07) 340–351www.elsevier.com/locate/geoderma

Geoderma 137 (20

Geochemical behaviour of Ni, Cr, Cu, Zn and Pb in an Andosol–Cambisolclimosequence on basaltic rocks in the French Massif Central

M. Soubrand-Colin a,⁎, C. Neel a,1, H. Bril a,1, C. Grosbois a,1, L. Caner b,2

a Laboratoire HydrASA UMR 6532 CNRS, Equipe “Eléments traces métalliques”, Université de Limoges,123 Avenue Albert Thomas, 87060 Limoges Cedex, France

b Laboratoire HydrASA UMR 6532 CNRS, Equipe “argiles et environnement”, Université de Poitiers, 40 Avenue du recteur Pineau, 86022 Poitiers cedex, France

Received 7 July 2005; received in revised form 8 May 2006; accepted 16 August 2006Available online 19 October 2006

Abstract

The objective of this study is to identify parameters influencing trace elements (TE) vertical distributions in an Andosol–Andic Cambisolclimosequence. In order to understand processes involved in the current distribution of trace elements from basaltic rocks to associated soils, sixpedons developed on Tertiary massive basaltic formations, located at similar position in the hill-slopes, submitted to the same wet mountainousclimate but under various microclimatic conditions (altitude and annual rainfall), were considered. The six studied soils are andic and rich inorgano–metallic complexes. Two different and quantitative methods were used to compare the geochemical distributions of major and traceelements, both being calculated with Ti as invariant element: isovolumetric mass balance and enrichment factors. Anthropogenic, pedogenetic,microclimatic, inherited factors were discriminated. Anthropogenic influences are pointed out for K in two pedons and for Pb in all studied sites.Distributions of other TE are affected by discontinuities between underlying rocks and soil horizons. Zinc and Cu distributions are controlled byorganic matter whereas Cr and Ni distributions are influenced by pedogenetic processes. Results emphasize the relative trace elementimmobilization in andic soils under temperate climate.© 2006 Elsevier B.V. All rights reserved.

Keywords: Trace element distribution; Pedogenesis; Andosol; Cambisol; Basaltic rocks; Temperate climate

1. Introduction

Neogene basaltic formations outcrop over large areas inWestern Europe (Spain, France, Germany, Italy…). Basalts andbasanites from the French Massif Central present high contentsin trace elements (TE), mainly Ni, Cr, Cu and Zn (Downes,1983). Part of pedological processes in accumulating ormobilizing TE is of major interest since the geochemical TEbackground is taken into account for numerous land uses suchas sewage sludge which is the most usual way to manage wastecoming from water treatment.

⁎ Corresponding author. Tel.: +33 555 45 74 22; fax: +33 555 45 74 13.E-mail addresses: marilynecolin@hotmail.com (M. Soubrand-Colin),

catherine.neel@unilim.fr (C. Neel), hubert.bril@unilim.fr (H. Bril),cecile.grosbois@unilim.fr (C. Grosbois),laurent.caner@hydrasa.univ-poitiers.fr (L. Caner).1 Tel.: +33 555 45 74 22; fax: +33 555 45 74 13.2 Tel.: +33 549 45 36 70; fax: +33 549 45 42 13.

0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geoderma.2006.08.017

Pedological processes are first driven by factors directlyderiving from parental rock properties: mineralogy, structure(discontinuities, prismation, fractures) and texture (grain sizepores). They are also driven by environmental conditions (rain-fall, temperature, topography, water circulation), vegetation andland use. On basaltic parent material, as soon as the meanannual rainfall is sufficient, andosolisation is a usual the com-mon expression of lithological control on pedogenetic processes(Hétier, 1975; Shoji et al., 1993). In either tropical, Mediter-ranean (Fujikawa et al., 2000; Palumbo et al., 2000) or tem-perate regions (Hétier, 1975; Aran et al., 1998; Kleber et al.,2003; Quantin, 2004), several modalities of andosolisation havebeen identified, depending on local morphoclimatic conditionsand topographic positions. However, few studies provide in-formation about natural TE background levels and aboutinfluence of pedogenetic processes on TE distribution.

Studying three Japanese soils (Andosol and dystric Cambi-sol) developed on the same glassy volcanic material underdifferent morphoclimatic conditions, Fujikawa et al. (2000)

341M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

suggested a strong influence of pedogenetic processes for ex-plaining differences observed in the TE distributions. On thecontrary, Palumbo et al. (2000) pointed out weak pedogenicprocesses influence in the Cr, Ni, Pb, Zn and Cu distributions intwo Sicilian Vitric Hapludands developed on pyroclastic ma-terial. Based exclusively on statistical treatments of bulk che-mical compositions and sequential extractions analyses, thelatter study only concluded to organic matter influence in the Cuallocation. Relative part of rock inheritance versus pedogenicinfluence on TE distributions in soils was more recently in-vestigated by comparing andic soils developed from volcanicpyroclasts, ranging in composition from phonolitic tephrites totrachytes (Martinez Cortizas et al., 2003). Ni is mainly presentin primary phases, such as olivine, indicating a strong mine-ralogical control, whereas for Zn, systematic concordanceswith organic matter enrichments evoke a pedogenic influence.Semlali et al. (2001) showed, in a young andic soil from theFrench Massif Central, that Cu and Zn are strongly adsorbed byorgano–allophanic complexes while Cr and Ni remain stablefrom the basaltic pyroclastic materials to the topsoil suggestinga rock inheritance for the latter elements. These authors also

Fig. 1. Locations and rainfa

indicated disturbances in vertical distributions of Cu, Zn andmore often Pb, due to anthropogenic inputs. However, thesestudies were mostly based on enrichment factors profiles with-out any mineralogical nor geochemical investigations. In ad-dition, most studies dealing with the andic soils TE distributionhave been carried out on deep soils, well developed and onunconsolidated volcanic formations. Little is known about andicprocesses occurring in soils developed over massive volcanicrocks (Kleber et al., 2004).

In the French Central Massif, six andic soils developed onalkali basalts and basanites, quite in similar bulk chemicalcomposition and showing different rock textures and structureswere chosen. The sites were also differently affected bysuperficial reworking during late and post-glacial periods. In aprevious work, Soubrand-Colin et al. (2005) demonstrated themineralogical control on the current TE contents found in theserocks and their weathering products. Attempts of the presentstudy are to better understand the respective part of pedogenesisand rock inheritance on the TE distributions in these soils.Based on comparison of Cr, Ni, Cu, Zn and Pb distributionswith those of major elements in the six soil profiles, this study

ll of the 6 studied sites.

342 M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

aims at discriminating the inherited, morphogenic, pedogeneticand anthropogenic factors.

2. Materials and methods

2.1. Studied area

In the French Massif Central, Tertiary volcanism coversmetamorphic and granitic rocks. Basalts and basanites consti-tute around 2/3 of these volcanic areas, and mainly outcrop asplateaux 800–1400 m of altitude, known as “planèzes”. DuringWürm and tardi-Würm periods, due to the covering of thevolcanic area by mountain glaciers, earlier soils were submittedto erosion, creeping or solifluxion. Currently, soils found on theplanèzes can be mainly considered as the result of post-Würmpedogenesis, developed either on basaltic rocks, on fragmentsof former soils or on translocated superficial formations(Quantin, 2004). These soils are covered by fertile grasslandsintensively used during spring and summer for pasture andfodder.

2.2. Sampling

Six soil profiles were examined in 3 different volcanicsystems: (i) Cantal (MC, PFT and LBS), (ii) Devès (PC andSCD) and (iii) Monts Dore (CHZ). Located at elevationsranging from 700 to 1180 m all the six sites are submitted to awet mountainous temperate climate. However, microclimaticconditions change from one site to another (Fig. 1): annualrainfall vary from 760 mm/year to 1900 mm/year and averageannual temperatures from 6.4 to 10.4 °C depending on siteelevation and on inland distance from the Atlantic coast. Profilelocations were specifically chosen in flat areas or gentle slopesin order to minimize influences of lateral transfers in the TEdistribution in soils. For each site, the microclimatic conditions,the accurate topographic position and the profile description, arespecified in Fig. 2.

2.3. Sample analyses

Each horizon of the six soil profiles was collected and parentrocks were also taken at the profiles bottom. Chemical analyseswere performed on air-dried crushed rock and soil samples(b2 mm fraction), after a first digestion by melting with LiBO2,followed by a second digestion in nitric acid. Total chemicalcompositions were determined by ICP-AES for major elementsand by ICP-MS for trace elements (CRPG, Nancy). Loss ofignition (LOI) was determined by heating the sample for 3 h upto 980 °C.

Soil pH was measured in water and in 1 N KCl, respecting asoil:solution ratio equal to 1:2.5. The cation exchange capacity(CEC) was determined by the 0.05 N cobalthexamine method atthe soil pH (Orsini and Remy, 1976). This method was chosenbecause of actual soil pH values lower than 6. Exchangeablecations were measured by atomic absorption spectroscopy andexchangeable acidity by potentiometry. Organic C and Ncontents were determined by dry combustion with a CHN

elemental analyser. Particle size distribution of b2 mm fractionwas determined by sieving and using the pipette method afterdestruction of organic matter by H2O2 and dispersion by Na-resins. Bulk densities of soil horizons were determined by thecore cylinder method, with at least three replicates per horizon.Rock bulk densities were obtained by hydrostatic doubleweighting method. Phosphate retention was obtained by aStuder method (Studer, 1987). Soil pHNaF was determined using1:50 soil/solution ratio (Blakemore et al., 1987). Active formsof Al, Fe and Si were extracted using 0.2 M ammoniumoxalate–oxalic acid (pH=3) with a soil:solution ratio of 1:40(Schwertmann, 1964). Optical densities of the oxalate extracts(ODOE) were measured at 430 nm (Daly, 1982). Aluminium andiron complexed by organic matter were extracted by 0.1 M Napyrophosphate (1:100 soil:solution ratio). Extracted amounts ofAl, Fe and Si were measured by Flame-AAS. Allophane contentswere calculated using extraction results using the formula pro-posed by Parfitt and Wilson (1985) and refined by Mizota andVan Reeuwijk (1989); allophane=100*Sio/{−5.1[(Alo−Alp) /Sio]+23.4}. The total contents of alkaline and alkaline-earthcations were summed as the total reserve of base (TRB), in orderto estimate the content of weatherable minerals in soil (Herbillon,1988).

2.4. Data calculations

To allow quantitative comparisons between the studied soilprofiles, two methods have been used for major and trace ele-ments: (1) isovolumetric mass balance from rock to soil, (2) soilenrichment factor (EF).

(1) Stocks of elements are calculated by surface unit of eachparent rock (SROCK) and soil horizon (SSOIL HORIZON) using anisovolumetric method (Millot and Bonifas, 1955). Thiscalculation includes bulk density (D; Table 1), bulk concentra-tion (X; Table 2) and rock or soil horizon thickness (T; Fig. 2):

SSOIL HORIZON ¼ D� X � T

SROCK ¼ Do � Xo � To

½g:m−2� ¼ ½kg:m−3� � ½g:kg−1� � ½m� ð1Þ

where X and Xo are element concentrations respectively of soilhorizon and the parent rock, D and Do are bulk densities, T andTo are thickness.

Total input or output of elements in soil were calculated bydividing the cumulated stock of element in the pedon (SSOIL=sum of all the soil horizon stocks) by the stock of element in theparent material or in the deepest horizon considered as a refer-ence level. However, in andic soil profiles, original structure andtexture of basaltic rocks are not preserved, so that mass balancecalculations require the use of an invariant element such as Ti orZr to obtain theoretical thickness (To) of weathered rock, requiredto build up the measured soil thickness. The ratio of element lossor gain from the parent rock to the whole soil profile was cal-culated by the Nesbitt method (Nesbitt, 1979). Equalising stocksof invariant element in soil and parent rock allows to calculate a

343M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

theoretical thickness of parent material needed for soil formation(To):

To ¼ Rði� D� TÞ=ðio � DoÞ ð2Þwhere i and io are invariant element concentrations, D and Do

are bulk densities respectively of soil horizon and parent rock.The percentage of relative loss or gain (% change) of element

during weathering and pedogenesis from rock to soil is obtainedas follows:

% change ¼ ½100−½ðSSOIL=SROCKÞ � 100� ð3Þ

(2) Enrichment factor (EF) was calculated as defined byRahn and McCafrey (1979) to evaluate additional sources ofelements compared to a reference level.

EFðX Þ ¼ ½ðX=iÞ=ðXo=IoÞ� ð4Þwhere i and io are the same parameters as in Eq. (2) and X andXo are concentrations of an element respectively in soil horizonsand corresponding parent rock.

For both isovolumetric mass balances and EF ratio calcu-lations, parent rock samples always correspond to blocks of

Fig. 2. Profile description of each studied profile, indicating: I. Topographic position osoil morphology with horizon thickness (cm), III. Main pedological features.

basalt taken at the profile bottom, either directly as theyoccurred in the subsoil layers (MC, PC, PFT and CHZ) or in theperiglacial deposits found just below the pedons (LBS, SCD).

Mass balance and EF methods used to compare major andtrace elements distributions in the different soils are bothsubmitted to significant errors induced by calculation uncer-tainties. Hernandez et al. (2003), considered that the naturalvariability provides EF values ranging between 0.5 and 2,whereas EF values higher than 2 indicate soil enrichments byanthropogenic inputs. Precisions on EF according to analyticalerrors are about 20% for trace elements and 5% to 10% formajor elements. In our case, due to uncertainties in the deter-mination of TE contents, and in the estimation of the weatheredrock thickness, calculated losses and gains are consideredsignificant when attaining values greater than 10% for majorelements and 15% for trace elements.

3. Results

3.1. Morphology

The soil profiles were excavated until predominance ofstones (N40% of coarse elements) or occurrence of hard blocks

f the soil profiles (A=altitude; R=rainfall; T=air temperature), II. Macroscopic

344 M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

usually covered by a weathering corona, excepted for SCDwhere the profile bottom consisted in a thick (N1 m) hetero-geneous sandy layer. Main morphological features of the sixstudied pedons are summarized in Fig. 2. The studied soilspresent the following common characteristics: dark colour insurface and deep horizons, clay–loamy texture, fluffy structure,significant stones occurrences (10% to 40%), and high rootdensity in A horizons. However, the studied soils differ by theirtotal thickness from 24 to 160 cm. Weathering degree of parentrock blocks or stones in C horizons also vary from one site toanother.

3.2. Chemical and physical properties

Results of selective extractions and other soil parametersrequired for determining andic properties are presented in Table 1.The six studied soils present common features of andic soils: lowbulk density, high phosphate retention, general dark colour, largecontent in active forms of Al and Fe and in organic C, littlehorizon differentiation, specific pHNaF values. Bulk densities,inferior to 0.9 kg dm−3 for A horizons increase with depth.Phosphorus retention exceeds 85% in all soils. The optical density

Fig. 2 (cont

of oxalate extract (ODOE) gradually decreases with depth. TheseODOE evolutions do not reveal any occurrence of spodichorizons. The six pedons present the same decrease of carboncontents with depth. pHwater ranges from 4.7 to 7.6, pHKCl from3.9 to 5.9, expressing ΔpH values (pHwater−pHKCl) from 0.6 to2.2 pHNaF values greater than 9.5 in all horizons confirm thepresence of active aluminium.

In LBS, MC, CHZ and PFT pedons, the sum Alo+1/2 Feoexceeds 20 g kg−1. For PC and SDC pedons, this sum rangesbetween 7.3 and 13.7 g kg−1. According to the RéferentielPédologique (AFES, 1995) and the World Reference Base (FAO,1998), LBS, MC, CHZ and PFT are Andosols. LBS and MCpresent aluandic characters (Alp /AloN0.5; Siob6 g kg−1,exchangeable AlN2 cmol (+) kg−1 and pHNaFN10.5) whileCHZ and PFT rather belong to the silandic group (Alp /Alob0.5;SioN6 g kg−1 and pHNaFN10.5). By contrast, PC and SCDpresent cambic eutric properties due to larger base saturation(N80%), lower pHNaF values (b10), neutral pHw and C/N ratiosbetween 12 and 15. PC and SCD are thus identified as AndicCambisols (WRB, FAO, 1998) assimilated to the andic brownsoil class defined by Hétier (1975). Andosols present highcontents of organic carbon (up to 220 g kg−1) which is much

inued ).

Table 1Main chemical and physical properties of the six soils

Profile Hz Organic C C/N pH AlO+1/2 FeO Sio Bulk density Alp /Alo

Allophane CEC Exchangeables cations(cmolc kg

−1)

(g kg−1) H2O KCl NaF (g kg−1) (g kg−1) (g cm−3) (g kg−1) (cmolc kg−1) Ca2+ Mg2+ Al3+ H+

LBS A1 220 13.5 4.7 3.9 10.7 24.07 1.4 0.43 1.14 4.3 12.0 4.21 1.50 3.66 0.20A2 165 12.9 4.9 4.2 10.8 31.54 3.5 0.39 1.24 11.3 6.7 0.36 0.28 3.85 0.08B1 123 12.5 5.0 4.3 11.4 41.53 5.0 0.57 0.82 28.2 4.8 0.32 0.17 2.05 0.05B2 101 13.6 5.1 4.5 11.3 45.44 7.1 0.62 0.79 39.1 3.9 0.14 0.06 1.00 0.05C 20 14.5 5.4 4.5 11.4 31.63 9.3 0.80 0.25 77.9 3.2 0.19 0.05 0.03 0.05

MC A1 107 11.5 5.3 4.3 10.2 12.33 1.0 0.79 0.68 7.8 19.7 10.70 5.61 0.81 0.28A2 39 12.9 5.3 4.3 10.3 13.72 1.4 0.89 0.79 8.2 14.0 5.51 3.64 2.68 0.05

PFT A1 74 11.9 5.6 4.6 11.3 24.40 6.7 0.89 0.33 49.6 12.5 9.59 1.19 0.13 0.32A2 14 12.4 5.8 4.3 11.0 20.12 6.5 1.09 0.18 48.0 6.6 2.95 1.48 0.35 0.12B 3 16.4 6.4 4.4 10.5 11.41 5.5 0.97 0.21 29.9 12.1 6.78 3.40 0.04 0.12B/C 2 16.7 6.9 4.7 10.3 10.79 5.2 1.20 0.15 28.3 13.4 7.93 4.06 0.03 0.16

CHZ A1 115 11.3 6.1 5.2 11.1 29.30 8.4 0.53 0.52 66.8 17.0 10.65 3.46 0.17 0.28A2 72 10.7 5.8 4.9 11.3 30.38 9.1 0.79 0.64 70.4 10.5 6.75 1.80 0.21 0.20B 24 11.7 5.2 5.0 11.4 30.55 9.8 0.99 0.57 77.7 7.6 3.94 2.06 0.11 0.05C 16 11.7 6.3 4.9 11.3 23.95 7.5 1.16 0.60 56.2 7.0 4.02 1.84 0.12 0.05

PC A 43 11.1 6.8 5.9 9.2 8.46 4.7 1.05 0.30 21.3 22.1 10.60 9.98 0.02 0.05B 17 10.4 7.3 6.1 9.4 12.16 6.2 1.50 0.22 28.1 19.7 8.43 9.65 0.02 0.05C 9 9.6 7.3 5.7 9.5 9.00 2.4 1.50 1.01 10.2 26.4 12.72 10.86 0.02 0.05

SCD A 39 11.6 6.6 5.5 9.5 7.32 2.7 0.97 0.27 13.0 17.1 10.70 7.35 0.02 0.16B 14 11.2 7.0 5.4 9.7 7.42 2.9 1.40 0.27 13.6 15.7 8.05 5.83 0.07 0.08C 12 11.3 7.1 5.5 9.7 7.50 2.6 1.39 0.20 12.3 15.9 8.24 5.71 0.05 0.16II1 13 11.7 7.1 5.5 9.7 7.91 2.8 1.15 0.23 13.3 16.8 8.85 6.23 0.08 0.08II2 9 10.9 7.6 5.9 10.0 13.73 2.6 1.30 0.44 13.1 20.6 8.79 9.14 0.06 0.28

Index o, p refer respectively to oxalate and pyrophosphate dissolutions.

345M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

lower in all andic Cambisols (39 and 75 g kg−1). AluandicAndosols (LBS and MC) present slightly acid pH, compared tosilandic Andosols (CHZ and PFT) and andic Cambisols (PC andSCD). Exchangeable cations are mainly Ca2+ andMg2+ (Table 1)excepted for aluandic Andosols, where Al3+ is the dominantexchangeable cation and overtake 2 cmolc kg

−1.Regarding particle size distribution (Fig. 3), pedons LBS and

MC show a decrease in proportion of thin fraction (b50 μm)with depth, whereas no obvious grain size evolution appears atPC and CHZ (Fig. 3). In SCD and PFT pedons, a texturaldiscrepancy is noticed at 40 cm and 35 cm deep respectively.

3.3. Geochemical vertical profiles of major and trace elements

Bulk chemical compositions of rocks and soil horizons aregiven in Table 2. Average TE amounts in soil horizons and parentrocks occur in the following order: CrNNiNZnNCuNPb. Parentrocks and associated soils are rich in Cr (249–455 mg kg−1), Ni(126–237 mg kg−1) and Zn (111–125 mg kg−1) and contain lessamounts of Cu (44–67 mg kg−1) and Pb (2–45 mg kg−1). Onlythe LBS pedon presents much lower contents for both Cr and Nithan other pedons (Table 2).

Because of great differences in bulk densities and in horizonsthicknesses (Tables 1 and 2), bulk chemical compositions cannot be directly compared from a pedon to another. Comparisonsof pedons were performed using the results of mass balance andEF calculations which gave quantitative values of elementslosses or gains and trace element vertical distributions. Thesetwo methods can use Ti and Zr as invariant element (Eggleton

et al., 1987; Veldkamp et al., 1990; Kurtz et al., 2000; Hill et al.,2000) during basaltic rocks weathering and showing similarvariations of the EF profiles and of the isovolumetric mass ba-lance. Like in recent studies applied to volcanic soils (Nieuwen-huyse and van Breemen, 1997; Martinez Cortizas et al., 2003), Tiwas chosen because a previous mineralogical study by Soubrand-Colin et al. (2005) identified as the main Ti-bearing minerals,Fe–Ti oxides which are preserved during pedogenesis andweathering.

Concerning major elements, the isovolumetric mass balanceindicates the following general sequence of relative mobility(Table 3): Fe=AlbSi≪K, Mg, Ca, Na. Major alkaline earthand alkali elements are leached in all the studied pedons exceptK which accumulates in the surface horizons at PC and CHZ(gain=+5 to +154%). Silicium is leached in all pedons (lost=−20 to −35%), with the exception of CHZ (+1%) and PC(−9%) where Si is conserved. As shown by isovolumetric massbalance, iron tends to be preserved in all pedons (−1 to 24%).Iron enrichment factors profiles remain stable (EF≈1) in all thestudied soils except at SCD (Fig. 4). Al presents more variableisovolumetric mass balance: LBS profile shows a loss of 23%between soil and the rock, PC, PFT, SCD, MC profiles presentconserved quantities of Al (−8 to +11%) and even accumulatedat MC and CHZ (+22 to +44%). EF(Al) also present avariability at a horizon scale and specially at PC and SCD wherethe deepest horizon is more enriched than the ones above(Fig. 4).

Regarding trace elements, no obvious relative mobility se-quence was found. The enrichment factors profiles (Fig. 4),

Table 2Bulk chemical compositions of parental rocks (R) and of soil horizons for major and trace elements

Profile Hz LOI(%)

SiO2

(%)Al2O3

(%)Fe2O3

(%)TiO2

(%)MgO(%)

CaO(%)

Na2O(%)

K2O(%)

TRB(cmolc kg

−1)Cr(mg kg−1)

N(mg kg−1)i

Zn(mg kg−1)

Cu(mg kg−1)

Pb(mg kg−1)

LBS A1 51 19.2 9.0 6.8 1.7 1.0 1.4 0.5 0.7 212 50 22 109 27 45A2 41 22.1 11.3 8.2 2.1 1.2 1.6 0.6 0.8 229 65 30 103 30 27B1 35 25.5 13.6 10.6 2.6 1.8 2.2 0.8 0.9 300 60 31 116 32 11B2 26 27.2 16.1 10.4 2.6 1.7 1.9 0.9 1.1 277 70 32 123 31 14C 17 40.0 19.6 10.4 2.5 2.7 3.8 1.9 1.8 434 71 44 130 37 11R 7 39.8 15.0 12.1 2.8 10.6 9.5 1.4 1.2 999 427 219 117 53 4

MC A1 30 32.2 16.1 10.8 2.4 1.4 1.0 0.2 0.7 166 293 168 143 52 22A2 22 37.1 20.2 13.5 3.0 1.8 0.9 0.2 0.7 174 376 213 151 68 18R 1 45.8 14.5 11.2 2.7 8.9 9.4 3.7 2.0 947 371 214 118 49 4

PFT A1 28 28.8 14.1 14.4 3.2 5.1 4.2 0.6 0.7 559 478 228 160 46 14A2 13 34.9 15.9 16.3 3.6 7.3 5.9 0.8 0.8 701 554 287 168 62 6B 14 34.6 17.4 18.4 4.1 6.1 3.0 0.6 0.8 507 663 307 178 74 7B/C

15 34.7 18.6 16.2 3.8 5.5 3.5 0.6 0.7 498 507 282 164 77 6

R 5 39.7 14.8 13.2 3.5 9.3 11.3 1.7 0.9 987 270 142 125 67 4CHZ A1 33 30.3 12.2 9.3 2.1 4.9 4.4 0.9 0.8 594 245 117 109 34 14

A2 25 33.1 13.9 10.6 2.4 5.7 5.0 0.9 0.8 636 300 141 110 33 14B 20 35.8 17.8 11.9 2.7 4.3 4.5 1.1 1.1 519 232 126 116 50 10C 17 38.1 18.0 12.8 3.0 4.2 4.6 1.1 1.1 501 273 125 123 54 8R 3 41.7 13.6 13.2 3.2 9.4 11.5 2.7 0.5 1003 381 218 111 45 4

PC A 16 39.4 13.7 13.6 2.9 4.5 5.4 1.5 1.3 573 330 155 156 49 14B 11 42.9 14.6 14.6 3.1 4.8 5.5 1.5 1.4 569 335 159 147 48 14C 14 41.5 19.2 12.5 2.6 3.1 4.0 1.2 1.0 409 236 147 113 50 9R 3 44.6 15.6 13.0 2.7 6.6 9.4 3.1 1.1 808 249 126 123 44 4

SCD A 18 35.3 12.4 18.4 3.3 6.5 3.8 0.8 1.0 596 695 288 156 67 14B 12 37.6 13.8 20.2 3.5 6.2 3.5 0.8 1.2 540 686 291 164 71 13C 12 37.3 14.0 20.8 3.6 5.9 3.3 0.8 1.2 514 691 295 171 74 13II1 12 38.1 14.4 20.2 3.7 5.4 3.0 0.8 1.2 477 675 273 167 73 13II2 15 35.9 18.2 17.4 3.1 4.7 2.9 0.6 0.9 429 467 259 134 68 9R 1 44.7 13.1 12.2 2.7 10.9 10.3 2.9 1.8 1040 455 237 114 53 2

LOI is the related Loss of ignition (LOI) and TRB is the Total Reserve in Bases (Herbillon, 1989).

346 M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

reflect a natural variability with EF values ranging from 0.8 to1.3, except for Pb for which EF range from 1 to 17 (not shownin Fig. 4). As indicated by the isovolumic mass balances(Table 3), the A horizons indeed accumulate Pb from 40% up to

Fig. 3. Grain size fractionation

500% in all the pedons. Copper and Zn tend to be preservedfrom the rock to the soil in the cambisols (0 to +13%) and LBS(−9 to −15%) while they accumulate in the andosols (17 to +25%). Chromium and nickel are also preserved in the cambisols

of thin earth soil samples.

Table 3Isovolumetric mass–balance results (Eq. (3)) with theoretical thickness of parent material (T ref) needed to build up the actual total profile thickness as determinedusing Ti as referent element (Eq. (2))

Profile To (cm) Si (%) Al (%) Fe (%) Mg (%) Ca (%) Na (%) K (%) Cr (%) Ni (%) Zn (%) Cu (%) Pb (%) Zr (%)

LBS 40 −34 −23 −4 −41 −47 −55 −44 −8 −31 −9 −15 +109 −34MC 8 −28 +22 +6 −82 −91 −96 −68 −11 −13 +17 +21 +500 +11PFT 74 −20 +11 +13 −43 −69 −68 −25 +78 +82 +22 0 +40 −20CHZ 19 +1 +44 +5 −42 −53 −55 +154 −19 −31 +24 +25 +227 +1PC 28 −9 +9 −1 −45 −52 −59 +5 +7 +17 0 +8 +172 −9SCD 112 −35 −8 +24 −60 −75 −81 −52 +12 −3 +13 +9 +314 −35

Negative percentages represent loss (−) of studied element, from the parent rock (R) to the whole soil profile (S) while positive values represent gain (+).

347M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

(−3 to +17%) while they tend to be partially leached in theandosols (−8 to −31%).

4. Discussion

4.1. Genesis of andic soils according to climatic and structuralproperties

All the studied soils are developed on massive basalticformations, poor in glass and presenting similar contents inweatherable minerals (i.e. similar TRB). As all the profiles arelocated in b1% slopes near the top of the hills (cf. Fig. 2),differences in pedological features mostly depend on localmicroclimatic factors (rainfall and thermal rate). All the soilsare developed under the current temperate mountain climate sincethey are younger than the tardi-Würmwithdrawal of ice. In Fig. 5,the six studied pedons presented according to soil types and soildevelopment degrees, are plotted according to site altitude andrainfall amounts. Typic Andosols (MC, LBS, CHZ and PFT) arepredominantly observed at uphill position, on volcanic formationswhere climate is permanently wet and cold with important annualrainfalls and significant snowfall periods. By contrast, at loweraltitudes, warmer and drier climates are factors limiting ando-solisation and inducing the development of cambic characters (PCand SCD). Quantin (2004) shows that factors enhancing ando-solisation are altitude, low temperature and important annualrainfall. The Andosol–Cambisol sequence is then commonlyobserved over a toposequence. In our case, it corresponds to aclimosequence for all soils deriving from similar basaltic rocksand located in similar topographic positions. Besides the differenttypes of andic soils, three stages of soils development wereidentified according to the number of horizons, depth of the parentrock, structure and thickness of the B horizon. As pointed out inFig. 5, the soil development sequence appears as follows: MC isthe less developed soil, followed by the Cambisols (SCD, PC) andPFT. In a third group, CHZ and LBS are the most developedprofiles. The differences in the development of the two aluandicAndosols (LBS, MC) are explained by differences in altitude andrainfall. Development differences between the two silandicAndosols (CHZ and PFT) result from various pedoclimatic con-ditions: higher temperatures occur in the PFT area. For the othersoils, differences in the development are related to intrinsic factorssuch as the structure of the basaltic rock. Within a basaltic flow,the upper part is structured in vertical columns and joints, where-as, near the bottom, discontinuities are mainly horizontal follow-

ing the pre-volcanic topography. In the latter case, lack of verticaljoints prevents penetration of meteoric waters and thus limitswater–rock reactions and weathering. For MC, due to the erosionof the upper part of the basaltic flow by quaternary glaciers, thesoil was developed on the massive flow bottom characterized byhorizontal joints and cracks (Fig. 2).

All the studied soils present an important accumulation oforganic matter (OM). The great stability of organic matter ismainly attributed to C stabilization in metal–humus complexes(Aran et al., 2001) or by oxide–humus interactions (Caner et al.,2000). Usually, alumina is complexed in organometallic com-plexes or/and short-range order phases such as allophane andimogolite (Dahlgren et al., 1993). As indicated by the low con-tents in allophanes and the Alp /AloN0.5 (Table 2), Al is mainlyimmobilised in Al-humus complexes rather than short-rangeorder phases. Andic Cambisols also contain large amounts oforgano–metallic complexes presenting high physical stability andlow mobility. However, the andic Cambisols differ from theAndosols by the low resistance of organo–metallic complexesagainst biodegradation leading to lower OM accumulation (Aranet al., 2001).

4.2. Factors affecting the major and trace elements distributions

Several natural factors may affect the distribution of majorand trace elements in soils: (i) bedrock to soil discontinuities (ii)pedologic factors due to the soil formation. These factors arediscussed on the basis of the geochemical calculation (massbalance and EF). In order to focus on the natural factors,anthropogenic contaminations are first identified.

4.2.1. Anthropogenic inputsThe mass balance and EF results highlight anthropogenic

inputs for potassium and lead. Regular fertilizer inputs explainEF values greater than 2 for K at CHZ. At PC, more irregular or/and older K inputs could explain the contrasted distribution of Kcompared with Ca, Na and Mg (Table 3). Anthropogenic in-fluence is evidenced for Pb in all soils. According to the massbalance (Table 3) as well as the EF profiles, Pb accumulates in Ahorizons whereas the other TE remain in the range of naturalvariations. Such anthropogenic Pb contaminations by atmo-spheric pollutions have already been mentioned in the FrenchMassif Central (Semlali et al., 2000; Roy and Négrel, 2001).The latter authors indicated that Pb is the most abundant tracemetal in rainwater (mean value=996 μg/m2 per year). Semlali

Fig. 4. Vertical evolution of enrichment factors (EF) for selected major and trace elements in the six studied sites.

348M.Soubrand-C

olinet

al./Geoderm

a137

(2007)340–351

Fig. 5. Situation of the six analysed pedons according to the site altitude and themean annual rainfall. Soil types are specified by the shapes of symbols andrelative degrees of soil development by the symbol size.

349M. Soubrand-Colin et al. / Geoderma 137 (2007) 340–351

et al. (2000) also highlighted progressive depletion with depthof exogenous Pb vs natural Pb showing the atmospheric originof Pb at a young andosol surface.

4.2.2. Bedrock to soil discontinuitiesNieuwenhuyse and van Breemen (1997) indicated that EF

calculation provides a correct estimation of possible TE transportin open soil systems if (1) there is continuity between parentmaterial and soil horizons and if (2) parent material is homo-geneous in composition. These authors added that theseconditions are rarely observed in volcanic contexts where soilsoften appear to be stratified and heterogeneous from both struc-tural and geochemical points of view. Palumbo et al. (2000) andMartinez Cortizas et al. (2003) pointed out that discontinuitiesbetween subsoil and soil surface affect the vertical distributionsof major and trace elements. Quantin (2004) mentioned suchkinds of heterogeneity in soils developed on volcanic formationsfrom the French Massif Central.

At PC, CHZ and MC, soils are directly developed on massivebasaltic flows whereas the continuity between bedrock and soil isquestionable in the three other profiles (PFT, SCD, LBS). Theselatter pedons have been affected either by recent superficialreworking (PFT) or by mass translocation (creeping) of thebasaltic alterites which occurred in the immediate post-Würmperiod before current pedogenesis (SCD, LBS). Superficialreworking at PFT is evidenced by uneven grain size profiles(Fig. 3) and a clear discontinuity was noticed at this depth (Fig. 2).

SCD appears as a polycyclic soil with a surface layer devel-oped on a sequence of periglacial materials belonging to a

former truncated paleosoil. These polycyclic features are shownby the grain size profile (Fig. 3), by pedological examination(Fig. 2) as well as by irregular EF profile (Fig. 4). However, atSCD, on the opposite to PFT, the discontinuity has not inducedany obvious discrepancy in the bulk content profiles (Table 2).

For LBS, the profile seems to be continuous regarding to thevertical variations of grain size, major and trace element con-tents. The EF (Fig. 4) and TRB values (Table 2) reveal a dis-continuity between B2 and C horizons.

4.2.3. Pedogenetic processesIn andic soils, pedogenetic processes, determining the pedo-

logical patterns (pH, allophanes…) and the soil type (aluandicand silandic Andosols or andic Cambisols), can be splitted intoseveral categories: those controlling evolution of primary TEbearing phases (mineral weathering, water drainage), thosedetermining nature and quantities of accumulated organic mat-ters (grass growth, biotransformations, pedoplasmation, humi-fication or stabilization of organic complexes) and those leadingto the development of amorphous phases or spodic features (Si,Al and Fe solubilization, leaching, complexation or trapping bysecondary phases).

In our case, all the studied soils are developed on similarparent rocks, characterized by the same TE-bearing primaryminerals: Fe–Ti oxides (spinels) and ferromagnesian silicates(augites and olivine). Previous mineralogical study (Soubrand-Colin et al., 2005) showed that the Fe–Ti oxides were preservedin soil horizons whereas the silicates (olivines) were hydrolyzedby weathering. Moreover, these studies revealed that the ironfraction, mobilized during breakdown of olivines, was trappedin authigenic phases (iddingsite and other oxides) or associatedto organic matter. In a quantitative point of view, the high TRBvalues obtained for each soil (Table 2) indicate significantamounts of preserved primary minerals in all studied pedons. Ata profile scale (balance and EF), our results highlight that,although initially carried by Fe-minerals (oxides and silicates),geochemical evolutions of all TE contrast with those of Fe.While Fe amounts, initially contained in parent rocks, remainalmost preserved (balance=−1% to 24%), and evenly distrib-uted along the soil profile (EF≈1), Zn and Cu tend to ac-cumulate in the soil, Cr and Ni are either preserved or slightlylost in soil horizons. Hence, TE elements are not immobilizedby the same soil constituents than Fe during pedogenesis.

Comparing the six studied pedons all together, Cu and Zncontent variations are linked to organic matter contents variations,which corresponds to the high complexing potential alreadyreported for these trace elements (Rahman et al., 1996, 1998;Fujikawa et al., 2000; Latrille et al., 2003;MartinezCortizas et al.,2003). Regarding the vertical distributions, EF(Cu) and EF(Al)present the same trends in all pedons. In andic soils, Al2O3 is ofteninvolved in organo–metallic complexes which implies that Cu ismostly immobilized by Al-complexes. Zn is the only TE forwhich the enrichment factor gradually increases from the rock tothe organic-rich surface horizon in all pedons, suggesting lessspecific associations with the humus fraction.

On the opposite to Cu and Zn, vertical variations of EF(Ni)and EF(Cr) present the same trend in the studied Andosols and

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they are opposite of EF(Al) variations. In the andic Cambisols,Ni and Cr are not correlated together and still present oppositetrends to Al. These results first indicate the low affinity of Cr andNi for the Al–organic complexes. They highlight the indicativequality of Al for discriminating Andosol from andic Cambisols.Moreover, our results underline differences in involvedpedogenetic processes governing the Cr and Ni distribution.Due to the similarities of the primary and secondary Cr and Ni-bearing phases shown for all pedons by Soubrand-Colin et al.(2005), only differences in leaching efficiency and in secondaryphases stability could explain the dissimilarities between the twosoil types for Cr and Ni. From a quantitative point of view withisovolumic mass balance, Cr and Ni are indeed rather lost in theAndosols profiles compared to the andic Cambisols, suggestingchanges in drainage degrees.

Finally, our results show that organic compounds and second-aryminerals globally result in Cu, ZnCr andNi immobilization inthe studied andic soils, while related major elements, initiallycontained in the same primary phases (Si,Mg), are partly leached.

5. Conclusions

This study allows understanding Cr, Ni, Cu Zn Pb dis-tributions in andic soils, developed on basaltic rocks under amountainous temperate climate. The six studied locationsconstitute a climosequence from west to east for where Ando-sols are present in coldest sites and andic Cambisols in driestsites. Three stages of soil development were identified. There-fore, pedogenetic processes, determining the soil types, are firstrelated to specific microclimatic factors (altitude, annual rainfalland temperature), and then, to time and weathering degrees ofparent rock.

Factors, influencing the TE distribution, are identified bytwo different and quantitative methods. Isovolumetric massbalance allows comparison of the TE amounts between the sixsoil profiles although their respective thickness is very different.Enrichment factors provide information about the factors go-verning each TE distribution, differencing trace elements (Ni,Cr, Zn and Cu) with a natural origin from Pb of anthropogenicorigin. However, this work also points out the necessity ofaccurate pedological characterizations for interpreting results ofthe two quantitative methods. Although all pedons are devel-oped on similar basaltic formations in similar topographicconditions, the TE geochemical evolution at the profile scale isaffected by occurrences of discontinuities between underlyingrocks and soil horizons.

By association of geochemical, pedological and mineralog-ical studies, a global diagnostic of factors explaining eachnatural TE distribution was done. Whatever the degree of theandic soil development, Zn and Cu are both controlled byorganic matter. Zn is immobilized by the humus fraction whileCu is associated to Al–organic complexes. Despite Cr and Niinitially carried by the same minerals, their distributions areinfluenced by pedogenetic processes which are well evidencedby the Al geochemical evolution at a profile scale.

A practical perspective of this study is to draw possiblegeochemical indicators for predicting vertical TE distributions

in andic soils. Although Fe is not a possible indicator, organicmatter contents can be used for Zn, combined with Al contentsfor Cu, or andosolisation features to forecast similarities bet-ween the Cr and Ni trends. Nonetheless, such qualitative per-spective assumes the natural lithogenic origin of elements andthus the continuity between the parent basaltic rocks and the soilhorizons.

In a quantitative point of view, the most important resultsconcern the relative TE immobilization in andic soils undertemperate climates. Results cannot be extended to other types ofclimate for which weathering processes of TE-bearing primaryminerals can differ. Nevertheless, these results involve onlybulk soil analyses to provide integrative view of the long termpedological turnover. Measurements in plants and water areneeded to assess the potential mobility and availability of the TEin a short term period.

Acknowledgements

This work was sustained by the Direction Regionale del'Environnement (DIREN) of the Auvergne region, the wateragencies Adour Garonne and Loire-Bretagne and the Conseilrégional du Limousin. Special thanks to Michel Peymirat for thesample preparations and Claude Fontaine for AAS analyses.François Martin, Emmanuel Fritsch and Jean-Luc Potdevin arealso acknowledged for their comments of an early draft of themanuscript.

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