Geoderma 202–203 (2013) 203–211

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Influence of rainfall and temperature on DTPA extractable nickel contentof serpentine soils in Turkey

İ. Ünver a,⁎, S. Madenoğlu b, A. Dilsiz b, A. Namlı aa Ankara University, Faculty of Agriculture, Department of Soil Science and Plant Nutrition, 06110 Dışkapı, Ankara, Turkeyb Ministry of Agriculture and Rural Affairs, Directorate General of Agricultural Research, Soil-Fertilizer and Water Resources Central Research Institute, P.O. Box 54, Yenimahalle, Ankara, Turkey

⁎ Corresponding author. Tel.: +90 312 5961164; fax:E-mail address: unver@agri.ankara.edu.tr (İ. Ünver).

0016-7061/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.geoderma.2013.03.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 August 2012Received in revised form 21 March 2013Accepted 28 March 2013Available online 25 April 2013

Keywords:Serpentine soilsNickelDTPA extractionWeathering

Influence of rainfall and temperature on DTPA (diethylene triamine pentaacetic acid) extractable nickel (DNi)and fractional (DNi / total Ni = F-DNi) concentrations of soils derived from ultramafic serpentine rock undertemperate semiarid continental and Mediterranean climates were studied. All serpentinite areas in WesternAnatolia and the East Thrace (ca. 400,000 km2 areas) were targeted. Meteorological data from 185 stationsversus so-called phytoavailable Ni concentrations of 192 serpentine soil sampleswere examined. Digital elevationmodel (DEM), ANUSPLIN and ARC GIS 8.1 software packages for generation of climatic surfaces and analysis wereemployed for extrapolation of the weathering conditions in preparing comparative maps. Total Ni concentrations(TNi) were in the range of 25.7–2680 mg kg−1, whereas DNi were between 0.08 and 143 mg kg−1. The correla-tion between Ni extractability and the pH was weak (R2 = 0.175). This restricted effect may be attributed to thesoil pH varying between neutral and slightly alkaline. Average DNi concentrations of the soil samples groupedwithin the province borders indicated that both precipitation and air temperature might be effective on theamount of DNi in the serpentine soils studied. The combined effect of annual precipitation andmean atmospherictemperaturewere significant (P b 0.01) onDNi. The differences between the climatic zoneswere distinct and gen-erally increasing with the increase of annual rainfall and mean air temperature.

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1. Introduction

Recent ideas about the role of nickel (Ni) as an essential nutrient inmany living organisms have resulted in increased interest on this ele-ment. On the other hand, Ni is considered more likely toxic than manyother metals because of its widespread existence (Burt et al., 2001;Navarro-Pedreño et al., 2003). Pedogenic or anthropogenic Ni contamina-tion is hardly avoided as it is a basic constituent element of many soils,particularly of less silisic (ultrabasic) serpentine soils. Average Ni concen-tration in soils is about 16 mg kg−1 (Nriagu, 1980) varying normally be-tween 2 and 750 mg kg−1 (Seregin and Kozhevnikova, 2006) with thehighest concentrations in basic igneous rocks (McGrath, 1995). Ure andBerrow (1982) reported the average Ni concentration of 4265 soil sam-ples as 33.7 mg kg−1. The geology and soil-forming processes stronglyinfluence the amount of nickel in soils (Kabata-Pendias and Mukherjee,2007). Distribution of Ni in serpentine soils depends on parent materialof the soil and on pedogenetic activities that influence the weatheringprocesses (Lee et al., 2004). Ultramafic formations can contain as muchas 0.3% Ni. The extreme weathering removes most elements except theleast soluble ones from the protolith. The residual soil material can aver-age as much as 5% nickel (Marsh and Anderson, 2011).

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Global distribution and total amounts of nickel in soils are wellestablished; however, more information about their phytoavailabilityis still needed. Governing weathering processes and pedogenesis ofserpentine soils differ from location to location with varying climaticconditions as well as the nature of the parent material and other soilforming factors due to the wide distribution and occurrence of thesesoils. Higher levels of Ni and/or Mg in ultramafic soils seem likely toaccount for vegetation change where Ni availability increases withdecreasing pH and lower pH on serpentine soils may be responsiblefor poor forest colonization (Robinson et al., 1996), suggesting highlycomplex interrelations.

Large number of reports on the effectiveness and reliability of extrac-tionmethods ondetermining phytoavailability of nickel to crop plants arenow available (Aydinalp and Katkat, 2004; L'Huillier and Edighoffer,1996; Sukkariyah et al., 2005), whereas information about the effects ofsoil properties onNi bioavailability is rather limited. A series of systematicstudies seem to be necessary on soils with a wide range of physicochem-ical properties with common plant species.

Many researchers focused onNi extractionmethods so as to establishrelationships between Ni concentrations of the soil and plants because anumber of extraction techniques simulating metal uptake process wereclaimed to be useful in estimating the amounts accumulated in plants(Misra and Pande, 1974; Shewry and Peterson, 1976; Wang et al.,2004). Although superiority of any extraction method over the othersseems to depend on plant species and soil types, any relevant analysis

204 İ. Ünver et al. / Geoderma 202–203 (2013) 203–211

method might to be useful at least in comparing the test soils. Thechemical used in this study was diethylene triamine pentaaceticacid (DTPA) as its ionized ligand, which has a high affinity formetal cations. Although the test is commonly used in determiningso-called phytoavailable Ni contents of the soils near neutral (Lindsayand Norvell, 1978; Quantin et al., 2008; Severson et al., 1979), a numberof soil properties including pH and organicmatter contentmay influencephytoavailability (Barančíková et al., 2004; Kukier et al., 2004; Li et al.,2003). Tracemetals extracted by this techniquewere shown to correlatewith plant metal uptake if soil pH variation of test soils is small (Bidwelland Dowdy, 1987; Sukkariyah et al., 2005). On the other hand, large-scale differences in uptake, translocation and accumulation of variouselements indicate the importance of plant species, subspecies and geno-types on the matter suggesting that an element at the rhizosphere couldbe available to one plantwhile not applicable to some others. Superiorityof any extraction techniquemay become controversialwhen consideringthe above mentioned multivariate factors (Kabata-Pendias, 2004). Theextractionmethod has a predictive value if there is a correlation betweenthe extractable pool of a metal in soil and the uptake of the metal by aplant (Kukier and Chaney, 2001). Labile pool of soil Ni could be a usefultool for the assessment of soil-to-plant transfer of Ni, based on thehypothesis that extractable Ni is well correlated with the labile pool,avoiding carrying out of time consuming experiments (Denys et al.,2002).

The objective of this research was to determine the influence of rain-fall and temperature on the DTPA (diethylene triamine pentaacetic acid)extractable (DNi) and fractional (DNi/TNi = F-DNi) nickel concentra-tions of soils derived from ultramafic serpentine rock under temperatesemiarid continental and Mediterranean climates from Western Turkeyand the East Thrace in about 400,000 km2 area.

2. Material and methods

2.1. Climate and vegetation

Precipitation and temperature are the principle climatic variablesinfluencing soil formation (Brady andWeil, 1999). Air and soil tempera-tures are functionally interrelated, and mean annual temperatures havebeen proven quite satisfactory in soil formation investigations. Signifi-cant quantitative correlations between soil properties and mean annualtemperatures were obtained. It should be recognized that the officialrainfall and temperature records deal with macroclimate (Jenny, 1994).

Extensive coverage of the study area did not facilitate establishinga detailed, reliable vegetation index. Less phytodiversity and commonNi tolerant temperate climate plant species including perennialshrubbery and herbaceous members of Brassicaceae were noted. Al-most no differences in relation to health degree and density betweenthe woods grown in the serpentinites and others were obvious.

2.2. Data collection

Weathering climatic agents were restricted to annual precipitationand mean temperature normal distributions, and the time span forthe formation of soils developed on serpentine. The data from the con-tinuously active stations from 1975 to 2010 (36 years) were employedin the statistical analyses. Average annual precipitation in the researcharea was between 321 mm (Konya) and 1147 mm (Muğla). Precipita-tion increases from the central plateau to the sea at the west andsouth. Projected average annual rainfall and mean temperature consid-ering topographic components were required and generated forreaching more reliable conclusions. Mean temperature normal in thearea was between 19.2 °C (Mersin) and 10.4 °C (Kayseri). Temperaturerecordings, which were historically monitored in urban areas, are gen-erally higher on the seashore. However, extrapolation of the climaticdata considering the elevation and slope aspect factors showed greattemperature differences within short distances. Discrimination of the

Mediterranean and continental climates with distinct lines seemedhardly possible due to the existence of broad transition zones.

2.3. Sampling and analyses

The contact between the soil and the serpentine parentmaterialwasrarely prominent to determine the effect of climatic conditions on soilformation. Nickel concentrations show relatively little variation withincreasing depth below 10 cm (Berrow and Reaves, 1986). Soil samples(0–15 cm) were collected from 192 serpentine soils in the WesternAnatolia and the Eastern Thrace of Turkey. Subsamples obtained fromeach stationweremixed on a polyethylene blanket to prepare a homog-enous sample. Total research area was about 400,000 km2, possiblycovering all visible serpentine soil formations in the region. Soil sampleswere taken every 25–100 km2 from extensive ultramafic rocks. Geolog-ical map of 1:100,000 scale prepared by Turkish State Mineral Research& Exploration was used for spotting serpentinite locations and fordesignating the survey routes. The coordinates and elevation of thesampling locations were determined by using a handheld MagellaneXplorist XL receiver, with 3–6 m accuracy. Soil subsamples from aserpentine location were mixed, homogenized, passed through a4 mmplastic sieve in situ and filled in polyethylene bags. Anthropogenicpolluted lands were rejected to avoid possible interferences (Massouraet al., 2006; Němeček et al., 2001). Cultivated lands developed onserpentiniteswere also not sampled considering possible pH etc. interac-tions which may arise from agricultural chemicals.

A laboratory meter was used for pH measurements of a 1:2.5 of soil:water dilution. Free carbonate contents of the soils were determinedwith a manometric pressure calcimeter (Loeppert and Suarez, 1996).Air dried ≤2 mm 50 g oven-dried soil was extracted with 100 mL of aDTPA solution consisting of 0.005 M diethylene triamine pentaaceticacid, 0.1 M triethanol amine, and 0.01 M CaCl2 at pH 7.3 (Lindsay andNorvell, 1978) and 25 °C ± 1, shaken for 2 h. Extracts were centrifugedat 5000 rpm and gravimetrically filtered from Whatman cellulose filtergrade 42. PerkinElmer® Optima™ 7000 DV ICP optical emission spec-trometer (ICP-OES) was used for determining DTPA extractable nickel(DNi) concentration (Miller, 1998). Analysis was carried out with threeparallels. Total elements of acid digested samples were determinedusing ICP-MS (ICP-mass spectrometer) (Hossner, 1996). For total Ni,0.1 g of the soil was digested in 3.5 mL of aqua regia (ultrapure mixtureof concentrated HNO3/HCl, 1/3 (v/v)) on an aluminum block and dilutedwith 5 mL of 0.2% HNO3. Total Ni analyses were double-checked by an-other method as follows: 0.2 g of soil sample was digested with wet di-gestion in 8 mL of aqua regia in 60 mL standard digestion vessels. Thesolution was placed in a Berghof Speedwave® microwave digestion in-strument, v 1.2.2 software, at 180 °C under 40 bar pressure for 20 minas was suggested by the manufacturer. Total Ni concentrations were de-termined by PerkinElmer®Optima™ 7000DV ICP optical emission spec-trometer. The total Ni context data obtained from the latterwas used andreported where necessary.

2.4. GIS studies and mapping

Climatic data (1975–2010) of 185 stations in the area studied wereobtained from the Turkish State Meteorological Affairs. ANUSPLIN andARC GIS 8.1 software packages were largely employed for generation oflocal climatic characteristics to prepare comparative maps using digitalelevation model (DEM) (Tunçay et al., 2006). Procedures from theANUSPLIN software were used to fit the thin plate spline functions,which were trivariate functions of longitude, latitude and elevation(Hutchinson, 1991). ANUSPLIN package allows for arbitrarily manysurfaces and introduces the concept of surface independent variables,so that they may change systematically from one location to another.

The DEM ready to use is a map converted from a 1:250,000 scaledigital topographic map with a resolution of 0.01° covering the studyarea. After the small scale DEM data were used in generating climatic

Table 1Selected parameters of the soils studied.

GPS coordinates DNimg kg−1

pH ECdS m−1

Texture Free carbonates%

OM%

N 37 06.786, E 28 36.181 14.5 7.75 0.12 SL udl 1.11N 37 06.385, E 28 39.250 35.7 7.65 0.13 SCL udl 5.04N 36 51.271, E 28 32.396 12.6 7.48 0.13 SCL udl 1.31N 36 54.525, E 28 35.726 143 7.69 0.12 CL udl 5.34N 36 44.682, E 29 00.391 10.1 7.81 0.10 SCL 1.14 1.21N 36 44.653, E 28 52.634 41.5 7.68 0.16 L udl 3.53N 36 46.586, E 27 59.917 13.0 7.96 0.18 SCL 2.81 2.12N 36 51.306, E 28 12.205 39.6 7.36 0.07 CL udl 3.88N 36 21.519, E 35 55.299 14.1 8.62 0.42 SL 1.82 1.26N 36 53.759, E 28 17.826 52.2 7.24 0.13 CL 1.14 2.55N 37 39.680, E 28 52.228 56.8 8.22 0.36 CL udl 4.25N 37 12.960, E 28 41.008 20.3 8.14 0.16 SL 2.27 3.19N 37 51.594, E 30 54.649 11.5 7.73 0.12 SL 1.79 1.49N 38 02.332, E 31 21.483 33.2 7.89 0.11 SCL udl 1.79N 38 36.695, E 27 45.059 24.2 7.91 0.24 SCL 10.1 3.80N 38 43.316, E 29 41.500 14.8 7.90 0.21 CL 3.25 5.82N 39 36.219, E 29 56.791 28.8 8.10 0.21 C 1.00 3.18N 39 35.364, E 29 57.452 10.3 8.22 0.30 L 3.14 5.08

pH and EC: from 1:2.5 suspension, udl: under detection limit.

0

10

20

30

40

50

60

70

80

<1 1-5 5-10 10-20 20-40 40-80 80-150

66

43

28 27

1511

2

Nu

mb

er o

f sa

mp

les

Ni content range (mg kg-1)

Fig. 1. DTPA extractable nickel concentration ranges of the soils.

205İ. Ünver et al. / Geoderma 202–203 (2013) 203–211

surfaces, all climatic maps were re-projected to the geographic coordi-nates with 0.5° × 0.5° resolution. DNi data were tabulated into 6 arbi-trary range classes to providefitting in ARCGIS 8.1 software formapping.

3. Results and discussion

3.1. Properties of the soils

The basic approach in searching for the climatic effects on soilproperties was that they have the common origin of the parent materi-al, i.e. serpentinite. Selected properties of a number of soils containingmore than 10 mg kg−1 DNi were identified (Table 1).

Soil pH in the area studied varied between 7.24 and 8.62 with mostvalues from 7.5 to 7.8. Those pH ranges support the view that theclimatic conditions in the area were conducive for the persistence ofbases in the soils. Salinity was not troublesome with any soils wheretheir free carbonate content was normally less than 1% with a fewexceptions, i.e. only one sample contained 10.1% carbonates. Thisdominant deficiency of free carbonates was not attributed to leachingas no acidic reaction was detected with the soil samples. Texturalclasses were usually around loamy groups. A few numbers of relativelyhigh organic matter containing soils suggested that serpentine soilsmay be fertile under favorable environmental conditions. Studying thepoor vegetation was outside of our research scope. Organic mattercontents of the surface soils were generally between 1 and 4%, whichis expected in arable soils under temperate climatic conditions (Botand Benites, 2005).

Comparing the sampleswith other regional soils, it appeared that thesoils studied have relatively lower organicmatter and lime contentswithmedium texture.

3.2. DNi concentration

DNi concentration of the soils was highly variable. TheDTPA extract-able pool is considered to be a measure of the amount of potentiallyplant-available metal in the soil (Bani et al., 2009; Li et al., 2003). Num-bers of arbitrarily selected ranges of the DNi concentrations are repre-sented in Fig. 1. The highest DNi concentration among 192 soilsamples, 143 mg kg−1 DNi was 5.75% (F-DNi) of total 2480 mg kg−1

(TNi) of its own soil. Many samples contained less than 0.1% DNi, corre-spondingly having very low F-DNi. These small ratiosmade it difficult toseparate the roles of climatic conditions, soil genesis, topographic com-ponents, geological and vegetative history in the development of theextractable nickel concentrations.

The first conclusion to be drawn was relatively weak (positive andexponentially significant, P b 0.05) relationship between TNi and DNiin the soils (Fig. 2). Previous surveys of serpentine sites provided totalnickel analysis, which were of restricted ecological value since suchfigures needed not directly reflect availability to plants. If DTPA extrac-tion resembles phytoavailability to an extent, seeking or establishingcorrelations with the total amount in soils and the accumulation inplant bodymight probably be difficult. The possible causes of consid-erable variation in soluble and total nickel of British serpentine soilswere discussed in terms of the influence of maritime conditions andweathering regimes (Slingsby and Brown, 1977).

3.3. pH dependency of DNi and F-DNi concentrations

Important factors affecting Ni phytoavailability in soils are pH aswellas the presence and abundance of organic materials, hydroxides, clayminerals, cations, and complexing ligands (NRCC, 1981). Soil pH has aclose relationship to the phytoavailability of heavy metals controllingtheir solubility and their capacity to form chelates in the soil. Soil pHversus DNi was evaluated as an accepted major soil parameter control-ling nickel solubility in the soil (Ge et al., 2000; Gray and Mclaren,2006; Tye et al., 2004). However, the degree of the relationship amongparameters was so formulated that site-specific modifications of valuesof partitioning coefficients (Kd) to input into environmental fate modelsshouldminimally be correct for pH effects especially when using an em-pirical approach (Sauvé et al., 2000), underlining the importance of otherenvironmental parameters as well as pH of the soil.

y = 0.233e0.002x

R² = 0.89P<0.05

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500 3000

DN

i, m

g k

g-1

TNi, mg kg-1

Fig. 2. The relationship between total and DTPA extractable Ni concentrations of thesoils, the curve indicates exponential relationship.

y = 83.774x2 -1327.4x + 5273.4R² = 0.175, n.s.

0

20

40

60

80

100

120

7.20 7.40 7.60 7.80 8.00 8.20 8.40

DN

i, m

g k

g-1

Soil pH

y = 7.7237x2-119.83x + 465.04R² = 0.285, n.s.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

7.3 7.4 7.5 7.6 7.7 7.8 7.9 8F

-DN

iF

-DN

i

Soil pH

Soil pH

b

c

a

4

3.5

3

2.5

2

1.5

1

0.5

07.3 7.5 7.7 7.9

Fig. 3. pH dependency of D Ni (a) and F-DNi (b–c) concentrations of the soils.

206 İ. Ünver et al. / Geoderma 202–203 (2013) 203–211

The pH ranges of the soils studiedwere between neutral and alkaline.The relationship of pH of 1:2.5 soil suspensions versus DNi concentrationindicated a negative tendency, i.e. higher pH values resulting in lowerDNi concentration (Fig. 3a). Less alkaline soil reaction also resulted inhigher F-DNi (Fig. 3b). However, a weak and inconsistent correlation be-tween Ni extractability and soil pH was observed (for DNi: R2 = 0.175,n.s. and for F-DNi: R2 = 0.285, n.s.). The polynomial curve, which fittedthe best, indicated a less sloping line above pH 8.03 likely suggesting lessdependency of DNi with soil reaction within these small ranges. Theclose similarity of the graphs, one of which showed the soil pH vs. DNi(Fig. 3a) and the other pH vs. F-DNi (Fig. 3b) relationship indicates theeffect of pH on Ni extractability and perhaps potential influence ofother soil components on eventual soil reaction. If varying Ni releasewas attributed to soil pH ranges based on the similar trends of thesetwo figures, it could be concluded that TNi content hardly influencedweathering processes and corresponding Ni release under studiedconditions.

Results showed a more pronounced effect of soil pH on nickel ex-tractability with acidic conditions as documented by several studies:The amount of Ni extracted by DTPA was found dependent on pH ofthe soil sample, with retention dramatically increasing above pH 7.0to 7.5 and Ni was somewhat less extractable than Pb, Cu and Zn, withthat sorbed by the highest pH soils being the least extractable (Harter,1983).

Higher pH values (above 8) or organic matter content increased theamount of Ni adsorbed on the soil particles (Ge et al., 2000). DecreasedNi toxicity at 7.2 pH compared to pH 6.2 on corn (Wallace et al., 1977)was supported by similar findings with tomato and barley, where vary-ing results from soil to soil was attributed to different cation exchangecapacities of the soils (Rooney et al., 2007). Nickel concentrations inshoots of two Alyssum species were reported to increase with higherpH soil despite a decrease in water-soluble soil Ni (Kukier et al.,2004), opposite to that seen with agricultural crop plants, this resultagain indicating complexity of the effects of soil pH on Ni availabilityand plant growth. Staunton (2004), discussing these pH dependent var-iations, proposed a radiotracer technique to explain the changes.

Soil pH is naturally expected to influence almost all weathering pro-cesses. The effect of pH on F-DNi was nonsignificant in this study, likelybecause of the narrow variation ranges. Then, a transcendental plotting,which could give positive relationships was also tried (Fig. 3c). Thefindings were interpretable enough as follows: R2 = 0.586 andP b 0.01. This specific graph, although seems scientifically accept-able, is deliberately reported with the reservations of limited literaturecitation on thematter, inadequate sample numbers and limitedpH rangesused for plotting.

3.4. Influence of rainfall and temperature

Meteorological data were gathered from the available centers. Thesoils were grouped in provinces to facilitate scrutinizing the influence

of climatic conditions on their phytoavailable Ni concentrations. Thesoils developed under Mediterranean climate with hot and dry sum-mers and rainy during the winter time had more DNi and F-DNithan those soils formed in the arid central Turkey (Table 2). If Bursa,located at a transition zone, were to be ignored, average DNi concen-tration of the Adana soils was the only exception and this may beexplained by being mountainous. The serpentine soils of the higherrainfall regions, including Muğla (1127 mm) and Hatay (1084 mm)had the highest DNi concentrations. Partial high variation coefficientsof a few provinces and positive skewness coefficients made them lessreliable. Negative and low positive skewness of several groups as wellas relatively small standard deviation and variation coefficients sug-gested higher reliability.

Both annual precipitation and mean temperature gave positivecorrelation with DNi and F-DNi (Figs. 4 and 5) indicating that thoseclimatic components remarkably influenced weathering processes.It was hardly possible to discriminate the roles of the temperature andrainfall due to frequent coincidence of higher temperature andmore pre-cipitation at the same stations. Higher R2 value of precipitation versus

Table 2The relationship between average precipitation, mean temperature and DTPA extractable nickel distribution.

Climate components Statistical components

R, mm T, °C A M

mg kg−1 Ni mg kg−1 Ni SD VC SC n

Bursa 674 14.5 0.19 0.24 0.11 0.01 −1.58 3Aksaray 350 11.9 0.34 0.27 0.13 0.01 1.73 3Adana 657 19.1 0.64 0.27 1.12 1.10 2.81 8Ankara 405 11.8 0.88 0.80 0.69 0.41 0.85 7Bolu 550 10.4 1.20 0.51 1.41 1.33 1.68 3Kayseri 397 10.4 1.56 1.96 1.20 1.19 −0.62 6Niğde 327 11.0 1.62 0.48 2.46 4.53 1.96 4Eskişehir 367 10.6 2.19 0.89 3.06 8.03 2.25 7Konya 321 11.4 3.31 2.49 3.27 9.60 1.28 10Çanakkale 591 14.9 3.93 3.79 4.06 12.3 0.06 4Manisa 702 16.9 4.16 0.28 8.93 68.3 2.54 7Kütahya 547 10.6 5.10 1.45 7.99 58.9 2.48 13Isparta 494 12.0 7.00 1.97 10.5 98.6 2.06 10Antalya 1052 18.2 8.05 10.3 6.04 33.2 0.31 11Mersin 572 19.2 8.28 8.49 1.09 0.95 −0.99 5Burdur 405 13.0 11.1 4.91 12.6 147 1.39 15Denizli 546 16.1 13.8 8.37 16.6 252 1.92 11Hatay 1084 18.2 18.3 19.8 3.66 8.92 −1.51 3Muğla 1127 14.9 26.1 12.8 29.7 865 2.02 48

R: Annual rainfall (mm), Temperature normal (°C), A: Average, M: Median, SD: Standard deviation, VC: Variation coefficient, SC: Skewness coefficient, n: Number of soil samples.Bold figures for prevailing Mediterranean climate.

207İ. Ünver et al. / Geoderma 202–203 (2013) 203–211

DNi concentration suggests that weathering of nickel containing min-erals in the serpentine soils could be more dependent on the rain ratherthan the mean temperature. The less twisted concave polynomial curveof the DNi vs. rainfall and F-DNi vs rainfall may likely indicate increasinginfluence of rainfall on Ni release, whereas convex curves were obtainedwith mean temperature recordings. The effect of mean annual tempera-ture normal on F-DNiwas statistically nonsignificant. This fact again sup-ports prevailing role of rainfall on weathering as a climate component.

Prevailing characters are highlighted to differentiate the Mediterra-nean from continental climatic for the sampling areas (Table 2). DNi con-centrations might change considerably within short distances becausestandard deviation and variation coefficient were high particularly in

y = 4E-05x2-0.037x + 12.8R² = 0.553, P<0.01

0

5

10

15

20

25

30

200 400 600 800 1000 1200

DN

i, m

gkg

-1

Annual rainfall, R, mm

y = 4E-06x2-0.0029x + 0.9296R² = 0.597, P<0.01

0

1

2

3

4

200 400 600 800 1000 1200

F-D

Ni,

%

Annual rainfall, R, mm

b

a

Fig. 4. The relationship between its annual rainfall and DNi (a) and F-DNi (b) concentra-tions of 19 provinces having at least three soil samples, straight line indicates relationship.

theMediterranean Zone, the south andwest regions of Turkey. Deviationswere attributed to varying ages of the soil development. None of theCentral Anatolian soils were found containing more than 10 mg kg−1

DNi (Figs. 6 and 7). As most of the meteorological stations were locatedin or near the towns, an additional GIS analysis was performed to providecrossing of climatic data to the sampling points, almost all of which werein rural areas.

Multiple regression analyses suggest that precipitation and meantemperature altogether were responsible for 52.6% (P b 0.01) of DNiextractability (Fig. 8a). Coefficient of the standard deviation for this anal-ysiswas 21.6 and the bestfitting linear equation confirms reliability of therelationship. It seems that mean temperature and the amount of annual

y =-0.33x2+ 10.4x -71.6 R² = 0.257, P<0.05

0

5

10

15

20

25

30

9 11 13 15 17 19 21

DN

i, m

g k

g-1

Mean temp normal, T, oC

y =-0.0469x2+ 1.4532x -10.067 R² = 0.2205, n.s.

0

1

2

3

4

5

10 12 14 16 18 20

F-D

Ni,

%

Mean temp normal, T, oC

b

a

Fig. 5. The relationship between its mean temperature normal and DNi (a) and F-DNi(b) concentrations of 19 provinces having at least three soil samples, straight line indicatesrelationship.

Fig. 6. The relationship between annual rainfall (mm) and DTPA extractable Ni concentrations of the soils.

208 İ. Ünver et al. / Geoderma 202–203 (2013) 203–211

rainfall are greatly responsible for the release of Niwhere its total amountin the serpentines is not considered.

3.5. Total Ni and percent DNi concentrations

The ratios of DNi to TNi concentrations were between 0.05 to 3.91%,with an average F-DNi value of 0.61%. These small percentages may notbe readily correlated relatively slow release of Ni from the serpentines.Ure and Berrow (1982) stated that Ni is relatively easily mobilizedduring weathering, due to its abundance in quick weatherability fromferromagnesian minerals. Yet, serpentine soils still have potentialavailable sites for further Ni adsorption and the presence of hydrousMn oxides in these soils as well as Fe oxides determine the Ni sorptioncapacity (Alves et al., 2011). Large variations of F-DNi suggested that the

Fig. 7. The relationship between mean annual temperature norm

climatic factors are the most significant factor in weathering of the ser-pentine soils (Table 3). The multiple regression analyses showed that59.8% of DTPA extractability from total Ni content could be explained bycombined effect of precipitation and mean temperature (Fig. 8b). Thebest fitting cubic relation was significant (P b 0.01) and the coefficientof standard deviationwas 0.61. This combined effect, relying on previous-ly mentioned limited effect of mean temperature on F-DNi may notcompletely define the historical background of study conditions.

Organicmatter content, texture (Siebielec et al., 2007), pH (Horak andFriesl, 2007; Weng et al., 2004), CEC, aeration degree (Kabata-Pendias,2004) and biological activities (Díaz-Raviña and Bååth, 1997) werereported to be the main factors affecting extractable Ni concentration inthe soil. The ratio of DNi to total Ni contents range could be associatedwith the accumulation in plant tissues.

al (°C) and DTPA extractable Ni concentration of the soils.

Table 3DTPA extractable (DNi), total (TNi), and fractional (F-DNi) nickel concentrations of selected

Location GPS coordinates El

Adana, Kozan-Feke N 37 34.661, E 35 50.433 3Adana, Saimbeyli N 38 00.261, E 36 05.707 10Aksaray, Ihlara, Melendiz N 38 24.348, E 34 07.663 11Ankara, Karagedik N 39 25.300, E 32 52.204 10Ankara, Nallıhan N 40 06 662, E 31 36.901 4Ankara, Gülhüyük N 39 05.938, E 33 33.746 9Antalya, Çandır, Demirciler N 37 02.146, E 31 03.533 5Bolu, Abant, Çepni N 40 35.051, E 31 16.319 11Burdur-Fethiye, Karaçal N 37 33.804, E 30 04.861 9Bursa, Orhaneli, Doğancı N 40 05.779, E 28 56.543 3Denizli, Karacahöyükavşarı N 37 32.201, E 29 26.680 9Isparta, Eğridir-Gelendost N 37 51.594, E 30 54.649 10Isparta, Sütçüler, Çobanisa N 37 29.000, E 31 01.147 11Isparta, Cankurtaran N 38 14.749, E 31 20.077 12Kayseri, Yeşilhisar, Araplı N 38 15.522, E 35 04.411 13Kırşehir, Akpınar N 39 21.957, E 34 00.608 11Konya, Altınekin, Sarıtaş N 38 18.870, E 32 54.037 10Konya, Beyşehir yolu N 37 51.779, E 32 23.027 13Konya, Çeşmelisebil N 38 37.815, E 32 32.503 10Konya, Kulu, Kandil N 38 58.267, E 32 31.064 12Konya, Taşkent N 36 55.512, E 32 29.485 15Kütahya, Çavdarhisar N 39 16.761, E 29 56.674 11Kütahya, Seyitömer N 39 33.770, E 29 53.593 11Kütahya, Tavşanlı, Andız N 39 29.906, E 29 53.138 9Manisa, Doğankaya N 38 59.856, E 27 57.022 5Muğla, Köyceğiz, Sultaniye N 36 54.525, E 28 35.726Muğla, Balandağı N 36 53.128, E 28 18.941 7Muğla, Marmaris, Çetibeli N 36 59.539, E 28 19.881 1Niğde, Pozantı-Çamardı N 37 39.979, E 34 59.901 12

Fig. 8. a. The combined effect of annual precipitation andmean temperature onDNi concen-trations. b. The combined effect of annual precipitation and mean temperature on F-DNi.

209İ. Ünver et al. / Geoderma 202–203 (2013) 203–211

The highest DNi concentrations of 9 soils were found inthe Mediterranean region. The highest TNi concentration was2840 mg kg−1. It is expected that Ni leaching process is slower inthe temperate than hotter and rainy tropical regions or acidic soils.Massoura et al. (2006), reinforced the hypothesis stating that the Niconcentration was relatively limited in ultramafic primary mineralsand poorly enriched in secondary neoformed phyllosilicates (generally0.2–0.3%, with a maximum value of 0.5%) under temperate andMediterranean environmental conditions.

The highest DNi containing 3 soils (A1, A2 and A3) showed that DTPAextractability nickel concentration of the serpentine soil could be as highas 5.77% of the total under study conditions. Actually, thatmaximum ratioshould be expected smaller, as larger particles, of which specific surfacearea is limited were eliminated at a couple of locations by sieving thesoils to 4 mm during sampling.

3.6. Other heavy metals in the soils

Total analysis showed common properties of the serpentine soilswhich were high in Fe, Mn, and low in Al and Ca contents (Tables 4aand 4b). Total amounts, however, do not necessarily representphytoavailable heavy metal concentration. Extremely low Ca/Mg ratioismost likely due to lowCa contents of the soils derived from serpentine.

Iron, Cr, Co andMn contents of the selected serpentine soils are higherthan those reported in the literature (Alexander et al., 2007). Theseamounts may bemore attractive for mining rather than for plant growth.

Weak growth of normal plants on serpentine soils may be due to anumber of factors including a number of nutrient deficiencies (Ca, P, N,K, and Mo) (Brady et al., 2005; Chaney et al., 2008; Chiarucci et al.,2003; Spence and Millar, 1963) and in acidic serpentine soils due to Niphytotoxicity (Anderson et al., 1973; Brady et al., 2005; Halstead, 1968;Kazakou et al., 2008). Although many serpentine soils have anomalouslevels of Co, Cr and Mn compared to average soils as the results obtainedfrom the study soils showed (Table 5), there is no evidence that these el-ements actually limit plant growth on serpentine soils. Decreasing order

serpentine soils (Ni mg kg−1 soil).

evation (m) DNi TNi F-DNi

%

85 0.28 32.3 0.8758 0.08 37.4 0.2114 0.26 49.2 0.5390 0.8 566 0.1473 0.11 34.1 0.3275 0.14 25.7 0.5400 12.6 1780 0.7125 0.51 72.3 0.7127 0.6 372 0.1670 0.07 147 0.0536 12.2 1610 0.7654 11.5 1280 0.946 0.28 89.5 0.3100 0.28 49.4 0.5795 2.59 960 0.2725 2.18 737 0.330 2.93 805 0.3625 0.28 426 0.0777 3.45 930 0.3748 9.89 1490 0.0729 1.16 344 0.3450 0.24 75.2 0.3234 3.89 843 0.4680 0.48 142 0.3474 0.28 58.7 0.4711 32.8 2610 1.2606 52.2 2550 2.0534 105 2680 3.9130 5.29 1970 0.27

Table 4aTotal macro elements metal and metalloid concentrations of the selected soils (g kg−1).

Soil➔ A1 A2 A3

Ni 2660 2490 2660Fe 80.9 88.8 77.9Ca 3.30 1.60 6.60Mg 74.1 74.9 131Na 0.05 0.04 0.23K 0.80 0.70 0.60P 0.23 0.09 0.16S b0.50 b0.50 b0.50Al 6.80 7.00 5.00Ti 0.10 0.12 0.08

Table 5DTPA extractable heavy metal concentrations of the selected soils, mg kg−1.

A1 A2 A3

Ni 143 105 75.6As 0.07 0.06 0.04Co 2.36 1.22 0.57Cr 0.02 0.01 0.01Cu 0.78 0.79 0.41Fe 96.7 39.1 18.4Mn 82.9 62.9 13.2Pb 0.42 0.28 1.58Zn 2.29 1.38 5.64

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of As, Co, Cr, Fe, Mn, and Pb from the soil A1 to A3, with the similartendency of Ni, implied that weathering conditions may affect the DTPAextractable concentrations of those metals. Three soil samples were cer-tainly not enough to draw a conclusion, but this seemed to deservefurther attention.

4. Conclusions

It was confirmed that DTPA extractable nickel concentration of thesoils studied could be an indication of the degree of weathering pro-cesses in serpentines. The soil pH, changing within relatively narrowranges, between neutral and moderately alkaline had a weakimmobilizing effect on the soils studied. Annual precipitation andmean air temperature have remarkable influence on the DNi andF-DNi amounts. As expected, the effect was more pronounced underMediterranean conditions than semiarid temperate continental cli-mate regions. DNi concentrations of the serpentine soils may changefrom trace to 143 mg kg−1 dry soil suggesting that the range ofDTPA-extractable Ni is a poor indicator for estimating total Ni concen-tration in serpentine soils.

Acknowledgments

This study was funded by TUBITAK (The Scientific and Technolog-ical Research Council of Turkey) as project no: TOVAG-105O635. Wegratefully acknowledge the cooperation and guidance of Dr. A.

Table 4bTotal micro elements metal and metalloid concentrations of the selected soils (mg kg−1).

Soil➔ A1 A2 A3

Mo 0.2 0.2 0.3Cu 11.9 14.5 15.9Pb 13.1 7.5 47.7Zn 63 87 164Ag b0.1 b0.1 b0.1Co 154 166 160Mn 1340 1570 1270La 4 4 4As 2.1 2.0 2.2U 0.3 0.4 0.3Cr 631 536 211Th 1.0 1.3 1.2Sr 13 7 13Sb b0.1 b0.1 b0.1Bi b0.1 b0.1 b0.1V 29 26 17Ba 33 27 29B b20 b20 b20W b0.1 b0.1 0.1Hg 0.11 0.09 0.10Sc 7.7 8.0 5.7Tl b0.1 b0.1 b0.1Ga 2 2 2Se 0.6 b0.5 b0.5Au 10−3 0.6 0.8 1.6

Karabulut Aloe and Dr. İ. Bayramin in the GIS studies and in generat-ing the maps. We are deeply indebted to Dr. E. Başpınar and Ms E.Özgümüş for their generous knowledge-sharing and stimulating ad-vice in statistical evaluation.

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