the ratio (%) of metal contained in humic acid fraction to totalamount of metal in soil, and results obtained by selective extraction of organicphase by other workers (also given as the ratio to total amount of each metal insoil). As shown in Table V, percentages of metals extracted with acidified H2O2 inthis work are within the range of values reported, except for Al regarding which anadequate reference could not be found.

The ratios of metals present as contained in humic acid to total metal contentsof soil were less than 1% except for Cu (Tables IV and V). From Table V, Cushowed high ratios in both acidified H2O2 extraction and inclusion into humic acid.Among the metals tested, ratios of Pb in soil-b, soil-d and soil-KUR, and that ofMn in soil-b extracted with acidified H2O2 were high, but ratios of inclusion ofthese metals in humic acid were rather low. This indicates either that Pb and Mnwere associated with soil organic matter other than humic acid, or that these metalswere associated with humic acid by weaker chemical bond than in case of Cu. Thelatter explanation is also supported by some laboratory experiments, which suggeststronger chemical association of Cu with humic acid than that of Mn. Namely,an affinity of metal with humic acid is reported to be higher for Cu than for Zn,in agreement with Irving-Williams series (Ashley, 1996), whereas Zn has higheraffinity toward humic acid than Mn does (Takahashi et al., 1997). Desorption ofCu once sorbed to humic acid was found to be relatively low, indicating strongassociation of Cu with humic acid (Slavek et al., 1982).

3.3. ACCUMULATION OF METALS IN TOPSOIL

The mechanisms that promote the accumulation of metals in topsoils are the de-position of metals from natural or anthropogenic sources onto the soil surface,retention of metals as organic complexes in shallow horizon, and plant removal ofelements from the mineral soil in deeper horizon and transfer to the organic horizonin litter (e.g. Zöttl, 1985).

Results of extraction test in this work can be applied to evaluate the effect ofmetal association with SOM on accumulation of the metals in shallow, organic-rich horizons compared with deeper horizons of soil. Concentrations of Mn, Zn,Cu, Cr, Ni, Co and Pb in soil-b, those of Mn, Zn, Cu, Cr and Pb in soil-d, and thoseof Mn, Ba, Zn, Cu and Pb in soil-KUR were found to be higher at depths less than15 cm from the surface (Fujikawa et al., 2000). Among these metals, Mn, Cu, Pband Zn in soil-b, Cu in soil-d and Pb in soil-KUR could have been influenced bytheir association with SOM, resulting in their accumulation in shallow horizons, asthese metals had significant ratios for being extracted with acidified H2O2 (higherthan 10% as listed in Table III). As for Pb, however, its accumulation in shallowhorizons was not only because of its association with SOM but also because ofdeposition of atmospheric lead fallout on soil surface. Presence of such fallout was


indicated by different lead isotopic ratios between shallow and deep layers of soil(referred to in Part I of this study). The factors that caused the accumulation of theother elements will be the topics of future investigations.

Duration of metal accumulation in organic-rich horizon induced by metal as-sociation with SOM is important in predicting the long-term behavior of metalsin soil profiles. Association of metal with humic acid can provide a clue to thisaspect. Because humic acid is relatively stable in oxidative environment (Changand Berner, 1988) and also relatively immobile in soil (Schnitzer, 1986), metals thatare strongly associated with humic acid can be retained within humus-rich horizonsfor a long time. Among the metals studied, Cu exhibited significant associationwith humic acid and also showed high extractability with acidified H2O2 (refer toTable V). Long-term as well as short-term retention of Cu with SOM is thereforelikely to occur in the soil profiles studied. Pb, on the other hand, showed relativelylow association with humic acid while its extractability with acidified H2O2 washigh. This suggests that although significant portion of Pb may have been retainedwith SOM, its retention in organic-rich horizons may last for a shorter period thanretention of Cu.

4. Conclusion

The present study aimed at clarification of the effect of SOM on the vertical distri-bution of some metals (Al, Mn, Fe, Ba, Zn, Cu, Cr, Ni, Co and Pb) in three differentsoil profiles in Japan by applying selective extraction techniques. Selectivity of theacidified H2O2 extraction technique to metal fraction associated with SOM wasevaluated before interpreting the extraction results. The binding of Zn, Cu, Ni andPb in soil-b (a Dystric Cambisol), Cu in soil-d (a Humic Andosol), and Pb and Cuin soil-KUR (an Orthic Acrisol) with SOM was shown to have contributed to theiraccumulation in organic-rich layers of soil. High ratio of association of Cu withhumic acid suggested that retention of Cu in humus rich layer may last longer thanthat of the other metals.

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Documents

Vertical Distribution of Trace Metals in Natural Soil Horizons from Japan Part 2: Effects of Organic Components in Soil