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Scientia Agricola

versão On-line ISSN 1678-992X

Sci. agric. (Piracicaba, Braz.) vol.78 no.4 Piracicaba  2021  Epub 08-Jul-2020

https://doi.org/10.1590/1678-992x-2019-0115 

Soils and Plant Nutrition

Sombric-like horizon and xanthization in polychrome subtropical soils from Southern Brazil: implications for soil classification

Mariane Chiapini1 
http://orcid.org/0000-0001-7111-0555

Jairo Calderari de Oliveira Junior2 
http://orcid.org/0000-0003-3818-0513

Judith Schellekens1 
http://orcid.org/0000-0001-8364-7034

Jaime Antonio de Almeida3 
http://orcid.org/0000-0001-5808-9421

Peter Buurman4 
http://orcid.org/0000-0001-6267-5378

Pablo Vidal-Torrado1  * 
http://orcid.org/0000-0001-9228-9910

1Universidade de São Paulo/ESALQ – Depto. de Ciência do Solo, C.P. 09 – 13418-900 – Piracicaba, SP – Brasil.

2Universidade Federal do Paraná – Depto. de Ciência do Solo – R. dos Funcionários, 1540 – Curitiba, PR – Brasil.

3Universidade do Estado de Santa Catarina – Depto. de Solos e Recursos Naturais – Av. Luis de Camões, 2090 – Lages, SC – Brasil.

4Wageningen University and Research/Earth System Science Group, P.O. Box 47 – 6700 AA Wageningen – The Netherlands.


ABSTRACT

The occurrence of dark subsurface horizons rich in organic matter (OM) associated with polychrome in the B horizon (yellowish over reddish hue) is common in soils from Southern Brazil. The formation of these horizons and the combination with such morphological attributes has not been properly documented, and neither has the cause effect relationship. Four soil profiles with such sombric-like horizons with a yellowish color at the upper part of the B horizon over red subsoil were studied in Southern Brazil. Results from micromorphology, extractable sesquioxide minerals, clay mineralogy and isomorphic substitution of Fe by Al in iron minerals showed that melanization, xanthization, bioturbation, moderate shrinking/swelling and moderate ferralitization were the most evident pedogenetic processes in role. Xanthization is closely related to the sombric-like horizon formation. In the studied area the findings demonstrated that no clay and OM illuviation had taken place. Therefore, the classification of these soils was revisited, so as to take into account the processes that underlie their genesis with emphasis on xanthization, clay illuviation and soil aggregation. The results suggest that the sombric horizon may need redefinition, unless profiles can be found in which illuviation of clay and/or OM can be proven.

Key words: Argissolos; goethite; dark subsurface horizon; shiny peds; matte aggregate faces

Introduction

Sombric and sombric-like horizons are found in well-drained landscape positions with high soil moisture, on high plateaus and/or mountains in tropical and subtropical regions, and several theories exist as to their formation (Almeida et al., 2009; Bockheim, 2012; Almeida et al., 2015). Except for Faivre (1990), sombric horizons described in the literature lack evidence of OM illuviation, and are therefore frequently referred to as sombric-like horizons (De Craene and Laruelle, 1955; Sys et al., 1961; Gouveia et al., 2002; Caner et al., 2003; Almeida et al., 2009; Velasco-Molina et al., 2010). Recently, Chiapini et al. (2018) studied the sombric-like horizons in Southern Brazil from a site that is representative of such conditions. For this area the authors found that the sombric-like horizon is a remnant of an earlier phase of soil formation in which grass vegetation and frequent natural fires caused deep accumulation of black carbon (BC). During subsequent wetter conditions, from the Late Holocene until the present, forest vegetation replaced grass vegetation and fire incidence declined. A similar mechanism may explain the formation of sombric-like horizons in other areas, and was also proposed by Caner et al. (2003).

Taxonomic soil classification systems are based on morphological properties and recognition of key processes responsible for soil genesis (Lebedeva et al, 1999; Bockheim, 2012). Our improved understanding of the formation of the sombric-like horizons from Southern Brazil (Caner et al., 2003; Chiapini et al., 2018), inhibits the correct classification of these soils, in particular, in the Brazilian Soil Classification System (Embrapa, 2018). Associated with sombric-like horizons in subtropical soils the yellowish color at the upper part of the B horizon over red subsoil can be observed. This process is called xanthization (from the Greek xanthos, yellow) (IUSS Working Group WRB, 2015). Xanthization is the conversion of hematite into goethite influenced by constantly moist climatic conditions. In subtropical soils moist conditions and the presence of organic matter inhibit hematite formation or microbial oxidation and are responsible for the preferential reductive dissolution of originally formed hematite turning red soils yellow (Cornell and Schwertmann, 2003). In the present study, the classification of these soils is revisited, taking into account the processes that underlie their formation. The relationship with the formation of the sombric-like horizon and xanthization is highlighted. With this aim in mind, new data of micromorphological, physicochemical, mineralogical and iron and aluminum dissolution analysis were interpreted.

Materials and Methods

Study site

The study area is located in Tijucas do Sul (in the state of Paraná, Brazil; 25°55’41” S, 49°11’56” W) (Figure 1). The parent material was described as a colluvium derived from migmatites with local influence of other metamorphic rocks (Santos et al., 2006). The native vegetation was classified as a mixed ombrophylous forest with grassland. The climate is temperate and humid subtropical (Behling et al., 2001). The soils found in the region are classified as Argissolos, Nitossolos, Cambissolos mainly with some local occurrence of Latossolos (Embrapa, 2018). The pedologic system is made up of four soil profiles with a sombric-like horizon, previously studied by Chiapini et al. (2018) to assess environmental changes. In this study we used this pedologic system to examine the main soil forming processes in the context of their classification within the three soil classification systems. These included three profiles from a toposequence (P1, P2 and P3) and a profile on the summit of a nearby hill (P4). Figure 2 illustrates a representative profile. Pedogenic horizons were described in the field. Morphological description was based on the field guide elaborated by the FAO (2006). The soil samples were collected according to pedogenic horizon (Table 1). The samples were dried and sieved and the fraction < 2 mm was used for physical (granulometry; Table 2) and chemical analysis ( pHH2O, pHKCl; Al3, H+Al; Ca2, Mg2, K+, total C; base saturation (BS), clay activity and C/N ratio; Table 3) that were presented in Chiapini et al. (2018).

Figure 1 – Location of the study area. 

Figure 2 – Representative soil profile with sombric-like horizon (P1; Chiapini et al., 2018). 

Table 1 – Morphology of the studied soil profiles. 

Profile Hz. Depth Color1 Structure2 Consistency3 Waxiness4 Coatings surfaces5

Dry Moist Wet
cm
P1 A1 0–10 7.5YR 3/2 SB/G SO FR PL/ST NO NO
A2 10–20 7.5YR 3/2 SB/G SO VFR PL/ST NO NO
A3 20–40 7.5YR 3/2 SB/G SO VFR PL/ST NO MAS
A4 40–65 7.5YR 4/4 SB/G SO VFR PL/ST NO MAS
A5 65–70 5YR 3/2 SB/G SO VFR PL/ST NO MAS
A6 70–85 5YR 3/2 SB/G SO VFR PL/ST NO MAS
A7 85–95 5YR 3/2 SB/G SO VFR PL/ST NO MAS
AB 95–100 5YR 3/3 SB SO VFR PL/ST NO MAS
BA 100–110 5YR 4/4 SB HA FR VPL/VST SA SAS
B1 110–130 5YR 4/6 SB HA FR VPL/VST SA SAS
B2 130–170 2.5YR 4/6 SB HA VFR VPL/VST SA SAS
BC 170–200+ 5YR 4/6 SB HA VFR VPL/VST NO SAS

P2 A1 0–13 7.5YR 3/2 SB/G SO FR PL/ST NO NO
A2 13–26 7.5YR 3/2 SB/G SO VFR PL/ST NO NO
A3 26–70 7.5YR 3/2 SB/G SO VFR PL/ST NO NO
A4 70–105 7.5YR 2.5/1 SB/G SO VFR PL/ST NO NO
AB 105–120 7.5YR 3/2 SB SO VFR PL/ST NO NO
BA 120–135 7.5YR 3/4 SB HA FR VPL/VST SA SAS
B1 135–180 7.5YR 4/6 SB HA FR VPL/VST SA SAS
B2 180–210 5YR 4/6 SB HA FR VPL/VST SA SAS
B3 210–250 2.5YR 4/6 SB HA VFR VPL/VST SA SAS
B4 250–320 2.5YR 4/8 SB EHA VFR VPL/VST SA SAS
2B5 320–380 2.5YR 4/6 SB EHA VFR VPL/VST SA SAS
2BC 380–450+ 5YR 6/8 SB SO VFR VPL/VST NO NO

P3 A1 0–10 7.5YR 3/2 SB/G SO FR PL/ST NO NO
A2 10–42 7.5YR 3/2 SB/G SO VFR PL/ST NO NO
A3 42–60 7.5YR 3/2 SB/G SO VFR PL/ST NO NO
A4 60–75 7.5YR 3/2 SB/G SO VFR PL/ST NO NO
A5 75–94 7.5YR 2.5/1 SB/G SO VFR PL/ST NO NO
A6 94–101 7.5YR 3/2 SB SO FR PL/ST SA MAS
AB 101–118 5YR 3/2 SB HA FI PL/ST SA SAS
BA 118–135 5YR 5/6 SB HA FR VPL/VST SA SAS
B1 135–165 2.5YR 4/6 SB HA VFR VPL/VST SA SAS
B2 165–200+ 2.5YR 3/6 SB HA FR VPL/VST SA SAS

P4 A1 0–20 7.5YR 3/2 G SO VFR PL/ST NO NO
A2 20–50 7.5YR 2.5/1 G SO VFR PL/ST NO NO
A3 50–60 7.5YR 2.5/1 SB SO VFR PL/ST SC MAS
AB 60–65 7.5YR 3/2 SB HA VFR PL/ST SC MAS
BA 65–75 7.5YR 3/2 SB HA VFR VPL/VST SC SAS
B1 75–110 7.5YR 4/4 SB/P HA FR VPL/VST SA SAS
B2 110–140 7.5YR 4/6 SB/P HA FR VPL/VST SA SAS
B3 140–180+ 5YR 4/6 SB/P HA FR VPL/VST SA SAS

1Munsell Color, from Chiapini et al. (2018); 2Structure: SB = subangular block, G = granular, P = prismatic; 3Consistency: SO = soft, HA = hard, EHA = extremely hard, FR = friable, VFR = very friable, PL = plastic, ST = sticky, VPL = very plastic, VST = very sticky; 4Waxiness described in the field: NO = not observed; SC = strong and common; SA = strong and abundant; 5Coatings surfaces described in the field: NO = not observed; MAS = matte aggregate surface; SAS = shiny aggregate surface. The sombric-like horizon is indicated by grey bands.

Table 2 – Granulometry of the studied soil profiles. 

Profile Hz. Depth Clay1 Silt ------------------------------------------------------------------------- Sand2 ------------------------------------------------------------------------- Silt/Clay1
Total VC C M F VF
cm -------------------------------------------------------------------------------------------------g kg–1 -------------------------------------------------------------------------------------------------
P1 A1 0–10 530 183 287 27 54 78 98 29 0.4
A2 10–20 527 159 314 27 71 87 91 38 0.3
A3 20–40 527 167 306 29 61 83 102 31 0.3
A4 40–65 552 123 325 30 60 89 112 35 0.2
A5 65–70 556 121 323 39 67 96 98 24 0.2
A6 70–85 494 209 297 32 62 95 101 8 0.4
A7 85–95 510 131 359 41 59 99 125 34 0.3
AB 95–100 535 124 341 43 58 88 116 35 0.2
BA 100–110 567 135 298 22 52 83 106 34 0.2
B1 110–130 541 169 290 22 58 0 98 33 0.3
B2 130–170 426 232 343 29 67 90 105 51 0.5
BC 170–200+ 303 382 315 9 47 81 118 60 1.3

P2 A1 0–13 543 124 333 43 65 89 102 34 0.2
A2 13–26 562 135 303 29 55 83 103 33 0.2
A3 26–70 587 98 315 27 66 90 99 34 0.2
A4 70–105 595 65 340 21 81 100 104 34 0.1
AB 105–120 597 96 307 27 58 84 104 34 0.2
BA 120–135 620 62 318 29 61 88 129 11 0.1
B1 135–180 652 37 311 34 64 85 95 33 0.1
B2 180–210 657 43 301 31 62 81 95 33 0.1
B3 210–250 641 49 310 31 68 83 95 33 0.1
B4 250–320 634 60 306 42 56 77 100 32 0.1
2B5 320–380 428 324 248 14 34 54 85 61 0.8
2BC 380–450+ 276 407 317 8 23 47 120 119 1.5

P3 A1 0–10 373 298 329 18 82 98 99 33 0.8
A2 10–42 520 170 311 17 66 90 101 36 0.3
A3 42–60 542 150 308 22 67 89 98 32 0.3
A4 60–75 546 124 330 22 68 93 103 44 0.2
A5 75–94 577 103 321 25 61 88 109 38 0.2
A6 94–101 576 99 325 32 60 89 104 41 0.2
AB 101–118 583 95 322 26 60 88 102 45 0.2
BA 118–135 606 85 309 28 55 79 100 47 0.1
B1 135–165 620 99 282 19 55 81 93 33 0.2
B2 165–200+ 654 70 276 22 54 77 90 34 0.1

P4 A1 0–20 531 188 281 21 51 92 109 8 0.4
A2 20–50 505 222 274 21 52 85 111 5 0.4
A3 50–60 581 125 294 40 55 83 109 7 0.2
AB 60–65 556 171 273 32 49 89 102 1 0.3
BA 65–75 581 141 278 24 48 86 110 10 0.2
B1 75–110 586 97 317 15 52 100 111 38 0.2
B2 110–140 669 61 271 17 45 78 97 34 0.1
B3 140–180+ 691 62 248 18 43 68 86 32 0.1

1From Chiapini et al. (2018); 2Sand: VC = very coarse; C = coarse; M = medium; F = fine; VF = very fine. The sombric-like horizon is indicated by grey bands.

Table 3 – Chemical properties of the studied soil profiles. 

Profile Hz. Depth pH1 H2O H+Al Al 3+ CEC1 BS1 Ct1 C/N Clay activity1, 2
cm --------------------------- cmolc kg–1 -------------------------- ------------------ % ------------------
P1 A1 0–10 3.8 10.6 5.6 12.1 12 4.9 17 23
A2 10–20 3.9 8.0 4.5 8.8 9 4.3 14 17
A3 20–40 4.2 7.8 4.2 8.1 3 2.9 14 15
A4 40–65 4.5 6.1 3.2 6.6 7 2.9 16 12
A5 65–70 4.6 6.3 3.1 6.3 0 2.3 20 11
A6 70–85 4.7 6.4 3.0 6.8 5 1.8 21 14
A7 85–95 4.7 5.7 2.9 5.7 0 1.3 18 11
AB 95–100 4.8 5.0 2.7 5.4 8 1.1 16 10
BA 100–110 4.8 4.4 2.6 4.8 9 0.8 16 8
B1 110–130 5.0 3.3 1.9 4.1 20 14 8
B2 130–170 5.0 3.0 2.3 3.7 19 13 9
BC 170–200+ 5.1 3.4 3.2 3.9 12 13

P2 A1 0–13 4.2 8.7 4.2 10.1 14 6.1 15 19
A2 13–26 4.0 8.5 3.9 9.2 8 4.8 14 16
A3 26–70 4.3 7.8 3.5 8.2 5 4.0 19 14
A4 70–105 4.3 7.7 3.3 8.0 4 2.8 22 13
AB 105–120 4.3 6.5 3.3 6.8 4 2.0 21 11
BA 120–135 4.4 6.1 2.9 6.6 8 1.7 19 11
B1 135–180 4.5 4.4 2.4 4.8 9 15 7
B2 180–210 4.8 3.4 1.8 3.8 9 12 6
B3 210–250 4.9 3.1 1.2 3.4 10 5
B4 250–320 4.7 2.6 1.1 2.9 10 5
2B5 320–380 4.6 3.5 3.2 4.1 15 9
2BC 380–450+ 4.6 3.6 4.2 4.1 12 15

P3 A1 0–10 3.9 14.0 5.2 16.7 16 10.3 16 45
A2 10–42 4.1 10.1 5.0 11.0 9 7.0 17 21
A3 42–60 5.1 7.6 2.4 8.8 13 4.6 15 16
A4 60–75 4.5 7.0 3.2 7.1 1 3.3 19 13
A5 75–94 4.6 6.6 3.2 6.7 1 3.2 22 12
A6 94–101 4.6 6.2 3.3 6.2 1 2.4 22 11
AB 101–118 4.8 5.3 2.6 5.9 9 1.6 19 10
BA 118–135 4.8 4.6 2.4 4.6 1 1.4 18 8
B1 135–165 4.8 3.7 1.9 3.7 0 13 6
B2 165–200+ 4.7 3.5 1.6 3.5 0 13 5

P4 A1 0–20 4.6 9.1 3.1 9.5 4 4.8 19 18
A2 20–50 4.9 7.6 2.7 7.7 2 3.7 26 15
A3 50–60 5.1 6.2 2.4 6.5 4 2.3 25 11
AB 60–65 5.0 5.4 2.0 5.8 7 2.7 33 10
BA 65–75 5.1 4.5 1.5 4.8 8 1.2 21 8
B1 75–110 4.9 2.6 0.5 2.6 1 26 4
B2 110–140 5.4 2.0 0.3 2.1 1 17 3
B3 140–180+ 5.4 2.2 0.1 2.2 1 3

1From Chiapini et al. (2018); 2Calculated by: (T value × 1000)/(clay content (g kg–1)); The sombric-like horizon is indicated by grey bands.

Fe and Al contents

Fe and Al contents were determined from samples of each profile. First, the OM of the soil samples was eliminated by the addition of 30 % H2O2 in a water bath. Free Fe and Al oxyhydroxides were extracted by sodium citrate-bicarbonate-dithionite (CBD) treatment (four times at 80 ºC for 30 min in a water bath) (Mehra and Jackson, 1960; Jackson, 1979). Amorphous Fe (Feo) and Al (Alo) oxyhydroxides were determined by extraction with 0.2 mol L1 ammonium oxalate in the dark at 3.0 pH (McKeague and Day, 1966). Fe and Al contents were determined by atomic absorption spectrometry. Sodium pyrophosphate (0.1 mol L1) was extracted at pH 10 according to USDA (1996) providing the Fe and Al that is bound to OM (Fep and Alp) (Table 4).

Table 4 – Iron oxides dissolution of the studied soils. 

Profile Hz. Depth Fed Feo Fep Feo/Fed Fep/Fed Ald Alo Alp
cm mg kg–1 mg kg–1
P1 A1 0–10 48860 4538 20150 0.093 0.412 54650 3520 1701
A2 10–20 50650 5750 27670 0.114 0.546 19538 4832 1585
A3 20–40 53880 4670 14220 0.087 0.264 32868 4762 1543
A4 40–65 52372 4470 18790 0.085 0.359 19488 5182 1979
A5 65–70 51860 4278 24990 0.082 0.482 34680 5340 2446
A6 70–85 57755 4590 28840 0.079 0.499 27478 5902 2356
A7 85–95 63355 3275 19010 0.052 0.3 19018 5182 1914
AB 95–100 68573 2285 13960 0.033 0.204 24373 4987 1344
BA 100–110 71371 1537 8380 0.022 0.117 24710 4810 972
B1 110–130 72163 1139 450 0.016 0.006 15065 4605 244
B2 130–170 77894 1107 36 0.014 0 8765 4075 176
BC 170–200+ 77894 666 64 0.009 0.001 19123 3867 223

P2 A1 0–13 49500 5320 20550 0.107 0.415 40135 5765 4583
A2 13–26 55280 5350 25610 0.097 0.463 26720 5660 3576
A3 26–70 49595 6355 31100 0.128 0.627 18005 6455 2758
A4 70–105 59410 6820 38750 0.115 0.652 8735 7135 638
AB 105–120 59665 7225 60070 0.121 1.007 44490 6740 4578
BA 120–135 57285 8055 290 0.141 0.005 25235 6735 804
B1 135–180 64466 2214 487 0.034 0.008 8013 5187 311
B2 180–210 71529 1691 0 0.024 0 19618 4492 321
B3 210–250 75004 1796 0 0.024 0 12818 4442 364
B4 250–320 80277 1833 15 0.023 0 3381 4040 148
2B5 320–380 91530 980 11 0.011 0 7868 3712 213
2BC 380–450+ 56382 778 30 0.014 0.001 26768 3742 254

P3 A1 0–10 40745 4975 17530 0.122 0.43 34353 5677 2607
A2 10–42 47022 6898 19640 0.147 0.418 5508 5912 2878
A3 42–60 52535 7485 21710 0.142 0.413 3674 6217 3028
A4 60–75 52465 5965 17850 0.114 0.34 22353 5957 3818
A5 75–94 55615 6975 23190 0.125 0.417 15510 6100 3931
A6 94–101 55002 4958 23000 0.09 0.418 12838 4512 2731
AB 101–118 58282 4098 5660 0.07 0.097 12103 5067 2035
BA 118–135 57762 2208 780 0.038 0.014 5614 4135 1311
B1 135–165 60235 1685 437 0.028 0.007 20573 4337 621
B2 165–200+ 64052 2168 36 0.034 0.001 23273 4557 455

P4 A1 0–20 63097 9313 39940 0.148 0.633 17138 7192 3996
A2 20–50 66667 7753 47460 0.116 0.712 5520 7340 3639
A3 50–60 70432 6488 37050 0.092 0.526 10363 6337 3553
AB 60–65 70697 7403 37930 0.105 0.537 1972 6450 3274
BA 65–75 70607 7283 33680 0.103 0.477 7018 6632 3730
B1 75–110 84902 2148 397 0.025 0.005 16843 4347 672
B2 110–140 97685 2205 18830 0.023 0.193 16762 6568 455
B3 140–180+ 97665 3435 13560 0.035 0.139 28630 4630 453

The sombric-like horizon is indicated by grey bands.

Mineralogical analysis in the clay fraction

After dispersion with 0.2 mol L1 NaOH, the sand fraction was separated from the silt + clay fractions by sieving (0.053–mm sieve opening), the soil samples (P1: B2; P2: B3; P3: B2 e P4: B3), after which the clay was separated from the silt by decantation, following Stokes’ law (Jackson, 1979). Clay samples were saturated with Mg2(Mg 25 ºC) and solvated with ethylene glycol (Mg + EG). In another aliquot, the samples were saturated with K and then heated to 500 ºC. Slides of oriented clay were prepared for X-ray diffraction (XRD) using a Rigaku Miniflex II device, with Cu (CuKα) radiation, a graphite monochromator, operated at 10 mA and 15 kV, at a rate of 0.02 º 2θ and a speed of 1 sec/step, in the range of 3 to 45 ° 2θ.

Furthermore, in samples from the A and B horizons from all profiles (P1: A6, B1, B2; P2: A4, B1, B3; P3: A5, AB, B2; P4: A3, B1 and B3) the Fe-oxyhydroxides in the clay fraction were concentrated with 5 mol L1NaOH, removing clay minerals and Al-hydroxides (Norrish and Taylor, 1961). Sodium metasilicate was added to reach 0.2 mol L1 Si concentration in solution to avoid the formation of iron oxides with high Al isomorphic substitution (Kämpf and Schwertmann, 1982). Sodalite [Na4Al3Si3O12(OH)] formed during extraction was removed by washing twice with 50 mL of 0.5 mol L1 HCl solution and washing once with 50 mL of deionized water (Norrish and Taylor, 1961; Singh and Gilkes, 1991). The mineral components of the concentrated Fe residue were identified by XRD, carried out on un-oriented powdered samples on glass slides, using a scanning range of 3 to 45 ° 2θ. An internal standard was added to the samples (5 % NaCl) for correction of distortions and mounted on glass slides (non-oriented).

The goethite/hematite ratio [Gt/(Gt + Hm)] was estimated using the main diffraction peak areas (Torrent and Cabedo, 1986). Fe isomorphic substitution by Al (IS) in Gt was calculated according to Schulze (1984) and in Hm according to Schwertmann et al. (1979). The Gt and Hm contents were estimated based on the crystalline Fe content (FeCBD - Feo), considering the Gt/(Gt+Hm) ratio, the IS level, and the least mineral formulas (Melo et al., 2001) (Table 5).

Table 5 – Iron oxides quantification of some studied horizons. 

Hz. WHH+ Gt111 Corrected d-spacing Gt/(Gt+Hm) MCD# IS* Hm Gt


Gt110 Gt111 Hm104 Hm110 Gt110 Gt111 Hm104 Hm110 ISGt ISHm
(º 2θ) --------------------------------- nm --------------------------------- --------------------------------- nm --------------------------------- --------------------------------- % ---------------------------------
P1 A6 1.10 4.15 2.43 2.67 2.51 0.69 21.77 8.92 22.34 18.85 18.01 11.84 2.85 6.35
B1 0.97 4.16 2.43 2.69 2.51 0.76 10.05 10.59 20.86 18.50 21.29 13.10 3.05 9.70
B2 0.66 4.17 2.44 2.70 2.51 0.56 12.29 18.95 9.40 9.07 14.09 7.14 5.57 7.01

P2 A4 0.91 4.13 2.42 2.67 2.48 0.84 12.98 11.53 19.55 23.41 23.79 - 1.66 8.70
B1 1.01 4.15 2.43 2.75 2.47 0.78 11.59 9.99 13.94 14.09 19.64 - 2.79 9.99
B3 0.86 4.15 2.44 2.69 2.47 0.62 20.34 12.65 23.23 24.34 9.86 - 5.52 8.84

P3 A5 1.03 4.16 2.42 2.66 2.50 0.90 8.03 9.80 20.88 26.41 27.35 - 0.97 8.62
AB - 4.13 2.41 2.67 2.51 - - - - - 32.38 8.90
B2 1.03 4.18 2.43 2.70 2.52 0.56 14.05 9.72 24.86 10.47 18.70 1.32 4.50 5.63

P4 A3 1.06 4.17 2.44 2.68 2.47 0.85 7.74 9.40 15.40 31.30 9.24 - 1.70 9.75
B1 1.37 4.15 2.42 2.66 2.50 0.87 12.21 6.80 15.63 31.91 30.41 - 2.09 14.56
B3 1.03 4.18 2.43 2.70 2.51 0.62 14.05 9.72 13.20 11.39 16.53 10.92 6.08 9.89

+ = width at half peak height; # = mean crystal diameter; * = isomorphic substitution. The gray bands are sombric-like horizons.

Micromorphological analysis

Thin sections (5 × 9 cm) were obtained from Chiapini et al. (2018), and additional data were presented in relation to pedogenetic processes to assist in soil classification (Table 6 and Figures 3, 4 and 5); these processes included clay illuviation, soil aggregation and, in particular, xanthization.

Table 6 – Micromorphological characteristics of the studied soils. 

Profile P1 - A4/A5/A6 Profile P2 - A4 Profile P3 - A5/A6 Profile P4 - A2
Microstructure Subangular blocky and granular Granular Subangular blocky and granular Subangular blocky and microgranular
Coarse material Quartz (95 %); Iron nodules (2 %); Charcoal (3 %) Quartz (95 %); Iron nodules (2 %); Charcoal (2 %); Mica (< 1 %) Quartz (96 %); Iron nodules (2 %); Charcoal (2 %) Quartz (95 %); Iron nodules (3 %); Charcoal (2 %)
Micromass Speckled, Porostriated Speckled Speckled, Pore and Grain Striated Speckled, Pore and Grain Striated
c/f related distribution Porfiric-Enaulic Enaulic-Porfiric Enaulic-Porfiric Enaulic-Porfiric
Pedofeatures Excrement infillings, Incomplete infillings and iron nodules Incomplete infillings and iron nodules Excrement infillings, Incomplete infillings and iron nodules Incomplete infillings and iron nodules

Figure 3 – Photomicrographs. A), D) (polarized light), B) and D) (cross polarized light) Porostriated b fabric (shiny peds) due to moderate shrinking and swelling of aggregates in B horizon of P1; E) and F) Soil matrix in B horizon (variegated color) and microscopic fragments of charcoal of P2, representing the xanthization process below the ‘sombric’ horizon (sombric-like horizon). Q: quartz; P: pore and M: soil matrix. 

Figure 5 – Increase in structure density with depth. A) and B) Subsurface horizon with subangular block and granular structure (P3 transition A3/A4; 57–72 cm); C), D) and E) Subsurface horizon with subangular block structure (P3 transition A5/A6; 77–90 cm); F) Subsurface horizon with subangular block structure (P3 horizon B2; 170–183 cm). 

Results and Discussion

Soil morphology and micromorphology

Surface horizons have a yellowish color, with 7.5 YR hues, which suggests the predominance of goethite rather than hematite (Tables 1 and 5). The darker color observed in sombric-like horizons compared to overlying horizons is related to a larger contribution from BC in the sombric-like horizons (Chiapini et al., 2018), which has a strong pigmentation effect on the soil matrix (Silva and Vidal-Torrado, 1999; Macedo et al., 2017). Thus, the concentration of both BC and OM influence the melanization process in the upper part of the soil profiles (Macedo et al., 2017).

Field (Table 1) and micromorphological (Table 6) descriptions showed that the soil structure of the surface horizons and sombric-like ones are characterized by granular and subangular blocks with a moderate to strong degree of pedality. This is a result of the high OM content (Table 3; Ct) and an intense bioturbation process (Figure 4 and 5) (De Craene and Laruelle, 1955; Bennema et al., 1970; Macedo et al., 2017). On the other hand, the structure in the subsurface horizons was characterized by subangular blocks with strong pedality indicating a much denser structure (Figure 5). This higher density was related to lower OM content, lower contribution from roots, lower faunal activity, and higher clay content at depth. Shiny and matte aggregate faces were observed in the field (Table 1). On the micro scale, a porostriated b-fabric’s features were described and are the result of swell-shrinking of soil in alternating dry and humid periods (Figure 3A, 3B, 3C and 3D). Thus, both the shiny and matte surfaces described in the field are actually in fact compression features (shrinking and swelling process) and are not due to clay illuviation.

Figure 4 – The bioturbation process. A) and B) Loose discontinuous infillings in the A4 horizon of P2 and the granular structure; C) Loose discontinuous infillings in the A5 horizon of P3; D) Detail of C). 

Thin sections showed a variegated color (red/yellow) in the matrix of B horizons, which were not observed on the macro scale (Figure 3E e 3F). This variegated color is due to xanthization in the A, AB and upper B horizons (Cornell and Schwertmann, 2003). The Gt/(Gt+Hm) ratio would reflect this process, its values varying between 0.423 and 0.754 and indeed indicate predominance of Gt over Hm in the surface horizons of all profiles (Table 5). In fact, the Gt/(Gt+Hm) ratio decreased with depth, indicating xanthization (yellow colors; hues 5 YR and more yellow) in the upper part of the soils (Table 5). The high organic matter content and/or microbial oxidation in the sombric-like horizon inhibits hematite formation at the point of contact with the B horizon, causing preferential reductive dissolution of originally formed hematite turning red soils yellow in the lower subdivisions of the A horizon (Cornell and Schwertmann, 2003). Furthermore, the formation of Gt in soils is favored by wetter conditions (Schwertmann and Taylor, 1989), high OM content, low temperature and low contents of Fe in solution (Camêlo et al., 2018), all of which are evident in the soils from the study area.

Chemical and physical properties

The pH in water of the studied soil profiles ranged from 3.8 to 5.4 and increased with depth (Table 3). Extractable aluminum contents (Al3) in the surface horizons of profiles P1, P2 and P3 were high enough (> 4 cmolc kg1) for an aluminic qualifier (Embrapa, 2018); however, in the subsurface horizons this was not observed. Bases (K+, Na+, Ca2, and Mg2 data not shown) and base saturation (BS) were low in all horizons, due to a significant contribution of H+Al ions, as well as low clay activity, which are typical for such highly weathered soils. Similar to Dalmolin et al. (2006), we observed that high Cation Exchange Capacity (CEC) values in the A horizons were a result of the high OM content (Ct; Table 3).

The contents of Fe and Al forms are given in Table 4. The Fed contents showed a clear gradual increase with depth, while the opposite was observed for Feo (r2 = 0.93, 0.44, 0.36 and 0.84 for profiles P1–P4, respectively). This reflects that low OM content favors pedogenic Fe oxyhydroxides formation at depth (mainly hematite) (Curi and Franzmeier, 1987). The Feo/Fed ratio showed generally higher values in the A than in the B horizons because of the high influence of OM in the A horizons (Table 4). The Feo/Fed ratio indicated that the Fe in the A horizons was in poorly ordered form (values > 0.05), while in the B horizons the major part of Fe oxides were present in crystals with greater ordered form (values < 0.05; Inda Junior and Kämpf, 2003). The Fep was substantially higher than the Alp, and both showed positive correlation (r2 = 0.76). The high values of Fep and of the Fep/Fed ratio indicate that Fe is bound to soil OM and confirms that part is in the non-crystalline form.

High Ald values were observed in surficial horizons, which can be related to several methodological factors during the sodium citrate-bicarbonate-dithionite (CBD) extractions. First, successive extractions with CBD (80 °C) may dissolve a certain amount of kaolinite and gibbsite thereby releasing Al3 and causing an increase in Ald (Inda Junior and Kämpf, 2003). Second, the high Ald values can be due to complexation to sodium citrate catalyzed by the high temperature during the extraction procedure (Zhang et al., 1985). Third, the high values of Ald can be related to the extraction of the Al3 resulting from the isomorphic substitution of Fe3 in the iron oxides (Tables 4 and 5).

All four profiles presented a high clay content that was regularly distributed with depth (Table 2). The highest clay contents were observed in B horizons but this was not sufficient to characterize a textural difference (clay increase in depth) (IUSS Working Group WRB, 2015; Soil Survey Staff, 2014; Embrapa, 2018). The B horizons presented a low silt/clay ratio (~ 0.3), which is typical of highly weathered soils (Fox, 1982) where the clay content increases due to weathering of minerals in the silt fraction. Thus, properties such as low activity clays, silt/clay and Feo/Fed ratios characterizes the occurrence of a moderate ferralitization process in our studied soils (Kämpf and Curi, 2012).

Clay mineralogy

Clay components identified with XRD in the iron-free clay fraction of B horizons of all profiles were kaolinite (Kt), gibbsite (Gb), hydroxy-interlayered 2:1 minerals (2:1 HI) and quartz (Qz) (Figure 6). The intensities of Kt001 and Gb002 decreased from the profile at the summit (P1) towards the profile at the footslope (P3). 2:1 HI showed similar intensities in profiles P1 and P2, but was very weak in profiles P3 and P4. The absence of significant asymmetry of Kt001 in profiles P1 and P2 indicates no or little interstratification with smectite, which is in agreement with observations on similar soils in the region by Oliveira Junior et al. (2014).

Figure 6 – Diffractograms of the clay content of the B horizons, saturated with Mg (Mg 25 °C), solvated with ethylene glycol (Mg + EG) and K heated until 500 ºC (500 °C). All profiles (P1, P2, P3 and P4), there is the presence of kaolinite (Kt), gibbsite (Gb), 2:1 hydroxy-interlayered (HI) and quartz (Qz). 

Goethite and hematite were identified in the concentrated oxide fraction (Figure 7). The contents of goethite varied from 5 % to 11 % and that of hematite from 2 % to 7 %. Goethite predominated in all studied soil profiles, with the exception of the B3 and B2 horizons from P2 and P3, respectively (Table 5). The data showed the highest goethite content in soil profiles was in the first B horizon (B1) (Table 5). The yellow color was observed reflecting the morphology of xanthization (Figures 3E3F) which occurs in the interface of the sombric-like horizon that is rich in OM (Ct) and the B horizon; and the action of soil fauna in this interface aid which is responsible for the distribution of OM in depth (melanization), that consequently expands xanthization. According to Chiapini et al. (2018), the sombric-like horizon of these soils is a remnant of an earlier phase of soil formation under grass vegetation and frequent natural fires resulting in considerable accumulation of black carbon (BC) at depth. During wetter conditions, from the Late Holocene period to the present, xanthization was influenced by this accumulation of OM and contributed to the development of polychrome of soil profiles.

Figure 7 – Diffractograms after concentration of iron oxides in some soil horizons. 

The Hm content showed a clear increase with depth, which was the reverse for Gt (Almeida et al., 2000). The values for isomorphic substitution (IS) varied from 9 % to 32 % for Gt and from 1 % to 11 % for Hm. The IS for Gt presented similar values in both the A and upper B horizons for all profiles, with the exception of profile P2 where the A horizon presented a higher IS value (Table 5). This high Al goethite substitution is related to elevated, dessicated non-hydromorphic conditions that were observed in our soil profiles similar to other highly weathered Brazilian soils (Schwertamenn and Kämpf, 1985; Motta and Kämpf, 1992; Almeida et al., 2000). The deepest horizon showed lower IS values for both Gt and Hm (Table 5). In P2 and certain horizons of P3 (A5) and P4 (A3 and B1) it was not possible to obtain IS values for Hm.

The mean crystal diameter (MCD) of Gt showed a similar growth tendency for both directions (110 and 111), with the exception of the B3 horizon from profile P2. In Hm, the crystals showed a tendency to grow in a 104 rather than a 110 direction, again with exception of the B3 horizon from profile P2, in which similar dimensions for both directions were observed. The MCD for Gt and Hm in the studied profiles was smaller than that observed by Melo et al. (2001) in tropical conditions (hot and humid). Furthermore, the size of Hm and Gt crystals was smaller (< 5 nm; Table 5) than that observed in Oxisols (Melo et al., 2001; Lima et al., 2017). Thus, high Kt content favors the development of a block structure rather than a granular microstructure in the B horizons (De Wispelaere et al., 2015). No evidence of coalescence of microstructures nor transformation from small granular to blocky or prismatic aggregates were observed, as was proposed by Cooper et al. (2010).

Soil Classification

Important items for classification at the Order level are the morphological features related to the B horizon. These include particle rearrangement on the surfaces of structural units, shiny peds, low textural difference, clay-rich subsoil and structure from moderate to strong subangular blocks with waxiness characterizing a Nítico and Textural horizon at the same time as according to the Brazilian Soil Classification System (Embrapa, 2018). At WRB-FAO (IUSS Working Group, 2015) the B horizon is classified as a Nitic horizon, presenting the same morphological features and chemical characteristics such as ≥ 4 % Fed and ≥ 0.2 % Feo. In relation to Soil Taxonomy (Soil Survey Staff, 2014) the B horizon is classified as a Kandic horizon.

Another important classification item is the polychrome character of the soil profiles. As per the Brazilian Soil Classification System (Embrapa, 2018), in the Nitossolos Order (Nítico B horizon), the polychrome indication is only allowed in specific situations, none of which fit in our soils. The soil profiles showed colors of more than one hue page, and variation in value and chroma in A and B horizons (Table 1). Similarly, in the suborder, Nitossolos Brunos, this variation in color is also not allowed. Because of this, the profiles must be classified as Argissolos in the first Order.

Classification of the four soil profiles (P1–P4) under the Brazilian Soil Classification System (Embrapa, 2018) would result in the following reasoning:

Due to the presence of the Nítico and Textural B horizon with shiny peds that cannot be attributed to clay illuviation, nor polychrome characteristic, the first category level (Order) is classified as Argissolos. Because the soils presented a red-yellow color (P1), dominant yellow color (P2 and P4) and dominant red color (P3) in the BA and B horizons, they were classified as Vermelho-Amarelo (Red-Yellow) in the second level in P1, Amarelo (Yellow) in the second level of P2 and P4, and Vermelho (Red) in the second level in P3. The soil profiles showed low base saturation (BS < 50 %). Thus the third category level they were classified as Distrófico (Dystrophic). At the fourth category level the soil profiles P1 and P3 were classified as nitossólico, because of the soil morphology similar to Nitossolos and soil profiles P2 and P4 such as típico (typical) (Table 7). Additional problems with the Brazilian Soil Classification System (Embrapa, 2018) were observed at the subgroup level, where the sombric feature (caráter sômbrico) is not allowed because i) no OM illuviation was confirmed in the micromorphological analysis (organic or clay-organic coatings) and ii) no increase in carbon content in the sombric-like horizons was observed. The impossibility of classifying it as a sombric horizon thereby neglects an important process related to its formation, i.e., decomposition of the upper part of a humic horizon (Caner et al., 2003; Chiapini et al., 2018). As such, a great effort dispensed in pedogenic studies that use the soil as a record of past climatic conditions will be missed, as well as one of the main soil forming factors, i.e. time, is neglected.

Table 7 – Soil Classification in the Brazilian Soil Classification System (SiBCS; Embrapa, 2018), WRB (IUSS Working Group WRB, 2015) and Soil Taxonomy (Soil Survey Staff, 2014). 

Soil profile Classification

SiBCS WRB Soil Taxonomy
P1 ARGISSOLO Vermelho-Amarelo nitossólico Umbric Nitisol (Dystric, Humic) Typic Kandiudox
P2 ARGISSOLO Amarelo distrófico típico Umbric Nitisol (Dystric, Humic) Xanthic Kandiudox
P3 ARGISSOLO Vermelho distrófico nitossólico Umbric Nitisol (Rhodic, Dystric, Humic) Rhodic Kandiudox
P4 ARGISSOLO Amarelo distrófico típico Umbric Nitisol (Dystric, Humic) Xanthic Kandiudox

This situation must be revisited once all of the soil classification systems used in this work have assumed the taxonomic approach, in other words, considered the pedogenic process to fit a soil in a given class.

In the WRB classification (IUSS Working Group WRB, 2015), the polychrome characteristic is not a problem and the sombric horizon can be assigned to this soil order, but it cannot appear in principal qualifiers nor in supplementary ones. Therefore, the presence of a sombric horizon or a sombric qualifier could not be identified in the studied soil profiles and their classification in WRB would result in Umbric Nitisol due to the presence of a nitic and umbric horizon. As regards qualifiers, the soils P1, P2 and P4 were classified as Dystric and Humic and P3 as Rhodic, Dystric and Humic (Table 7). Our classification differs from Bockheim (2012) who found the sombric horizons primarily in Umbric Ferralsols (Sombric).

In Soil Taxonomy (Soil Survey Staff, 2014) no problems with morphological characteristics were found and the soils were classified as Oxisol in the Order level because of the low base saturation and high mineral weathering (moderate ferralitization). A Udic moisture regime was identified as the Suborder level. At Great Group the soil profiles were classified as Typic Kandiudox (P1), Xanthic Kandiudox (P2 and P4: dominant yellow color) and Rhodic Kandiudox (P4: dominant red color) (Table 7). Similar to those described above, Bockheim (2012) classified soils with a sombric horizon primarily as Sombriudox and Sombrihumult in Soil Taxonomy, which differs somewhat from our soils.

Thus, the sombric-like horizon in the soils from our study area cannot be classified as sombric horizons according to the Soil Taxonomy and WRB classification systems, nor as a sombric qualifier according to the Brazilian Soil Classification System.

The process of OM illuviation (Soil Survey Staff, 2014; IUSS Working Group WRB, 2015) in the formation of the sombric horizon was not observed in any of our four soil profiles (Figure 3A and 3B). Nevertheless, the other characteristics belonging to the sombric horizon were present, and included base saturation < 50 %, lateral tracing that distinguished them from some buried epipedons, and a lower color value and chroma than the overlying horizon.

Conclusions

The distinctive properties of subtropical soils with sombric-like horizon from the studied soil profiles included: the presence of shiny peds, a base saturation < 50 %, a lower color value and chroma than the overlying horizon, clear and sharp lateral tracing, the yellowish color in depth, and the polychrome characteristic. Melanization, xanthization, bioturbation, moderate shrinking/swelling and moderate ferralitization are the most evident pedogenetic processes. Xanthization is closely related to the formation of the sombric-like horizon during the past change from drier to wetter conditions.

The current definition of the sombric horizon in the soil classification systems (Soil Taxonomy, WRB-FAO and Brazilian Soil Classification) can be improved in terms of soil forming processes and polychrome characteristic. We propose developing classification criteria for the sombric-like horizon, or alternatively accept them as a special type of sombric horizon.

Acknowledgments

The authors thank the São Paulo Research Foundation (FAPESP) for financial support (Projects 2015/03577-2, 2014/23969-0 and 2013/03953-9) and the Coordination for the Improvement of Higher Education Personnel agency (CAPES). The Brazilian National Council for Scientific and Technological Development (CNPq) also supported this work through a grant 301818/2017-7 given to the sixth author. We thank Nivanda Maria de Moura Ruiz and Luiz Antonio Silva Junior for analytical activities, José Luiz Vicente and Sonia Aparecida de Moraes for thin section confection, and three anonymous reviewers, in particular reviewer #2, for their critical comments.

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Received: May 06, 2019; Accepted: February 08, 2020

* Corresponding author <pvidal@usp.br>

Authors’ Contributions

Conceptualization: Chiapini, M.; Vidal-Torrado, P.; Data acquisition: Chiapini, M.; Oliveira Junior, J.C.; Schellekens, J.; Almeida, J.A.A. Data analysis: Chiapini, M.; Vidal-Torrado, P., Oliveira Junior, J.C., Schellekens, J.; Almeida, J.A. Design of methodology: Chiapini, M.; Vidal-Torrado, P. Writing and editing: Chiapini, M.; Schellekens, J.; Vidal-Torrado, P.; Oliveira Junior, J.C.; Almeida, J.A.; Buurman, P.

Edited by: Paulo Cesar Sentelhas

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