Soil parent material is the main control on heavy metal concentrations in tropical highlands of Brazil

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Soil parent material is the main control on heavy metal concentrations in tropical highlands of Brazil ⁎

Yuri Lopes Zinna, , Jéssica Amaral de Fariab, Marla Alessandra de Araujoa, Alba Lucia Araujo Skorupaa a b

Depto. de Ciência do Solo, Universidade Federal de Lavras, Lavras, MG 37200-000, Brazil Depto. de Química, Universidade Federal de Lavras, Lavras, MG 37200-000, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: Geochemistry Soil micromorphology Lithology Trace elements Forest soils Pedogenesis

Despite recent efforts to assess heavy metal contents in soils in Brazil, little is known about the effect of parent material and sampling depth, both critical factors controlling soil geochemical composition, especially in areas with complex geology. We determined concentrations of Cr, Cu, Mn, Ni, Pb and Zn in soil profiles developed from eight contrasting rocks (quartzite, mica schist, gabbro, gneiss, limestone, phyllite, itabirite and ironstone after serpentinite) in a relatively small area under similar climate and native forests in Minas Gerais, Brazil. The hypotheses tested were that soil metal concentrations vary with parent material and depth. Soil samplings were done in triplicate at the 0–5, 30–40 and 90–100 cm depths, and metal concentrations assessed after nitric acid digestion and atomic absorption spectrometry determination. Generally, metal concentrations varied little with depth, and Pb concentrations were low and very similar among soils (always < 2.5 mg kg−1). Cu, Mn and Zn concentrations were also low (respectively, < 32 mg kg−1, < 330 mg kg−1 and < 27 mg kg−1) in all soils, except for that on itabirite, which was enriched in these metals, especially on Mn (6.850 mg kg−1). Conversely, soil Cr levels in all soils except for that on quartzite were high (> 75 mg kg−1), reaching 210–258 mg kg−1 in the soils derived from mafic and ultramafic rocks. Ni was generally < 18 mg kg−1, except in the soils on itabirite (36–43 mg kg−1) and ironstone after serpentinite (ca. 600 mg kg−1). Metal concentrations were best correlated with Fe oxide contents. The hypothesis that heavy metal concentrations vary with depth was rejected, whereas the hypothesis of parent material effect was confirmed, mostly due to the geochemically unique rocks itabirite and ironstone after serpentinite.

1. Introduction Heavy metals occur in soils in various forms, and their quantitative partition throughout these forms critically controls their activity in the environment. In increasing order of biological availability, heavy metals are present in soils as components of primary minerals in rock fragments; as substituting ions in crystal lattices of secondary minerals (e.g, carbonates, oxides and clays); sorbed at mineral and organic surfaces; as organic chelates and complexes; immobilized in microorganisms and plants, and finally in the soil solution (Schmitt and Sticher, 1991; McBride, 1994). Despite this variability of forms and availabilities, the chief control of heavy metal concentrations in pristine environments is mostly the soil parent material (Wilson et al., 2008; Kabata-Pendias, 2011; Fabricio Neta et al., 2018). Such effect is basically due to the presence of heavy metals in primary minerals in igneous and metamorphic rocks, or to their segregation and deposition

⁎

processes in sediments and soils, as is the case for Mn, which soil concentrations are often unrelated to those in parent materials (McBride, 1994). However, the effect of parent materials on heavy metal concentrations is demonstrable only for autochthonous (or authigenic) soils, i.e. those developed directly from alteration of the underlying parent material, with little or no contribution from elsewhere sources. Although this assertion may seem self-evident, autochthony is however a considerably difficult assumption to validate using routine soil analyses. Autochthonous soils developed from weathering of basic rocks typically present higher concentrations of heavy metals when compared to soils formed on granites, gneisses, sandstones and siltstones (Tiller, 1989; Oliveira et al., 1999). Ultrabasic rocks are highly enriched in some heavy metals, namely Co, Cr and Ni, even when compared to basic rocks. Conversely, some quartzites are strongly impoverished in heavy metals. However, most of such research on soil heavy metals has

Corresponding author. E-mail address: ylzinn@ufla.br (Y.L. Zinn).

https://doi.org/10.1016/j.catena.2019.104319 Received 13 December 2018; Received in revised form 8 October 2019; Accepted 14 October 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Yuri Lopes Zinn, et al., Catena, https://doi.org/10.1016/j.catena.2019.104319

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Fig. 1. Map of sampling locations (see Table 1 for numbers designating sampled soils).

advanced degree of weathering due to the dominant tropical humid climate, resulting in a great depth (often > 5 m) to the weathering front, and d) even where weathering is not extremely advanced, it is so efficient that similar Inceptisols can be formed from interaction of highly contrasting factors of soil formation (e.g., Skorupa et al., 2017). In any case, the paucity of knowledge on the effect of soil parent materials limits our understanding of potential public health risks in Brazil and other tropical humid areas, as more marginal areas are incorporated into agriculture or development. In addition, the widespread occurrence of small areas of unusual rock formations, such as ultramafic rocks and mineral ores, often are unaccounted for and can result in many pristine areas showing high concentrations of potentially toxic elements, which can even result in liability to land owners if legal background concentrations are exceeded. The objective of this work was to measure the concentrations of selected chalcophile (Cu, Pb and Zn) and siderophile (Cr, Mn, Ni) heavy metals in soils derived from eight different parent materials under native forests near Lavras, Brazil. In addition, by studying heavy metals with low (e.g, Pb, Cr) to high mobility (e.g. Zn, Mn), we aimed to gain a better perspective on the processes involved in their mineralization, segregation and retention in soils. The hypotheses tested were that soil concentrations of the selected heavy metals vary according to (a) soil parent material and (b) soil depth within the 0–1 m interval.

been carried so far in temperate countries, where cooler climates result in mild weathering conditions, and soil development can be relatively recent (Holocene), resulting in relatively shallow soils (e.g., with depths < 2 m). As a general rule, the effect of parent material on soil properties becomes much weaker for pre-weathered parent materials, or after a long period of pedogenesis (Chesworth, 1973). Such conditions are common in the humid tropics, where most soils are highly weathered and often very deep (e.g., > 5 m, Lara et al., 2018). Thus, in tropical soils, the surface layers mostly used by men and organisms are very distant to the subsoil weathering front, where heavy metals are liberated, which makes difficult to assess the effect of parent material on soil heavy metals. In the last decade, multiple efforts have been undertaken in Brazil to assess soil heavy metal concentrations in a regional or state-wide scale, aiming to establish a reference background to indicate quality, prevention and intervention limits for land use planners. Typically, these works involve sampling areas under various land use systems, spread across a wide area within a region or state, and only for superficial soil (e.g., 0–20 cm depth) (e.g., Paye et al., 2010; Biondi et al., 2011; Souza et al., 2015). Only a few works focused strictly on pristine areas under native vegetation or advanced regeneration (e.g., Skorupa, 2013), and the few works sampling subsurface soil layers deal with few soils (e.g., Marques et al., 2004; Ferreira et al., 2018). In all these works, extraction of heavy metals is done by acid digestion and determination by spectroscopic techniques, and due to the large number of samples, results are often expressed as descriptive statistics, and sometimes correlation analyses and geostatistics. However, in these studies, a series of constraints hinder a proper appraisal of the effect of soil parent materials on heavy metals, such as: (a) frequently limited or no information on underlying lithology, as well as complex lithologies involving the occurrence of many rock formations in some areas, or imprecise or small-scale geological maps, and also absence of rock outcrops and fragments; (b) a typically unassessed occurrence of allochthonous soil parent materials, especially in old polygenetic surfaces, that occupy large areas in Brazil and the tropics (Zinn & Bigham, 2016); (c)

2. Materials and methods 2.1. Environmental settings and sampling The study area is among the most geologically diverse in Brazil, as there are 27 mapped lithologies in a relatively small (ca. 2500 km2) area (Quéméneur et al., 2002). Such unique setting is ideal for a ceteris paribus study of the effect of parent material as a factor of soil formation and geochemical composition, since it allows for a highly homogeneous, warm temperate climate and similar semi-deciduous forest vegetations in all sites. In addition, to keep the effect of topography as 2

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Table 1 Sampled soils, respective parent material and summary horizon characterization (adapted from Araujo et al., 2014). Soil/Parent material

Horizon/depth (cm)

pH

SB

Al

molc kg soil 1. Humic Dystrudept/Quartzite 2. Typic Haplohumult/Mica-schist

3. Rhodic Humic Acrudox/Gabbro

4. Typic Haplohumult/Gneiss

5. Anionic Acrudox/Metalimestone

6. Haplohumult/Phyllite

7. Typic Kanhapludalf/Itabirite

8. Petroferric Acrudox/Ironstone after Serpentinite

A (0–18) B (18–100+) A1 (0–26) A2 (26–62) B (62–100+) A (0–22) AB (22–42) B (42–100+) A (0–10) AB (10–24) Bt (24–50) BC(50–100+) A (0–15) AB (15–50) B (50–100+) A (0–15) AB (15–27) Bt (27–100+) A (0–8) AB (8–34) B (34–90) BC(90–100+) A (0–8) B (8–100+)

4.6 4.9 4.9 4.9 4.9 4.3 4.7 5.4 4.6 4.6 4.7 4.6 4.9 4.9 4.9 4.9 4.9 5.0 5.7 5.6 5.6 5.7 5.1 5.8

0.3 0.2 0.4 0.3 0.2 0.3 0.3 0.2 5.1 0.8 0.3 0.2 0.7 0.2 0.2 5.6 2.6 1.0 17.9 6.4 5.4 3.7 1.7 0.3

1.7 0.9 4.0 2.4 1.8 2.5 1.5 0.5 0.6 1.4 1.2 1.0 0.9 0.3 0.9 0.2 0.3 0.2 0.0 0.1 0.1 0.1 0.2 0.1

T

Clay

−1

SiO2

Al2O3

Fe2O3

K2 O

MnO

% weight

12.2 5.4 24.5 15.7 10.7 16.0 10.5 7.8 12.4 7.6 5.1 3.6 9.5 6.8 6.3 9.5 6.5 4.6 21.4 12.3 10.3 6.4 6.7 2.5

11.6 13.1 45.7 42.4 41.5 73.5 70.7 73.3 41.2 37.9 39.4 35.8 72.9 75.8 79.0 29.1 31.6 56.5 26.3 26.6 29.9 23.4 18.3 18.1

62.6 59.7 38.8 39.8 38.1 24.1 25.8 25.3 55.7 51.0 37.3 45.4 23.2 21.3 22.6 46.8 48.1 41.4 36.6 40.4 39.8 41.2 12.9 13.2

26.1 27.3 47.1 45.5 45.0 44.8 45.0 43.8 37.5 43.0 57.4 49.1 56.0 58.0 57.1 37.6 37.1 40.2 25.8 26.9 27.4 28.8 21.0 21.4

8.1 9.5 10.9 11.5 13.1 33.8 31.9 33.1 6.5 6.4 6.1 5.9 20.4 20.9 20.0 13.6 13.7 18.5 35.3 30.8 32.2 31.0 72.3 71.1

4.4 4.8 3.5 3.4 3.9 < d.l. < d.l. < d.l. 0.8 0.8 0.6 0.9 < d.l. < d.l. < d.l. 2.9 2.1 1.2 2.3 2.6 2.4 0.9 < d.l. < d.l.

< d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. < d.l. 1.4 1.2 1.0 < d.l. < d.l. < d.l.

Minerals Clay

Sand

K, I, HIV, Gi, Go

Qtz, Il

K, I, HIV, Gi, Go

Qtz, Mt

Gi, K, He, Go

Qtz, He

K, Gi, Go

Qtz, Mi

Gi, K, Go, He

Qtz

K, I, Gi, Go, He

Qtz, Mi

K, I, HIV, Gi, Go, He

Qtz, He, Mt, Mi

He, Gi, Go

He, Mt, Gi, Qtz

Obs. Numbers before each soil are respective locations in Fig. 1. SB: sum of exchangeable bases (Ca, Mg, K), Al extracted with KCl 1 M, T: total cation exchange capacity at pH 7.0. K – kaolinite, I – illite, HIV – hydroxyl-interlayered vermiculite, Gi – gibbsite, Go – goethite, He – Hematite, Qtz – quartz, Il – ilmenite, Mt – magnetite, Mi - mica. Minerals are listed in decreasing order of importance, and did not vary among horizons.

drying at 60 and 100 °C (24 h each), the samples were impregnated with an epoxy resin, hardened and cured at 100 and 140 °C. After cutting with a diamond saw, resin blocks were mounted on glass slides, and eventually lapped to a thickness of 30 µm for micromorphological description. Fragments of the parent material were also collected at each site.

constant, we chose to sample only the midslope position of the landscape. Thus, after a series of field trips, we selected eight soils derived from the weathering of the following parent materials: quartzite, micaschist, gabbro, gneiss, meta-limestone, phyllite, itabirite and ironstone after serpentinite (Fig. 1). Mean annual temperature and precipitation are 19.3 °C and 1530 mm, respectively, with summer rainfall and dry winters. Temperatures are relatively mild in comparison to other tropical areas due to the highland altitudes, ranging from 895 to 1275 m a.s.l. (Fig. 1). A detailed morphological, mineralogical and geochemical (major elements) characterization of both rocks and soils is available in Araujo et al. (2014), but the soils are basically described as acidic and with low fertility (Table 1). In addition, the soils showed a desired wide range in soil texture, with clay contents varying from 11.6 to 79%, and also in chemical composition: SiO2 contents varied from 13 to 62%, whereas Al2O3 and Fe2O3 varied from 21 to 58% and 6 to 72%, respectively. Clay mineralogy also varied widely, ranging from highly kaolinitic to mixed kaolinitic-illitic-vermiculitic, to very rich in gibbsite or hematite, and in the case of the Acrudox on ironstone/serpentinite, without kaolinite (Table 1). Thus, the sampled soils cover an ample range of conditions likely to affect heavy metal contents. In each soil, three pits with a 1.2 m depth were excavated and bulk samples were collected with wooden tools at the standard depths 0–5, 30–40 and 90–100 cm, from each pit. Thus, samplings were taken in three true replicates, distant approximately 10–30 m from each other, and the 0–1 m depth was chosen to assess any effects of depth on metal concentrations, either by proximity to organic-rich A horizons or to the parent material underneath. Soil samples were air dried, passed through plastic 2 mm sieves, and then milled in an agate mortar until completely passing through a 100 mesh sieve. In this work, ascertaining if the studied soils were indeed autochthonous is critical to the goal of assessing the effect of parent materials on soil heavy metals, and thus micromorphological description was also performed to support such diagnosis, vis-à-vis the mineral composition and micromorphology of the respective parent materials described by Araujo et al. (2014). Undisturbed samples were taken from the B horizons of one replicate for each soil, using Kubiena boxes. After air-drying for 2 months and oven-

2.2. Analytical and statistical methods In order to determine total concentrations of major elements and studied heavy metals in soil parent materials, rock samples were finely ground (< 105 µm) and analyzed by semi-quantitative X-ray fluorescence on a WDXRF – S8 Tiger (Bruker) apparatus, working at 4 kW, 60 kV and 170 mA. Soil particle size distribution was determined by the pipette method after dispersion with NaOH 1 mol L−1 and slow agitation for 16 h and sieving of the sand (Teixeira et al., 2017). Amorphous Fe and Al oxides were extracted using 0.2 M ammonium oxalate at pH 3.0 (Kämpf and Schwertmann, 1982), whereas total Fe oxides were extracted using sodium dithionite in a buffer of sodium bicarbonate and citrate at 80 °C, and centrifuged at 1800 rpm for five minutes (Mehra and Jackson, 1958). This procedure was repeated at least twice, and until all soil samples turned to a light gray color, which in the Fe-rich Acrudox developed on ironstone after serpentinite took 10 extractions. In the extracts, Fe and Al were determined by atomic absorption using a Varian SpectrAA spectrophotometer. None of the soils presented carbonates, thus total C analyses is indicative of soil organic carbon (SOC), which was determined in the bulk soil < 2 mm by the dry combustion method at 900 °C in a VarioMAX CN apparatus, Elemental Americas (Nelson and Sommers, 1996). Mid- to long-term availability of the heavy metals was assessed by digestion with 5 ml of concentrated HNO3, microwave-heated in sealed teflon vials, according to the method SW-846 3051A (USEPA, 2007). For each batch of 20 duplicate samples, a blank vial and a standard sample with certified heavy metal values (Montana Soil, NIST SRM 2710a) were included. After cooling, the extracts were passed through a 3

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Table 2 Major element and studied trace element concentrations in soil parent materials, as determined by X-ray fluorescence. Letters in parenthesis are mineral composition in decreasing order of importance (Araujo et al., 2014; Vilela et al., 2019). SiO2

Al2O3

Fe2O3

Cao

MgO

Cu

Mn

Ni

Pb

Zn

mg kg−1 rock

% weight Quartzite (Qtz, Mi) Mica-schist (Qtz, Mi, Mt) Gabbro (A, P, Mt) Gneiss (Qtz, F, Mi) Meta-limestone (Calcite) Phyllite (Qtz, Mi, Mt) Itabirite (H, Mt, Qtz) Ironstone after serpentinite (He, Mt, Qtz, C) Serpentinite (S, A, He, Mt, C)

Cr

72.6

2.2

0.3

0.01

0.03

–

16.8

–

11.8

–

12.1

34.9

31.1

12.9

0.04

0.96

10.3

8.0

162.6

70.7

–

8.0

33.9

9.6

17.0

8.34

3.44

6.8

8.0

162.6

78.6

–

16.1

51.4

10.6

0.6

1.72

0.29

–

–

7.7

–

–

–

0.2

0.1

0.1

76.00

0.25

–

–

–

–

–

–

43.0

4.0

0.9

0.02

0.24

–

–

–

–

–

–

3.0

1.1

85.9

0.05

0.19

3.4

–

968.1

7.9

–

–

7.1

11.0

66.9

0.02

0.04

1,279.4

32.0

131.7

125.7

–

24.1

42.3

2.0

22.5

2.31

19.90

640.0

–

200.0

170.0

–

–

Obs. Qtz: quartz, Mi: micas, Mt: magnetite, A: amphiboles, P: plagioclase, F: feldspar, He: hematite, S:serpentine, C: chromite.

0.45 μm paper filter. Heavy metal concentrations were determined by atomic absorption spectrophotometry, using a Perkin Elmer AAnalyst 800 equipment, and when concentrations were below the detection limit, a graphite furnace atomizer was used for determination at very low concentrations. The experimental design was completely randomized, comprising eight soils, sampled at three depths and with three replications, summing up to total n = 72. Data on the selected heavy metals were not normally distributed, but followed a lognormal distribution in all cases for P < 0.01. Thus, data were log-transformed prior to use of the Tukey test at p < 0.05 for comparison of means, testing if each metal varied among soils in each depth, and also assessing how depth affected each metal in an individual soil. In addition, correlations and regressions between heavy metal contents were performed with help of spreadsheets and the JMP 5.1 software.

considerably among the selected soils, which is important since these properties commonly affect heavy metal concentrations in soils (Kabata-Pendias, 2011). There was wide variation on particle-size distribution among the sampled soils, although typically each soil varied little along the top 1 m depth. Mean clay contents were low (~16%) in the quartzite-derived Inceptisol and ranged between 78 and 83% in the limestone Oxisol (Table 3). Five out of the eights soils sampled had mean clay contents > 30% in all depths, which can be considered as an indicative of moderate to advanced weathering. Mean sand contents were generally less variable among soils, ranging between 8 and 50% Table 3 Mean (n = 3) particle size distribution, soil organic C (SOC), and Fe-Al oxides in the studied soils. 0–5 cm

Sand

Clay

SOC

Fe2O3o

Al2O3o

Fe2O3d

497 234 158 408 110 129 213 461

173 487 747 392 778 438 303 256

31.4 62.3 69.4 33.3 41.3 30.9 55.4 35.4

2.14 5.74 8.56 2.31 3.86 3.31 8.58 11.4

2.50 9.70 6.91 3.23 8.83 2.33 4.36 4.46

25.1 102.4 175.1 33.5 67.3 109.1 305.8 190.8

483 244 136 374 77 168 178 255

158 371 716 368 792 524 321 237

8.8 30.7 36.6 14.2 15.7 12.3 15.4 15.1

2.09 4.66 6.79 1.56 2.80 3.42 10.7 9.85

2.43 8.39 7.92 3.20 9.45 3.00 3.59 4.23

27.2 110.4 187.5 34.9 71.5 128.8 306.6 215.4

497 328 139 379 77 125 231 394

158 302 737 226 827 238 272 311

6.3 6.7 20.4 5.4 11.9 4.7 6.3 12.7

1.92 3.98 6.07 0.46 2.70 3.38 11.3 11.6

2.42 7.10 8.08 1.68 9.51 2.71 2.84 3.72

26.7 123.5 189.3 26.5 69.9 148.5 385.9 205.0

g kg−1

3. Results and discussion Dystrudept/Quartzite Haplohumult/Schist Acrudox/Gabbro Haplohumult/Gneiss Acrudox/Metalimestone Haplohumult/Phyllite Kanhapludalf/Itabirite Acrudox/Ironstone 30–40 cm Dystrudept/Quartzite Haplohumult/Schist Acrudox/Gabbro Haplohumult/Gneiss Acrudox/Metalimestone Haplohumult/Phyllite Kanhapludalf/Itabirite Acrudox/Ironstone 90–100 cm Dystrudept/Quartzite Haplohumult/Schist Acrudox/Gabbro Haplohumult/Gneiss Acrudox/Metalimestone Haplohumult/Phyllite Kanhapludalf/Itabirite Acrudox/Ironstone

3.1. Characterization of the studied parent materials and soils Table 2 shows the results of total chemical analysis (X-ray fluorescence) of the eight parent materials, plus the serpentinite that gave origin to the ironstone. As previously desired, the eight parent materials present a wide range in chemical composition of both major elements and heavy metals: SiO2 contents varied from 0.2% in the meta-limestone to 72% in the quartzite, whereas Al2O3 ranged from 0.1% also in the meta-limestone, to 31.1% in the mica-schist. Fe2O3 contents varied from 0.1% in the meta-limestone to 67% in the ironstone and 86% in the itabirite. CaO and MgO also varied widely, reaching the highest contents respectively in the meta-limestone and the serpentinite, although Mg was nearly totally removed from the ironstone formed from its alteration. Pb concentrations were below detection limits in all parent materials, whereas Cu and Zn were also generally low and in many cases below detection limits. Conversely, Mn concentrations varied widely from 7.7 mg kg−1 in the gneiss to 968 mg kg−1 in the itabirite. The concentrations of Cr and Ni were very high in the serpentinite and also in the ironstone after serpentinite. However, Cr concentrations increased in the ironstone in comparison to the serpentinite, probably with a large proportion as chromite (Vilela et al., 2019), whereas Ni was depleted. The contents of clay, SOC and secondary Fe or Al oxides varied

Obs. Fe2O3o and Al2O3o – extracted with ammonium oxalate; Fe2O3d extracted with dithionite. 4

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(Araujo et al., 2014). Table 2 shows that silica and alumina occur in small concentrations in the meta-limestone, suggesting they comprise clay minerals, which were probably accumulated in the soil as the rock dissolved. In any case, this Acrudox is representative of highly-weathered soils derived from limestone in a warm humid climate, thus we believe that a likely partial allochthony was not a hindrance to our interpretation.

sand, and followed the inverse trend of clays, i.e. soils richer in clay were poorer in sand and vice-versa. An important observation is that although conventional Soil Science assumes that higher sand contents typically indicate a lower weathering degree, this was not the case in the Oxisol on ironstone, in which the sand fraction is actually ironstone, chiefly composed of secondary Fe oxides (notably hematite), with little quartz (Araujo et al., 2014), and thus can be actually considered the most weathered soil. SOC concentrations were considered high, which is consistent with the native forest vegetation on tropical highlands > 900 m a.s.l. (Araujo et al., 2017), although there was important variation among soils at the same depth. The soils on mica-schist and gabbro reached 6–7% SOC at a 0–5 cm depth, whereas the soils on quartzite and phyllite had ca. 3%. At the 90–100 cm depth, the three Oxisols showed SOC contents > 1%, reaching 2% in the soil on gabbro, which are considerably high values for such deep layers. The average content of total Fe oxides extractable by dithionite varied at least one order of magnitude among the eight sampled soils: the lowest values were noted in the quartzite- and gneiss-derived soils, always < 3.5% Fe2O3, whereas the soils formed on itabirite (a type of iron ore) and ironstone after serpentinite ranged from 19 to 38% Fe2O3. However, in regard to the latter soil, these values refer to Fe oxides extractable by CBD, chiefly comprised within silt and clay particle sizes, whereas most sand-sized or coarser Fe oxides are left intact, since this soil has ca. 72% total Fe2O3 as determined by electron dispersion spectroscopy (Araujo et al., 2014). As expected from aerobic, tropical humid soils, amorphous Fe2O3 was low, with Fe2O3ox/Fe2O3cbd rates typically < 0.1, especially in the Fe-richer soils. Similarly, amorphous Al oxides were always low, never reaching 1% Al2O3ox and typically varying little with depth. The basic assumption of autochthony was satisfactorily fulfilled: micromorphological analyses showed fragments of each parent material interspersed within the fabric of all B horizons, with the exception of the limestone soil (Fig. 2). The quartzite-derived soil contained many fragments of coarse fragments of accommodated quartz grains (Fig. 2a,b), observed earlier in fresh samples of this massive quartzite (Araujo et al., 2014). The large fragment of mica-schist shows coarse, unaltered muscovite, which is the source of the sand- and silt-sized micas imparting the crystallitic b-fabrics of this soil (Fig. 2c,d) and the illite/vermiculite in the clay fraction (Table 1). The same micas are present in much smaller proportion in the quartzite, explaining the similar clay mineralogy of both soils, which occur alternated along the landscape (Fig. 1). Coarse grains of opaque magnetite and hematite occur commonly in the sand fraction of the soil on gabbro (Fig. 2e,f), as the most resistant minerals in that rock. Similarly, coarse muscovite and prolate quartz were visible in the soils on gneiss (Fig. 2g,h) and phyllite (Fig. 3c,d), the muscovite being responsible for the crystallitic b-fabrics in both soils and the illite in the clay fraction in the phyllite soil (Table 1). The coarse, opaque itabirite fragments seen in plane polar light (Fig. 3e), when between crossed polars (Fig. 3f) show clearly the opaque, coarse hematite and magnetite interspersed among and unusually reddish matrix, composed by fine quartz with microcrystalline hematite and mica, the latter not visible either on the rock or soil, although shown by X-ray diffraction (Table 1). The globular remnants of the hematite-rich, opaque ironstone formed from residual Fe accumulation from the weathering of serpentinite are surrounded by hydration haloes (Fig. 3g,h), contributing to the spheroid granular microstructure to this Acrudox. Finally, a very different type of granular microstructure is visible in the Acrudox on limestone, apparently formed by disintegration of coarse spheroid granules (Fig. 3a,b) of reddish (gibbsitic and Fe-oxidic) and yellowish (mostly gibbsitic) colors. In this soil, the total absence of parent material fragments is ascribed to the relatively low resistance of calcite to acid weathering, and to the > 5 m, probably > 10 m depth of this soil to the bedrock. Thus, it has been interpreted that the gibbsitic clay of this soil derives from the weathering of clays included within the limestone and/or in associated layers of clayrich limestone, weathering in a well-drained, low-silica environment

3.2. Soil heavy metal concentrations The nitric acid extraction process as performed in the present work showed adequate efficiency, with recoveries for individual metals varying between 88.3% for Mn and 113% for Cr concentrations in the Montana standard soil certified (Table 4). For Ni and Pb, recovery was 93–95%, and for Cu and Zn, 106–109%. In addition, although the standard soil sample presents concentrations of some metals much higher than our soils, the method detection limits are rather low and thus compatible with values expected in common, pristine soils under native vegetation. Thus, our quasi-total metal concentrations can be considered as adequately determined. The concentrations of the three chalcophile metal followed two somewhat different trends. Due to rather low standard errors, there were significant differences at p < 0.05 in Pb concentrations among soils at a same depth, but in practical terms Pb was similarly low in all soils and depths (always < 2.5 mg kg−1, Table 5), which is consistent with very low concentrations in all parent materials (Table 2). Such values are well below the average 10.1 mg kg−1 determined for 336 soils under native forest (Skorupa, 2013) or the median 16.1 mg kg−1 for other 697 sites (Souza et al., 2015) interspersed along the entire state of Minas Gerais. Typically, the highest Pb concentrations in Minas Gerais occur in soils formed from limestone (Burak et al., 2010; Skorupa, 2013; Souza et al., 2015; Ferreira et al., 2018) under semi-arid climate, in the northern area of the state. Although a calcareous parent material was included in the design of the present study, the low values reported are likely attributable to the humid climate, resulting in advanced weathering and eventual leaching of the otherwise rather immobile Pb. In addition, part of mineral components in this soil (mostly gibbsite, minor kaolinite) are probably inherited from weathering of associated siliciclastic rocks, clay layers or impurities (Araujo et al., 2014, Table 2), which typically are low in Pb in Minas Gerais (Piló, 1998). Conversely, Cu and Zn concentrations varied more widely: seven out of eight soils showed Cu concentrations lower than the statewide median of ca. 31 mg kg−1 (Souza et al., 2015), whereas the Alfisol on itabirite reached 84 mg kg−1 (Table 5). Similarly, Zn concentrations in six soils were below the state median of 35 mg kg−1 (Souza et al., 2015), but not for the soils on itabirite and ironstone, the latter the only case in which the state quality Zn standard of 46.5 mg kg−1 (COPAM, 2011) was exceeded. Such high concentration in Fe-rich soils was not detected in statewide-scale studies, in which soils with naturally high concentrations of Zn (Burak et al., 2010; Skorupa, 2013) and Cu (Souza et al., 2015) occurred on neoproterozoic limestones, similarly to the trend reported for Pb. The concentrations of the siderophile metals also showed wide, significant differences among soils. However, unlike for Cu, Pb and Zn, state quality standards for regulated metals were often exceeded. For Cr, seven out of eight soils exceeded the state quality standard of 75 mg kg−1, with soils on gabbro and ironstone exceeding 200 mg kg−1 (Table 5). Median Cr concentrations in the state are 72.8 mg kg−1 (Souza et al., 2015), which aside with the present study suggests that the state standards (COPAM, 2011) were grossly underestimated and need revision. In the present study, only the Inceptisol on quartzite was within Cr state quality standards, due to its highly siliceous nature which resulted in the lowest metal concentrations. In regard to Ni, the concretionary Oxisol on ironstone/serpentinite showed much higher concentrations than the other soils, reaching 1462 mg kg−1 (Table 5), nearly two orders of magnitude higher than the 17.8 mg kg−1 statewide 5

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Fig. 2. Thin sections of subsurface soils formed from: a, b-quartzite; c, d-schist; e, f-gabbro; g, h-gneiss. All images show pedogenesis from in situ alteration of parent materials, either as rock fragments (c,d) or coarse mineral grains (quartz in a,b; magnetite in e,f; quartz and muscovite in g,h). On the left, images under plane polarized light; on the right, images between crossed or partially crossed polars.

prescribed standards and about which no statistics are available, were much higher in the soils on ironstone and especially itabirite, reaching nearly 7000 mg kg−1 at the 0–5 cm depth. Interestingly, concentrations of Cr, Mn and Zn in the present study were generally much higher than values observed in the Brazilian Amazon (Fernandes et al., 2018), which did not occur for Cu, Pb and Zn. Therefore, our initial hypothesis that heavy metal concentrations

median and 30-fold higher than the median values for itabirite-derived soils (Souza et al., 2015). The state quality standard for Ni is 30 mg kg−1 (COPAM, 2011), which was also exceeded by the soil on itabirite. The trends reported here for Cr and Ni support those by Souza et al., 2015, who also noted that soils derived from itabirite are generally 3- to 5- fold richer in Ni than those formed from other parent materials. Soil Mn concentrations, which are not subject to state6

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Fig. 3. Thin sections of subsurface soils formed from: a, b: limestone; c, d- phyllite; e, f-itabirite; g, h- ironstone from serpentinite. Except for the soil from limestone, all images show pedogenesis from in situ alteration of parent materials. On the left, images under plane polarized light; on the right, images between crossed or partially crossed polarizers. The granular cluster in c, d is comprised by unidentified organic matter and is a bioturbation feature.

A general trend observed for all heavy metals studied is the negligible variation in mean concentrations along the different depths of the same soil. Although significant differences occurred in few cases due to low standard errors, they were basically irrelevant: for instance, Cu in the soil on phyllite varied significantly from 11.4 to 17 mg kg−1 along the top meter, but these values essentially pose no relevance for soil

varied with soil parent material was confirmed for all metals in purely statistical terms. However, in practical terms, the soils on itabirite and ironstone differed considerably in Cu, Mn, Ni and Zn from the other soils, whereas Cr was higher in soils on itabirite and gabbro, and differences in Pb concentrations were negligible. Thus, it can be argued that our first hypothesis was validated for five out of six heavy metals.

7

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(Lara et al., 2018) reported little differences in Cr, Ni and Pb concentrations along 7 m-deep Ultisol profiles in perhumid Puerto Rico. Similarly, in East Amazon the average concentrations of Cu, Ni, Pb and Zn did not vary in depth (0–20 vs. 80–100 cm depth layers), and differences for Cr and Mn were of ca. 20% (Fernandes et al., 2018). The lack of depth effects reported in the present study can be due to high soil bioturbation, or to the advanced degree of weathering, resulting in homogeneous profiles (see Section 3.4). Bioturbation features are very common in soils under native vegetations, and are easily seen in thin sections of A horizons of all studied soils as channel voids, granular infillings, roots along pores and peds resulting from egestion by fauna (see images in Araujo et al., 2017). In B horizons, such features are less common due to less intense faunal activity and their obliteration, but are still present in all soils, as seen in Fig. 3c,d. In practical terms, the similarity of heavy metal concentrations along the 1-m depth suggests a great representativeness of regional soil metal surveys, which typically were based on superficial soil layers (e.g., 0–20 cm), such as Paye et al. (2010) and Skorupa (2013).

Table 4 Average concentration and recovery of heavy metals determined in the Montana Soil standard sample (SRM 2710a). Metal

Determined concentration mg kg−1

Certified concentration mg kg−1

Recovery %

Method detection limit ± st. deviation mg kg−1

Cr Cu Mn Ni Pb Zn

26.02 3,655.50 1,889.00 7.63 5,155.50 4,586.00

23.00 3,420.00 2,140.00 8.00 5,520.00 4,180.00

113.14 106.88 88.27 95.38 93.40 109.71

11.68 ± 0.12 3.82 ± 0.04 4.78 ± 0.04 6.79 ± 0.06 2.37 ± 0.02 3.47 ± 0.02

Table 5 Heavy metal concentrations as affected by soil parent material and depth. Means followed by the same capital letter do not differ significantly by the Tukey test (p < 0.05) for comparison among different soils at the same depth. Means followed by different minor letters differ significantly by the Tukey test (p < 0.05) for comparison among the three depths of the same soil. 0–5 cm depth

Cu

Pb

Zn

Cr

Ni

3.3. Correlation of heavy metal concentrations

Mn

As important as determining heavy metal concentrations is perhaps identifying which edaphic properties are likely to affect their retention in soils. Table 6 shows a correlation matrix of heavy metal concentrations with soil properties and also with the other metals. Despite the wide range in particle-size distribution, SOC and oxide contents among the eight soils, the number of significant or relevant correlations were unexpectedly low. For instance, clay contents were not significantly and almost always negatively correlated with concentrations of all metals (r = −0.39 to 0.28, with a mean of only |0.22|). Positive correlations between clay contents and these heavy metals have been reported earlier for soils mostly developed from sedimentary rocks in Minas Gerais (Marques et al., 2004), in Bahia (Gloaguen and Passe, 2017) and in the Amazon (Fernandes et al., 2018). More importantly, the trend for negative correlations was unexpected because sorption to mineral surfaces is thought to be a major retention mechanism for metals. This trend can be partly explained by the fact that low-activity clays predominate in the clayey soils (e.g., the Oxisols on limestone and gabbro) whereas high-activity clays occur in sandy (Inceptisol on quartzite) or loamy soils (Alfisol on itabirite). The absence of correlations between metal concentrations with clay and organic matter in soils of another area of Minas Gerais was interpreted by Guevara et al. (2018) as the result of a predominant control of parent material, i.e. of geogenic rather than pedogenic processes of metal retention in soils. Sand contents showed even poorer correlations (r = −0.08 to 0.42, with a mean of |0.16|), but these were more mixed, i.e. roughly equally distributed in positive to negative correlations, which is of difficult interpretation. Silt contents showed the highest correlations (r = −0.28 to 0.68, with a mean of |0.34|) with Cu, Mn and Zn in a lesser degree, and these were mostly positive, which can suggest the important occurrence of Fe oxides in this particle size fraction, as discussed below. Total and amorphous Fe oxides showed the highest correlations with heavy metals, and these were always positive, and mostly significant for dithionite Fe (Table 6). These trends reflect the high Fe oxide contents in the Oxisols on gabbro, itabirite and ironstone (Tables 1 and 3), which generally had intermediate to high concentrations of all metals except Pb. Thus, part of the apparently limited influence of texture on heavy metal retention may be due to the occurrence of Fe oxides not only in the clay of all soils, but also in the sand fractions of three soils (Table 1) and thus in their silt fractions also. The thin sections of these three soils showed that opaque, coarse Fe oxide grains are indeed a major component of the sand fractions. In any case, the overall affinity of the studied metals for Fe oxides is remarkable, since the chalcophile metals Zn and Cu were often more correlated with Fe oxides than the siderophile metals. This trend can be partly explained by their sorption to clay-sized Fe oxides, although sorption is typically

mg kg−1 Dystrudept/Quartzite Haplohumult/Schist Acrudox/Gabbro Haplohumult/Gneiss Acrudox/ Metalimestone Haplohumult/Phyllite Kanhapludalf/ Itabirite Acrudox/Ironstone 30–40 cm depth Dystrudept/Quartzite Haplohumult/Schist Acrudox/Gabbro Haplohumult/Gneiss Acrudox/ Metalimestone Haplohumult/Phyllite Kanhapludalf/ Itabirite Acrudox/Ironstone 90–100 cm depth Dystrudept/Quartzite Haplohumult/Schist Acrudox/Gabbro Haplohumult/Gneiss Acrudox/ Metalimestone Haplohumult/Phyllite Kanhapludalf/ Itabirite Acrudox/Ironstone

12.4B 30.7B 16.2B 3.6B 11.7B

1.6C 1.9B 1.7B 2.3A 1.9B

19.4B 6.8B 7.7B 5.0B 7.9B

21.9D 153.2B 210.0AB 117.3BC 121.8BC

9.5C 10.1C 17.2C 4.3C 9.8C

78.3C 35.7C 106.1C 19.9C 42.2C

17.0Ba 83.8A

2.4A 2.3A

16.6BB 64.5A

84.4CD 110.9BC

9.2Ca 43.0B

330.1BC 6,850A

11.2B

2.5A

26.4Bb

256.6A

665.4A

862.1B

11.7B 19.1B 10.5B 10.0B 9.5B

1.2C 1.9B 1.8BC 2.3AB 1.9B

8.6C 8.2C 8.3C 19.3BC 7.3C

19.8D 142.6B 214.3AB 122.4BC 112.3BC

10.8B 12.9B 11.3B 14.5B 8.3B

99.1C 177.4C 53.5C 17.8C 25.5C

14.1Bab 82.5A

2.2AB 2.2AB

13.8BC 44.2B

88.2CD 110.1BC

7.8Bb 36.0B

250.9C 5,559A

30.0B

2.5A

79.6Aa

257.5A

1,462A

1,944B

10.0B 17.2B 15.4B 8.1B 11.9B

1.3B 1.7AB 1.8AB 2.1AB 1.9AB

7.6C 6.3C 9.6C 16.2ABC 6.0C

16.5C 128.5B 218.1A 62.9BC 112.9B

3.3B 10.4B 18.0B 10.9B 11.0B

131.8B 63.1B 95.9B 38.5B 40.6B

11.4Bb 76.4A

2.1AB 2.3AB

11.9BC 56.3A

93.4BC 100.9B

6.2Bc 33.8B

247.2B 4,089A

31.5B

2.5A

50.4ABab

258.3A

1,304.7A

1,701B

fertility or environmental quality. The only vertical variation both significant at P < 0.05 and practically relevant occurred for Zn in the soil on ironstone, which varied from 26.4 to 79.6 mg kg−1 from the 0–5 to 30–40 cm. Therefore, the bulk of the data points to a general rejection of our second hypothesis. This was unexpected because: a) in shallower soils such as those on quartzite, schist and phyllite, higher metal concentrations were expected near at 90–100 cm, nearer to the parent material, and b) SOC, which had always higher concentrations on the 0–5 cm than in deeper layers (Table 5), is thought to sorb considerable amounts of many heavy metals, and thus higher concentrations were expected in the surface layers of deep soils (e.g., McBride, 1994). It has been demonstrated that concentrations of Zn and Cu can vary considerably within the top 1.5 m depth of soils in Europe or North America (Kabata-Pendias, 2011). Recent data for the humid tropics 8

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Table 6 Correlation matrix between determined heavy metals and selected soil properties (n = 24), by sampling depth. Significant correlations are shown with a * for p < 0.05, ** for p < 0.01, and *** for p < 0.001. 0–5 cm

sand silt clay SOC Feox Alox FeDCB SSA Zn Cu Pb Mn Cr

30–40 cm

90–100 cm

Zn

Cu

Pb

Mn

Cr

Ni

Zn

Cu

Pb

Mn

Cr

Ni

Zn

Cu

Pb

Mn

Cr

Ni

0,01 0,57** −0,39 −0,04 0,24 −0,31 0,48* −0,28

−0,26 0,52* −0,17 0,28 0,25 −0,07 0,61** −0,17 0,78***

−0,05 0,34 −0,19 −0,17 0,29 −0,23 0,34 −0,05 0,28 0,12

−0,12 0,56** −0,29 0,21 0,37 −0,15 0,64** −0,25 0,86*** 0,86*** 0,33

0,15 −0,11 −0,03 0,33 0,70*** 0,08 0,40 −0,04 0,03 0,05 0,36 0,03

0,42* −0,02 −0,28 −0,23 0,60** −0,06 0,23 −0,03 0,1 −0,16 0,47* −0,02 0,58**

0,28 0,37 −0,39 −0,22 0,60** −0,30 0,42* −0,22

−0,15 0,63** −0,26 −0,17 0,59** −0,26 0,62** −0,25 0,55**

−0,08 0,08 0,01 −0,14 0,39 −0,12 0,44* 0,12 0,62** 0,34

−0,10 0,68** −0,31 −0,16 0,69** −0,25 0,67** −0,28 0,58** 0,94*** 0,32

−0,10 −0,22 0,19 0,44* 0,49* 0,24 0,41* 0,40 0,50* 0,06 0,49* 0,09

0,40 0,16 −0,35 −0,13 0,50* −0,13 0,28 −0,08 0,85 0,11 0,49* 0,20 0,63**

0,09 0,35 −0,24 −0,06 0,77*** −0,33 0,42* −0,15

−0,06 0,49* −0,21 −0,14 0,71*** −0,22 0,74** −0,10 0,62**

−0,13 0,20 −0,02 −0,07 0,45* −0,23 0,38 0,10 0,47* 0,44*

−0,05 0,44* −0,19 −0,14 0,73*** −0,30 0,49* −0,14 0,89*** 0,81*** 0,37

−0,22 −0,28 0,28 0,58** 0,60** 0,40 0,33 0,46* 0,24 0,15 0,37 0,11

0,30 0,11 −0,24 0,16 0,59** −0,14 0,17 −0,01 0,51* 0,15 0,44* 0,25 0,65**

Obs. SSA: specific surface area (BET-N2).

the tropical highland climate of the present work, weathering conditions that eventually control leaching of mobile metals and residual concentration of Fe are more intense than under cool temperate climate, but are relatively mild in comparison to most tropical areas with altitudes < 500 m, where mean annual temperatures are often 2–3 °C higher. Thus, the degree of weathering of the soils studied, as indicated by depths (1 to > 6 m), clay contents (11–78%) and mineralogy, can be considered moderate to advanced, in comparison to warmer tropical humid areas where very deep and clayey Oxisols and Ultisols largely predominate (e.g., Quesada et al., 2011). The soils more highly weathered are those deeper and with higher clay contents, developed from fast-weathering gabbro and limestone, whereas the less developed soil has coarse texture and formed from highly-resistant quartzite. The soils formed from gneiss, phyllite and mica-schist can be considered as of moderate weathering. In consequence, these six soils present relatively homogeneous Zn, Cu, Pb and Mn concentrations, more subject to intensive leaching (Marques et al., 2004). However, these soils also do not differ greatly in Cr and Ni concentrations, since their parent materials were not originally rich in these metals as was the serpentinite. Future weathering, within the pedogenic time scale, would likely result in even more similar concentrations of all six metals among these six soils. In other words, the tropical humid weathering promoted soil convergent evolution (Skorupa et al., 2017), since these six soils show very similar concentrations of different heavy metals despite deriving from geochemically very different rocks. The absence of marked differences in Pb, Cr and Cu concentrations between soils is in agreement with the results of Biondi et al. (2011), who concluded that for soils of the entire state of Pernambuco, Brazil, the natural concentrations of these heavy metals could not be inferred directly from lithology, which probably is true for a considerable part of the global tropics and adds to the international relevance of both studies. The soils on itabirite and ironstone after serpentinite do not conform to the homogeneization pattern mentioned above, since they have formed from parent materials rich in Fe oxides, namely hematite, which are primary in itabirite and mostly secondary in ironstone. Thus, both rocks are likely to be enriched in heavy metals co-precipitated with Fe oxides during the pedogenic process of lateritization (soil on ironstone after serpentinite) or diagenetic/metamorphic process (soil on itabirite). In fact, the itabirite rock which originated the soil studied here has nearly 1000 mg Mn kg−1. Itabirite is relatively resistant to weathering, and Mn was effectively preserved in derived soils probably because it was associated with inherited coarse, opaque hematite grains (Fig. 3f). In consequence, the soil on itabirite was highly enriched in Mn, reaching nearly 7000 mg kg−1 (Table 5), but also showed the highest Cu and Zn concentrations of all soils. On the other hand, serpentinite is a rock enriched in Cr and Ni

weak at pH < 6 (Sparks, 1995). Thus, the strong correlation between the heavy metals studied here and Fe oxides, noted earlier in soils (Burak et al., 2010) and in mine tailing sediments (Queiroz et al., 2018) near our study area, can be chiefly ascribed to metal co-precipitation and inclusion in their crystal lattice. Due to recent, catastrophic collapses of mine spoil dams across Brazil (Queiroz et al., 2018), this information is critical as spoils from rocks rich in Fe, either as primary (itabirite) or secondary (ironstones) can retain high concentrations of heavy metals that can be ultimately mobilized by reduction in anoxic layers. Conversely, Al amorphous oxides were negatively (but not significantly) correlated with metal concentrations, which was also unexpected since this phase is highly and positively correlated with soil specific surface area (Zinn et al., 2017) and thus it could increase metal sorption. However, these negative correlations can be ascribed to the high oxalate-Al concentrations in the soils on schist and limestone (both poor in heavy metals) aside with the low oxalate-Al in the metal-rich soils on itabirite and ironstone. SOC contents were also not significantly correlated with metal concentrations (r = −0.23 to 0.44, with a mean of only |0.20|). The combination of all correlation data discussed above leads to the interpretation that, in the studied soils, low-energy sorptive processes are much weaker mechanisms for metal retention than coprecipitation or specific adsorption with Fe oxides. Due to generally low, non-significant correlations of heavy metal concentrations with soil properties, multiple regressions aiming to predict metal concentrations based on clay, silt, sand, oxide and SOC data were only partially effective, reaching determination coefficients between 0.28 (Pb) and 0.65 (Cu) as a function of particle-size distribution, SOC and oxide concentrations (not shown). However, even the best equations were not considered useful since observed vs. estimated plots were very scattered and poorly distributed due to the effect of the metal-rich soils on itabirite and ironstone. Mutual correlations between the studied heavy metals were also highly variable. Zn was especially well correlated with Cu and Mn, as also noted by Burak et al. (2010). Such trend can be ascribed to their relatively high mobility and, since these three metals are plant nutrients, by the effect of uptake and biochemical recycling. The typically immobile Cr and Ni were highly correlated mutually, but rather poorly with the other metals including Mn, which is highly mobile. This result can be explained by the well-known trend of Cr and Ni to be common components of mafic and ultramafic minerals and rocks, and to occur in much lower concentrations in other rocks (e.g., Garnier et al., 2009). 3.4. Parent material as the main control of heavy metal concentrations The bulk of data reported here can be interpreted as follows. Under 9

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matter, some pedogenic Fe and Mn oxide concentrations are distinctly observed and thus can greatly help interpreting correlations among these metals. In addition, soil micromorphology is instrumental in ascribing if Fe oxides detected by chemical analyses are inherited from parent material or pedogenic, which has clear implications if metal coprecipitation is considered.

(Corrêa, 2010, Table 2), and since it weathers rapidly, both metals can accumulate in soils since they are not very mobile. Thus, the soil on ironstone after serpentinite showed the highest Cr and Ni concentrations of all soils, reaching ca. 220 mg Cr kg−1 and 600–1400 mg Ni kg−1, and was also rich in Mn (reaching ca. 1900 mg kg−1), probably by co-precipitation with secondary Fe oxides. However, these values are not relatively high for tropical serpentinite soils, and comparable to or lower than in fresh serpentinites from other location in Minas Gerais (Vieira, 2016). However, other serpentinite soils can be heavily enriched in Cr and Ni. For instance, Garnier et al. (2009) reported up to 11,900 mg Cr kg−1 and 73,800 mg Ni kg−1 due to residual accumulation after extreme weathering of ultramafic rocks under hot, moist climate in Goiás, Brazil. Nevertheless, in the present study it is not clear how Cr and Ni concentrations would differ if soils were derived from fresh serpentinite or ironstone after serpentinite, since the few studies on less weathered serpentinite soils in Brazil do not offer data on heavy metal concentrations (e.g., Vidal-Torrado et al., 2007). It is also possible that even small outcrops of ultrabasic rocks can impart high Cr and Ni concentrations to neighboring soils, which deserves further investigation. Finally, since the formation of hexavalent Cr is favored by the presence of high Mn oxide concentrations (McBride, 1994), further studies must attempt to quantify Cr(VI) concentrations in the soils and waters derived from this ironstone after serpentinite.

Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments The Minas Gerais State Foundation of Research Support is in Brazil (Fapemig), Brazil, funded soil sampling and analyses (grants # CAGAPQ 720/12, 778-15) and a research assistantship to J.A. Faria. Scholarship grants were also funded by CNPq, Brazil, (Y.L. Zinn, A.L.A. Skorupa) and Capes (M.A. Araujo). The Soil Micromorphology laboratory was equipped with funding from Capes, Brazil (Ed. PróEquipamentos). Finally, we thank Drs. Luiz R.G. Guilherme and Geila Carvalho (Universidade Federal de Lavras) for material and technical support with lab analyses, and Emerson F. Vilela for the map in Fig. 1. References

4. Conclusions Araujo, M.A., Pedroso, A.V., Amaral, D.C., Zinn, Y.L., 2014. Paragênese mineral de solos desenvolvidos de diferentes litologias na região sul de Minas Gerais. Rev. Bras. Cienc. Solo 38, 11–25. https://doi.org/10.1590/S0100-06832014000100002. Araujo, M.A., Zinn, Y.L., Lal, R., 2017. Soil parent material, texture and oxide contents have little effect on soil organic carbon retention in tropical highlands. Geoderma 300, 1–10. https://doi.org/10.1016/j.geoderma.2017.04.006. Biondi, C.M., Nascimento, C.W.A., Fabricio Neta, A.B., Ribeiro, M.R., 2011. Teores de Fe, Mn, Zn, Cu, Ni e Co em solos de referência de Pernambuco. Rev. Bras. Cienc. Solo 35, 1057–1066. https://doi.org/10.1590/S0100-06832011000300039. Burak, D.L., Fontes, M.P.F., Santos, N.T., Monteiro, L.V.S., Martins, E.S., Becquer, T., 2010. Geochemistry and spatial distribution of heavy metals in Oxisols in a mineralized region of the Brazilian Central Plateau. Geoderma 160, 131–142. https://doi. org/10.1016/j.geoderma.2010.08.007. Chesworth, W., 1973. The parent rock effect in the genesis of soil. Geoderma 10, 215–225. https://doi.org/10.1016/0016-7061(73)90064-5. COPAM (Conselho Estadual de Política Ambiental). Deliberação Normativa COPAM n° 166, de 29 de junho de 2011, pp. 18–20 [Belo Horizonte]. Corrêa, V.F., 2010. Estudo do Comportamento do Ni em rochas ultramáficas de corpos portadores de garnierita, Liberdade, MG. Monography (B.S. in Geology, in Portuguese). Universidade Federal Rural do Rio de Janeiro, Rio de Janeiro. Fabricio Neta, A.B., Nascimento, C.W.A., Biondi, C.M., van Straaten, P., Bittar, S.M.B., 2018. Natural concentrations and reference values for trace elements in soils of a tropical volcanic archipelago. Environ. Geochem. Health 40, 163–173. https://doi. org/10.1007/s10653-016-9890-5. Fernandes, A.R., Souza, E.S., Braza, A.M.S., Birani, S.M., Alleoni, L.R.F., 2018. Quality reference values and background concentrations of potentially toxic elements in soils from the Eastern Amazon, Brazil. J. Geochem. Explor. 190 (2018), 453–463. Ferreira, E.P., Coelho, R.M., Valladares, G.S., Dias, L.M.S., Assis, A.C.C., Silva, R.C., Azevedo, A.C., Abreu, C.A., 2018. Mineralogy and concentration of potentially toxic elements in soils of the São Francisco Sedimentary Basin. Rev. Bras. Cienc. Solo 42, e0170088. https://doi.org/10.1590/18069657rbcs20170088. Garnier, J., Quantin, C., Guimarães, E., Garg, V.K., Martins, E.S., Becquer, T., 2009. Understanding the genesis of ultramafic soils and catena dynamics in Niquelândia, Brazil. Geoderma 151, 204–214. https://doi.org/10.1016/j.geoderma.2009.04.020. Gloaguen, T.V., Passe, J.J., 2017. Importance of lithology in defining natural background concentrations of Cr, Cu, Ni, Pb and Zn in sedimentary soils, northeastern Brazil. Chemosphere 186, 31–42. Guevara, Y.Z.C., Souza, J.J.L.L., Veloso, G.V., Veloso, R.W., Rocha, P.A., Abrahão, W.A.P., Fernandes Filho, E.I., 2018. Reference values of soil quality for the Rio Doce Basin. Rev Bras Cienc Solo 42, 0170231. https://doi.org/10.1590/18069657rbcs20170231. Kabata-Pendias, A., 2011. Trace Elements in Soils and Plants, fourth ed. Taylor & Francis, New York. Kämpf, N., Schwertmann, U., 1982. The 5-M-NaOH concentration treatment for iron oxides in soils. Clay Clay Miner. 30, 401–408. Lara, M.C., Bus, H.L., Pett-Ridge, J.C., 2018. The effects of lithology on trace element and REE behavior during tropical weathering. Chem. Geol. 500, 88–102. https://doi.org/ 10.1016/j.chemgeo.2018.09.024. Marques, J.J., Schulze, D.G., Curi, N., Mertzman, S.A., 2004. Trace element geochemistry in Brazilian Cerrado soils. Geoderma 121, 31–43. https://doi.org/10.1016/j. geoderma.2003.10.003. Mehra, O.P., Jackson, M.L., 1958. Iron oxide removal from soils and clays by a dithionite citrate system buffered with sodium bicarbonate. Clay Clay Miner. 7, 317–327. McBride, M., 1994. Environmental Chemistry of Soils. Oxford University Press, New York.

The objective of this work was to measure the quasi-total contents of selected chalcophile (Cu, Pb and Zn) and siderophile heavy metals (Cr, Mn and Ni) in three depths (0–5, 30–40 and 90–100 cm) of eight pristine, tropical soils derived from eight different parent materials. The concentrations of these metals were generally below the state quality reference values, with the exception of Cr and Ni in most soils, and Zn in two soils formed itabirite and ironstone after serpentinite. The hypothesis that heavy metal contents in soils vary with parent material has been accepted, largely due to the presence of the geochemically unique rocks itabirite and ironstone after serpentinite, rich in Fe-oxides, Mn and Ni. However, it is interesting to consider that, had this study focused on more common parent materials, i.e. not included itabirite and ironstone after serpentinite, the hypothesis would have been mostly rejected, which can bear significance for other areas in Brazil and other tropical humid areas where these rocks are not common. The hypothesis that heavy metal contents vary with depth was rejected, which was unexpected since soil organic matter contents and proximity to parent material would supposedly affect metal concentrations. From an ecological viewpoint, the data suggest that, for most pristine soils under tropical humid climate in Brazil and elsewhere, concentrations of the studied heavy metals are: a) generally similar and below concern concentrations in soils developed from “common” parent materials; b) conversely, soils developed from itabirite and ironstone after serpentinite, and probably serpentinite, are likely to pose problems for plants and fauna due to high levels of Cr and Ni but also Mn, all three siderophile elements. Despite the wide range in particle-size distribution, SOC and amorphous Al-oxide contents among the eight soils, there were few significant and relevant correlations between these properties and heavy metal concentrations. However, all heavy metals were consistently correlated with Fe-oxide contents, suggesting that metal sorption by clays and organic matter are less important than co-precipitation with Fe oxides. Generally, Cu, Pb, and Zn showed similar concentrations in most soils due to advanced leaching, whereas Cr and Ni were enriched by residual concentration in soils derived from ironstone after serpentinite, and the soil on itabirite was rich in Mn through association with primary hematite. Finally, one can argue that soil micromorphology has an untapped potential to be used in studies on natural heavy metal occurrences: although most heavy metals are obviously not visible in thin sections when sorbed to clays or organic 10

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