Mineralogy , Micromorphology , and Genesis of Soils with Varying Drainage Along a Hillslope on Granitic Rocks of the Atlantic Forest Biome , Brazil

Although the physical environment of the Atlantic Forest realm is well known, studies on the soil-landform relationships are fundamental to improve the management of soil resources to facilitate sustainable development. The purpose of this study was to evaluate a representative topossequence on the “Mares de Morros” landscape of deeply weathered regolith on leucocratic granite rocks and demi-orange convex slopes. The soils varied along the topossequence according to drainage and were classified as Acrudox, Pseudogleysol, and Epiaquent. The clay fraction was composed by kaolinite, in association with gibbsite, goethite, hematite, and traces of vermiculite and hydroxy-Al interlayered vermiculite (HIV). The kaolinite crystallinity index obtained by different methods showed high structural disorder throughout the sequence, indicating that long-term pre-weathering has produced a homogenous regolith with little differences in terms of mineralogy, despite the changes in drainage. On the other hand, micromorphological features showed a complete change from the typical, well-developed microaggregate structure of upland, well-drained soils, to a massive, poorly developed structure downslope, consistent with the morphological description. Changes in microstructure development and micropedological features occurred both vertically and laterally along the topossequence and indicate that mineralogy alone cannot account for the microaggregate structure of kaolinitic Latossolos (Oxisols) well-drained with low Fe contents. Soils from the “Mares de Morros” landscape of the Alegre river basin on leucocratic granitic rocks highlight an inheritance of a deep pre-weathered regolith, showing subtle chemical and mineralogical changes, but marked morphological and physical differences along the topossequence, basically controlled by soil drainage in the past or present.


INTRODUCTION
Atlantic Forest domain stretches from the northeastern to the southern regions of Brazil and to northern Argentina and southeastern Paraguay.The state of Espírito Santo, Brazil, is entirely comprised within this domain, with a typical physiography that changes from mountainous or hilly areas to coastal tablelands.In the southern part of the state, the Alegre river basin is representative of the dissected plateau and the demi-orange landscape known as "Mares de Morros".It is characterized by hilly landforms forming a dissected plateau, originally covered by tropical rain forest on crystalline rocks, mainly gneiss and granite (Ab'Sáber, 1970, 2012).
Morphogenesis of the "Mares de Morros" landscape involves a set of physiographic and ecological processes that resulted in the demi-orange, dome-like dissection on crystalline rocks with dominant convex slopes (Ab'Sáber, 1970, 2012).This morphoclimatic area is characterized by humid to sub-humid tropical environments, associated with deep weathering and rainforest (IBGE, 2012).Such physiognomy and edaphoclimatic characteristics form a rather homogeneous landscape that favors the widespread formation of dome-like, convex slopes as well as deep soil mantles (Embrapa, 1978).
The original semi-deciduous forest cover of the Alegre river basin is preserved in the form of small and scattered forest fragments due to intense deforestation in the late 19 th century (Novaes, 1968;Lani et al., 2008) for the establishment of coffee plantations and subsistence crops.Due to long-term soil nutrient exhaustion by coffee cultivation, the current land use type is mainly pasture, almost exclusively with Brachiaria sp.(SEAG, 2008), which is adapted to degraded soils.
The peculiar thick mantle of weathered materials (regolith), typical of the "Mares de Morros" landscape, can also be observed in the Alegre river basin.The thickness of the saprolite (C horizon) tends to be much greater than the thickness of the overlying solum (A and B horizons) (Corrêa, 1984;Lani et al., 2001;Ab'Sáber, 2012), reaching more than 100 m in some places.
Although the geology of the Alegre river basin is dominantly granitic-gneiss, variations in the mineralogical composition in these rocks may lead to different soils, depending on the performance of the other training factors and processes.Studies on crystalline rocks in various climatic conditions in southeastern Brazil have revealed kaolinite as the main secondary mineral, often associated with the alteration of feldspars (Meunier and Velde, 1976;Melfi et al., 1983;Corrêa, 1984;Dixon, 1989;Melo et al., 2001).Most soil studies in the hilly landscapes of the "Mares de Morros" have been carried out on biotite gneiss, with little studies on leucogranites, normally associated with much-degraded pastures.In the first, a rejuvenation of some Latossolos is commonly observed, leading to a "cambic" character and enhanced nutrient status (Albuquerque Filho et al., 2008).
The regional geology is composed of Precambrian basement rocks of the Paraíba do Sul Complex, part of the Atlantic Mobile Belt (Oliveira et al., 1983).It mainly consists of granitic rocks (migmatized paragneiss) with a significant presence of quartz, feldspar, granite, and minor amphibole (CPRM, 2007).Rocks have an acid composition (70 % SiO 2 , 14 % Al 2 O 3 , and 2 % Fe 2 O 3 ), with low amounts of Fe and Mn (CPRM, 2007).
The geomorphology of the Alegre river basin is defined as part of the stepped hillslopes of southern Espírito Santo, as a geomorphological unit (Oliveira et al., 1983).Hence, the hills are distributed in successive steps at different altitudinal levels (Ab'Sáber, 1970;Corrêa, 1984).The structural control of the drainage is evident in the entire Alegre river basin (Figure 1).

Site selection and sample collection
The selected topossequence was identified through interpretation of a planialtimetric map at a 1:50,000 scale, a geological map at a 1:100,000 scale (Embrapa, 1978;CPRM, 2007), and a soil map at a 1:50,000 scale (Pacheco, 2011), using aerial photographs at a scale of 1:15,000.A Digital Elevation Model (DEM) was prepared, using the ArcGIS 10.1 software.Four representative soil profiles (P1, P2, P3, and P4) were selected and collected along the topossequence (Figure 2).
The positions of the soil profiles were georeferenced, and morphological descriptions were made based to Santos et al. (2015).Classification, to the categorical level of the subgroup, was based on the Brazilian System of Soil Classification (Santos et al., 2013) and Soil Taxonomy (Soil Survey Staff, 1999).

Physical and chemical analyses
The pH in water and KCl 1 mol L -1 were determined at a ratio of 1:2.5 v/v; the potential acidity (H+Al) was extracted with Ca(OAc) 2 0.5 mol L -1 buffered to pH 7.0 and quantified by titration with 0.0606 mol L -1 NaOH.Exchangeable Ca, Mg, and Al were extracted with KCl 1 mol L -1 , and Na + and K + were extracted with Mehlich-1 solution (Donagema et al., 2011).
The concentration of elements in the extracts were determined by atomic absorption (Ca 2+ , Mg 2+ , and Al 3+ ), flame emission (Na + and K + ), and photocolorimetry (P).Effective cation exchange capacity (CEC E ) was calculated by the sum of cations (Ca 2+ , Mg 2+ , Na + , K + , and Al 3+ ), and total cation exchange capacity (CEC T ) was estimated by the sum of bases (BS) and potential acidity.Base saturation (V%) and Al 3+ saturation (m%) were calculated via the sum of bases, CEC T , and Al 3+ content (Donagema et al., 2011).The remaining P was determined according to Alvarez et al. (2000).Soil texture analysis was performed by the pipette method (Ruiz, 2005;Donagema et al., 2011); soil was divided into coarse sand, fine sand, silt, and clay.Total soil organic carbon (TOC) was determined according to the Walkley-Black titration method (Mebius, 1960) by wet oxidation with K 2 Cr 2 O 7 0.167 mol L -1 in the presence of sulfuric acid with external heating (Yeomans and Bremner, 1988).
Total contents of Fe, Al, Si, and Ti were determined in dried soil by the "sulfuric attack" method (Donagema et al., 2011) and expressed as oxides.We used a digestion of sulfuric acid 9 mol L -1 at 180 °C for 1 h, and with NaOH (30 %) on a heating plate for 2 min.Based on these results, we calculated the weathering indices Ki = [(SiO 2 × 1.70)/Al 2 O 3 ] and Kr = {(SiO 2 × 0.60)/[(Al 2 O 3 /1.02)+ (Fe 2 O 3 /1.60)]}molar ratios.The Fe and Al contents in the clay fraction were quantified after five sequential extractions, using the dithionite-citrate-bicarbonate method (DCB) at pH 7.3 (Mehra and Jackson, 1958) and after one extraction by the acid oxalate method (AOD) in darkness at pH 3.0 (McKeague and Day, 1966).The molecular Fe o /Fe d and Fe d /Fe s ratios were calculated using the Fe contents extracted by ammonium oxalate (Fe o ), dithionite-citrate-bicarbonate (Fe d ), and sulfuric acid attack (Fe s ).These were used as an index for the crystallinity degree of Fe oxides and the interpretation of pedogenic processes and the intensity of weathering (Kampf and Curi, 2000).The Al, Si, Fe, and Ti,

Mineralogical analysis
X-ray diffraction (XRD) analyses were conducted on the clay-size fraction (<2 mm) from selected samples, using a Panalytical X'Pert PRO (CoKα radiation) at the Federal University of Viçosa.X-ray diffraction patterns were collected between 4 and 50 °2θ, at a scan speed of 1 °2θ min -1 , with a potential 40 kV generator and a current generator of 40 mA.Crystalline and non-crystalline Fe oxides were removed using a sodium dithionite-citrate-bicarbonate (DCB) solution (Mehra and Jackson, 1958).The remaining phyllosilicate minerals, after the removal of Fe oxides, were analyzed in oriented and random powder clay minerals.The identification of 2:1 minerals present in the clay mineral fraction followed the methodology described by Whittig and Allardice (1986).
The degree of structural disorder of kaolinite of selected samples was determined by full-width at half-maximum (FWHM °2θ) for the position of the peaks 001 and 002 (Klug and Alexander, 1974), the R2 crystallinity index (Liétard, 1977), and the HB index (Hughes and Brown, 1979).Kaolinite standard samples obtained from the Source Clay Repository of the Clay Minerals Society at Purdue University, West Lafayette, USA, were used as references (Pruett and Webb, 1993;Moll Jr, 2001).The standards used were KGa 2, with a high structural disorder, and KGa 1b, with a low structural disorder, together with the KAm with the lower structural disorder from Amazônia, Brazil.
Thermogravimetric (TGA) analysis was conducted using a Shimadzu TGA 50 at the Embrapa Cerrado Research Center.The samples of the clay-size fraction (Fe-free) were placed in an alumina cell with a thermobalance under a constant nitrogen flow.Analysis was performed in the temperature range from 25 to 800 °C, at a heating rate of 10 °C per minute.Mass losses of gibbsite and kaolinite were identified individually at temperature ranges in which the dehydroxylation of these minerals commonly occurs (Mackenzie, 1982;Stucki et al., 1990).

Micromorphological analysis
Micromorphological investigations were performed on undisturbed samples of soil subsurface horizons.The thin sections were prepared according to Fitzpatrick (1984).
The recommendations of Stoops (2003) and Stoops et al. (2010) were used for micromorphological descriptions.Microstructures were described by the aggregates and porosity.Inside the aggregates or in the apedal microstructures, the coarse and fine materials of the groundmass were described.Finally, the pedofeatures were identified and characterized.A Zeiss ® Trinocular Optical Microscope (Axiophot model) with polarized light and an integrated digital camera was used for the descriptions.

Morphological and physical properties
Soil color along the topographic sequence varied from yellowish at the top and mid-slope (P1 and P2) to grayish at the foot-slope and the toe-slope under past (P3) and current (P4) hydromorphic conditions (Table 1).The P3 presented hydromorphic properties that occurred in the past, since it showed grayish colors, but no more saturation by water in any period currently.Therefore, these pale colors are related to the nature of the parent material (Santos et al., 2010), with low Fe contents of leucocratic granites (CPRM, 2007).The slope gradients to P1, P2, P3, and P4 showed values of 7, 22, 15, and 6 %, respectively (Table 1).
The terraces along the margins of the Alegre river basin display evidence of past hydromorphism processes, which resulted in efficient Fe mobility and losses (Table 1).Hence, grayish colors are common, and the soils are difficult to classify via the Brazilian System of Soil Classification (Santos et al., 2013), the Soil Taxonomy (Soil Survey Staff, 1999), and the FAO guidelines (WRB, 2015).The main difficulty is that the soil lacks sufficient plinthite to be classified as Plintossolo (Plinthic subgroup) and is not saturated by water to be classified as Gleissolo (Aquents) (Santos et al., 2013).Hence, based on its features, the proposed nomenclature for soil P3 was "Pseudogleysol" (Blume, 1988;Fanning and Fanning, 1989), although this term is not officially included in the soil classification systems.
Rev Bras Cienc Solo 2018;42:e0170291 Soil P4 was located in the lowest part of the floodplain, which is periodically water saturated, leading to the occurrence of Gleissolos (Aquents).Abundant mottling and soft plinthite are common in the horizon Cg, especially along root channels, where an aerobic rhizosphere environment allows the oxidation of Fe 2+ into Fe 3+ , corroborated by the positive reaction with α, α '-dipyridyl (Childs, 1981).
For soil P4, Gleissolo (Aquents), the texture was uniform (sandy clay loam) throughout the profile, whereas P3 (Pseudogleysol) showed a textural gradient between the sandy loam A horizon and the clayey C3 horizon (Table 1).
net negative charges, as inferred from the negative values of ΔpH , 1973).Contents of Ca 2+ , Mg 2+ , and K + in surface and subsurface horizons of all soil classes studied ranged from very low to low, according to the criteria defined by Ribeiro et al. (1999), except for the horizon A1 of soil P3.
The soils are dystrophic (V% <50), with a base sum below 0.5 cmol c kg -1 and low CEC, generally less than 7.0 cmol c kg -1 , resulting from the high degree of weathering and intense leaching (Table 2).Levels of Al 3+ were high, ranging from 1 to 2 cmol c kg -1 , but not high enough (<4.0 cmol c kg -1 ) to characterize them as aluminic (Santos et al., 2013).
The clay activity values below 27 cmol c kg -1 (CEC R ), observed in all soils (Table 2), indicate a dominance of low-activity clays (Tb), consistent with the results presented by the XRD patterns and the weathering index (Ki) (Donagema et al., 2011).
Organic carbon contents ranged from low to medium (Ribeiro et al., 1999) in the surface horizons (Table 2).Values of CEC T and H+Al were mostly dependent on organic carbon (OC), with a high positive correlation with OC values (Xu et al., 2016), since clays are of low activity.The available P content was very low (Ribeiro et al., 1999), ranging from 0.40 to 2.70 mg kg -1 .The remaining P levels were negatively correlated (r = 0.99, p<0.01) with the clay contents in all soils.The Ki and Kr ratios (Donagema et al., 2011) ranged from 0.86 to 1.48 and 0.64 to 1.43, respectively, for the subsurface horizons and from 0.94 to 1.43 and 0.70 to 1.34, respectively, for the surface horizons (Table 3).

Mineralogical properties
The clay fraction identified via the XRD pattern was composed by kaolinite [(Al , and traces of vermiculite and hydroxy-Al interlayered vermiculite (HIV) (Figures 3 and 4).The presence of hematite and goethite was confirmed in the clay fraction via interplanar spacing (d) of 0.413 nm and 0.265 nm (Figure 3), with a marked presence in upslope soils (P1 and P2), but absent in the bottom (P3 and P4) under current or past hydromorphism processes.These Fe oxides have very close reflections (0.265 nm), but with different hkl directions, and (130) of goethite and (104) to hematite (Figure 3).
Gibbsite was identified via interplanar spacing (d) of 0.485 and 0.437 nm (Figure 3) for the hkl directions 002 and 110 in all soils.This mineral indicates an intense weathering degree, which was confirmed by the low Ki ratio (Table 3).The TGA showed a much greater kaolinite content (Table 4), ranging from 75.4 to 94.42 dag kg -1 , whereas gibbsite accounted for 5.86 to 14.86 dag kg -1 in the soils (Table 4).
Presence of vermiculite and hydroxy-Al interlayered vermiculite (HIV) was confirmed by interplanar spacing (d) of 1.418 and 1.212 nm in the clay fraction (Figure 4).Further, treatments with Mg, Mg + Glycerol, K 25 °C, K 350 °C, and K 550 °C were performed in the clay-size fraction after Fe extraction (Whittig and Allardice, 1986).These minerals were only observed in the soils of the back-slope (P2) and the toe-slope (P3 and P4).Kaolinite crystallinity index proposed by Liétard (1977) and Hughes and Brown (1979) and full width at half maximum (FWHM) of the 001 and 002 directions of kaolinite (Klug and Alexander, 1974) suggested a high structural disorder in all profiles (Table 4).This observation was confirmed by the crystallinity indices for kaolinite standards with high (KGA 2) and low (KGA 1b and KAm) structural disorder (Table 4).Several other useful indices are presented in the literature (Hinckley, 1962;Plançon et al., 1988;Aparicio et al., 2006), but are not applied to kaolinite with a high structural disorder, such as those highly weathered soils investigated in the present study.Since these indices using the peaks of kaolinite in the directions 020, 110, 111, 021, which are not identified by XRD in kaolinite with the high structural disorder (Figure 5).

Micromorphological properties
Micromorphological properties varied both vertically and laterally along the topossequence, with varying degrees of microstructure development and micropedological features (Table 5).The upland Latosol (P1) has a typical isotropic micromass, with occasional anisotropic areas with weak pedality, consisting of composite sub-angular block peds, with strong medium granular at high magnification (Figure 6).The main micropedological features were channels and micro-galleries, currently filled with a secondary clay plasma, fecal pellets of termites ranging from 10 to 50 microns, and other indiscriminate excremental pellets of microarthropods.
In general, P3 and P4 showed evidence of strong coalescence of microaggregates (Figure 7), accompanied by Fe oxyhydrate removal by reducing conditions, leading to a collapsing effect of the structure, turning it virtually apedal and massive, forming a homogeneous dotted micromass almost completely leached and pallid.No weatherable minerals were found in the coarse sand, which is formed basically by poorly sorted quartz (sub-rounded to sub-angular), with rare fragments of oriented clay, similar to the welldrained upland soils P1 and P2.

DISCUSSION
In the Brazilian System of Soil Classification (Santos et al., 2013), P3 could be classified as Argissolo Acinzentado or Cambissolo, since it now occurs in an environment no longer saturated with water.Therefore, a better definition of the classification criteria for this soil class is necessary, maybe as a Gleissolo variation.The interpretation is difficult when the Fe-poor soil has been subjected to several environmental cycles, causing an overlap of gley and pseudogley features (Buurman, 1980;Blume, 1988).Since there are  no well-defined characteristics for the classification of these soils, Pseudogleysols are generally recognized as Gleissolos (Rubinić et al., 2014;Ndjigui et al., 2015), regardless of whether the hydromorphism process has occurred in the past or is currently occurring.Sometimes, these soils are recognized in the Soil Taxonomy (Soil Survey Staff, 1999) as the fragi great groups or as Albolls, Albaqualfs, Albaquults, Argialbolls, and Hapludults (Fanning and Fanning, 1989).However, there is no consensus regarding soils with such properties and their taxonomy in the Soil Taxonomy.
Pseudogleysols are soils that largely correlate with Stagnosols or Planosols, according to the FAO soil classification system (WRB, 2015) because of their stagnic properties and redoximorphic features.However, the P3 of the present study totally differed from the definition given by the FAO (WRB, 2015) since it has no more stagnic properties and the hydromorphism process has occurred in the past.This demonstrates the need for a better definition of the classification criteria for this soil class.Morphology described for the upland Latossolos (P1 and P2) is consistent with their advanced weathering degree, confirmed by the dominance of kaolinite, with minor gibbsite, typical for this type of composed block/granular aggregation in such soils (Ker, 1997;Ferreira et al., 1999;Nunes et al., 2001;Resende et al., 2002).Profiles P3 and P4 showed a massive structure with greyish colors in the subsurface (high value and low chroma), resulting from past or present hydromorphism processes (Table 1).Since P3 is no longer water-saturated, it showed a negative reaction in the alpha, alphadipyridyl field test for ferrous iron (Childs, 1981), highlighting efficient Fe-removal after drainage incision.
Textural gradient found in P3 is consistent with that reported by Fanning and Fanning (1989), who described primary and secondary "Pseudogleysols", depending on the genetic nature of the dense underlying horizon of these soils.The secondary type would have a thick clay layer formed by pedogenesis, representing a Bt horizon, while the primary one would have a dense layer inherited from the parent material, such as a dense shale bedrock underlying the soil.The P3 is a typical secondary "Pseudogleysol" with a texture gradient.Its position in the landscape, at the bottomland, favored clay translocation through leaching (Smeck and Runge, 1973;Blume, 1988;Fanning and Fanning, 1989;Zaidel'man, 2007;Rubinić et al., 2014).
Compared with previous studies of soils developed on similar basement rocks on the "Mares de Morros" region (Ab'Sáber, 1970;Corrêa, 1984;Lani et al., 2001;Nunes et al., 2001;Santos et al., 2010), in our study, the Al 3+ contents were greater and similar to values reported by Gomes (1976) from southern Espírito Santo.These results are explained by the leucocratic nature of the rock and the hydrolysis of minerals such as kaolinite, identified in the XRD patterns, favoring desalination and intense weathering (Loughnan, 1969;White and Buss, 2014).
The low TOC content of the samples with low-activity clay explains the positive correlation with CEC, mainly since the negative charges of organic matter derived from dissociation of carboxylic and phenolic H compounds at high pH values.Thus, soils with high organic carbon have a high CEC at pH 7.0; under the natural acid condition, they have a low CECe (Parfitt et al., 1995;Ciotta et al., 2003).
Extremely low P content is consistent with the nature of the leucogranite (CPRM 2007), corroborating previous studies (Ker, 1997;Lani et al., 2001;Santos et al., 2010).Higher values in the surface horizons are due to organic matter, reducing P adsorption sites in the mineral fraction and increasing P availability (Mesquita Filho and Torrent, 1993;Guppy et al., 2005).
The Ki and Kr ratios were low (Table 3).Similar values have been reported for soils from southern Espírito Santo (Gomes, 1976;Lani et al., 2001), indicating a kaolinitic/oxidic mineralogical composition, typical of highly weathered soils on acid rocks (Ker, 1997;Melo et al., 2001).Soil P3 and P4 showed low levels of Fe 2 O 3 due to current (P4) and past (P3) poor drainage.Although the Pseudogleysol is no longer subjected to waterlogging, its shows evidence of past hydromorphism, with inherited past greyish colors (Kampf and Curi, 2000).The Al 2 O 3 /Fe 2 O 3 molecular ratio was greater than 1.80 (Table 3), indicating low contents of Fe oxides in all soils and high Al contents in the parent material (CPRM, 2007).
The Fe 2 O 3 content extracted by DCB (Fed) was consistent with the sulfuric acid data and decreased from the top to the bottom of the topossequence, highlighting that a changing Rev Bras Cienc Solo 2018;42:e0170291 soil moisture regime can lead to Fe losses downslope.These results are consistent with previous studies of topography influence on soil properties (Vidal-Torrado et al., 1999;Silva et al., 2001;Ghidin et al., 2006).
Ratio of Fe d /Fe s can be a proxy for the soil development degree, being higher in weathered soils (Pereira and Anjos, 1999;Cornell and Schwertmann, 2003).It showed a negative correlation with the Fe o /Fe d ratio (r = 0.70, p<0.01), since Fe-oxides are good pedogenetic indicators (Kampf and Curi, 2000).The Fe o /Fe d ratio decreased with depth for P3 and P4, which can be explained by the resistance of Al-rich goethites to dissolution upon reduction (Torrent et al., 1987).Also, part of the Fe can be assigned to Fe in kaolinite or as Fe-oxides in the coarse fractions (sand and silt), determined by sulfuric acid digestion (Resende et al., 2011).
Iron contents extracted by Fe o were higher in the surface horizons, indicating the inhibitory effect to Fe-oxide crystallinity exerted by soil organic matter (Schwertmann and Taylor, 1989).In addition, the increasing trend in Fe o /Fe d along the topossequence corroborates the decreasing amount of crystalline Fe oxides (P1 > P2 > P3 > P4) (Table 3), suggesting that increasing hydromorphism downslope accounts for higher contents of low-crystalline Fe oxides, such as ferrihydrite (Vidal-Torrado et al., 1999;Silva et al., 2001;Park and Burt, 2002;Ghidin et al., 2006).
The Fe o /Fe d ratio was lower in P3 (Pseudogleysol) compared to P4 (Gleissolo), showing that the signs of past hydromorphism process are retained in the well-drained terraces (Table 3).Since the Gleissolo is subject to redox conditions that lead to the formation of low-crystallinity and/or amorphous Fe-phases (Schwertmann, 1985;McBride, 1994), these values decreased towards the soil bottom, near the water table (Table 3).
Compared to P2, the well-drained P1 soil at the summit has a more uniform water retention profile and greater moisture levels, suggested by yellowish colors, indicating the dominance of goethite (Curi and Franzmeier, 1987;Kampf and Curi, 2000;Corrêa et al., 2008a).This evidence may be noticed by the increased intensity relative to the directions 110 and 130 of goethite (Figure 3) and more yellowish colors in P1 (7.5YR) than P2 (Table 1) in all diagnostic horizons evaluated.
The formation of hematite and goethite is consistent with humid climate conditions and low amounts of Fe in the parent material (leucogranite), as indicated by Fes and Fed values (Table 3).The Feo/Fed ratio and Kr (Table 3) along the topographic gradient were lower in soils at the summit (P1) in relation to the soil of the back-slope (P2), so that poorly crystallized Fe oxides increased downslope (Schwertmann and Kämpf, 1983;Curi and Franzmeier, 1987;Kampf and Curi, 2000), as corroborated by the lower intensities of hematite and goethite peaks in P2 relative to P1 (Figure 3).The Fed/Fes ratio was higher than 0.69 in all horizons of P1 and P2, corroborating the deep weathered nature of these soils (Schwertmann and Kämpf, 1983;Cunha et al., 2005).In both soils, with evidence of current or past hydromorphism (P3 and P4), the Fe d /Fe s ratio decreased with depth (Table 3).
On the other hand, the presence of 2:1 minerals (vermiculite/HIV) in the lower parts of the landscape indicates moderate drainage, maintaining a high Si activity in the soil solution (Dixon, 1989), despite the low Ki values (Table 3 ).Also, these 2:1 minerals (vermiculite and HIV) can be formed via chemical weathering of feldspars in the silt fraction, as suggested by Vidal-Torrado et al. (1999 a,b).The occurrence of vermiculite and HIV in highly weathered soils has also been observed by Gomes (1976) and Lani et al. (2001) in soils from southern Espírito Santo as well as from elsewhere in Brazil (Moura Filho, 1970;Alleoni and Camargo, 1995).
Increasing kaolinite and decreasing gibbsite (Table 4) downslope the topographic gradient are controlled by local soil drainage (Curi and Franzmeier, 1987;Kampf and Curi, 2000;Corrêa et al., 2008 a,b).Higher leaching at the upland soil (P1) reduces Si activity in the solution, leading to the formation of gibbsite (Norfleet et al., 1993;Sommer et al., 2006;Buol et al., 2011), whereas some Si mobility and accumulation in the lower parts of the topossequence helps the maintenance, or neoformation, of kaolinite (Curi and Franzmeier, 1987;Dixon, 1989).The TGA analysis indicated its abundance at the summit and back-slope profiles -with 83.78 and 75.40 dag kg -1 , respectively (Table 4).Soils at lower parts of the landscape (P3 and P4) had contents of up to 94.42 dag kg -1 , due to the low Fe oxides contents caused by hydromorphism in this sector.This fact is corroborated by the high values of the Feo/Fed ratio at P3 and P4, indicating the formation of low-crystallinity Fe oxides (Kampf and Curi, 2000).
Evaluating the HB index (Hughes and Brown, 1979) along the topographic gradient, decreasing values, P1 < P2 < P3 < P4 were observed (Table 4), indicating that the structural disorder of kaolinite decreases towards the bottom.If we compare this with standards values, these range from 28.7 to KGA 2 with the high structural disorder to very high values, symbolized by the infinity symbol (Table 4), for KGA 1b and KAm having a lower structural disorder.Therefore, the lower crystallinity index indicates materials with a high structural disorder.The values reported are consistent with those observed by several other authors (Ker, 1995;Melo et al., 2002;Corrêa et al., 2008b) in kaolinites of Latossolos related of Brazilian soils, as well as by Hughes and Brown (1979) and Singh and Gilkes (1992) for humid tropical regions worldwide.
Comparing the R2 crystallinity index (Liétard, 1977) and FWHM (Klug and Alexander, 1974) (Table 4) with the values obtained from kaolinite standards, all soils contained kaolinites with a high structural disorder (Figure 5).The HB index and the FWHM were highly correlated (r = 0.95; p<0.01), indicating a reduction in structural disorder downwards the topossequence (Table 4).This correlation was not observed for the R2 index, indicating the low sensitivity of this index for kaolinite from deeply weathered soils, with a high degree of structural disorder.
The HB index was negatively correlated with of Fe s (r = 0.91, p<0.01) and Fe d amounts r = 0.92, p<0.01) and positively with the FWHM (r = 0.84 and 0.86, p<0.01).This suggests that higher Fe contents are associated with an increasing structural disorder of kaolinite (Mestdagh et al., 1980;Fysh, 1983;Brindley and Kao, 1986;Melo et al., 2002;Iriarte et al., 2005;Ghidin et al., 2006).This in further corroborated by the Fe d contents in standard kaolinite (KGa 2, KGa 1b, and Kam equal to 0.12, 0.03, and 0.02 dag kg -1 , respectively), with a progressively greater structural disorder with increasing Fe d amounts (Table 4); this finding agrees with the previous observation of Singh and Gilkes (1992) in Australian soils.
Structural disorder of kaolinite showed no correlation with soil chemical and physical properties.This contrasts with reports of Murray and Lyons (1959) and (Singh and Gilkes, 1992), who showed higher CEC for kaolinites with a higher structural disorder.This may be explained by methodological differences for CEC determination; previous studies used the clay fraction for CEC determination, whereas in our study, the CEC was calculated for the <2 mm soil (fine earth).
Maximum dehydroxylation temperature of kaolinite (Table 4) for all soils was positively correlated with the HB crystallinity index (r = 0.98, p<0.01) and negatively with the FWHM (r = 0.97, p<0.01).This fact is attributed to the small crystal size and the high structural disorder of kaolinite (Melo et al., 2001;Ghidin et al., 2006), consistent with Rev Bras Cienc Solo 2018;42:e0170291 negative correlations with Fe s (r = 0.97; p<0.01) and Fe d (r = 0.99, p<0.01).The presence of structural Fe promotes the structural disorder and reduces the crystal size, resulting in a lower temperature for kaolinite dehydroxylation (Singh and Gilkes, 1992) because the thermal stability of Fe-structural OH groups is lower than that of the Al-OH groups in the octahedral sheet of kaolinite.
The described microstructure agrees with the morphological description of the structure at field scale and represents the common micropedological syndrome of most Latossolos Vermelho-Amarelo developed on crystalline rocks of the "Mares de Morros" landscape in southeastern Brazil (Corrêa, 1984;Carvalho Filho, 1989;Nunes et al., 2001;Schaefer, 2001).Microped structure, the b-fabric, and most pedofeatures are fully consistent with the "oxic syndrome" of Buol and Eswaran (1978), further corroborated by Stoops and Buol (1985), suggesting a polycyclic genesis on colluvial parent material (Muggler and Buurman, 1997).
In subsurface Bw horizons of P1 and P2 (Figure 6), the pedality degree was enhanced, with a strong medium-sized granular microstructure, typical of B horizons of Brazilian Latossolos.Recently, different studies have investigated the origin of the granular structure of Latossolos, either emphasizing a basic physicochemical interaction between kaolinite and Fe and Al oxi-hydroxides under extreme Si leaching conditions (Eswaran and Daud, 1980;Santos et al., 1989) or intense, long-term biological activity, especially by termites (Rezende, 1980;Schaefer, 2001).The presence of abundant termite fecal pellets with high porosity, contributing to the intense desilication of these soils, suggest microstructure formation, as reported by Lani et al. (2001) in soils from southern Espírito Santo.
In both Bw horizons, minute Fe concretions, charcoal particles, reddish hematite fragments, and fecal pellets were dispersed in the clay micromass, where easily weatherable primary minerals were completely absent.Titanium minerals (ilmenite), gibbsite nodules, Fe-impregnated lithorelics, and abundant biological channels were observed in the Bw2 horizon of P2.All features above support the observed desilication of these upland soils, favoring the formation of gibbsite (Hsu, 1989).
The A2 and C2 horizons of P3 and A and Cg2 of P4 (Figure 7) had a coarse fraction dominated by fractured quartz grains, without Fe-coatings or infillings along fractures and with many poorly connected voids and weak to absent pedality.The single grain structure at P2 is associated with the complete absence of easily weatherable primary minerals, with rare charcoal micro-fragments and ferruginized roots remaining.A general bleaching process was clearly observed in the two profiles (P3 and P4), forming depleting, reducing zones with a greyish clayey micromass (10YR) at the pedoplasmation front, where the soil forming process is active (Stoops and Schaefer, 2010).This corresponds to a monic (c/f) related distribution pattern.
Pedogenesis under hydromorphic conditions at P3 and P4 profiles has led to the massive and compact structure at all scales.This is consistent with the macro-morphological description at the field scale, as postulated by previous studies (Curmi et al., 1993a,b;1993b;Vepraskas et al., 1993).

CONCLUSIONS
Soils from the hilly Atlantic Forest zone of the Alegre river basin developed from leucogranites indicate that they were formed from homogeneous pre-weathered saprolite.Soils on past or present floodplain/terraces are, despite the depositional environment, highly weathered regardless of past or present hydromorphism.This highlights that erosion and sedimentation acted upon a deeply weathered regolith mantle, with limited contribution of fresh, little-weathered materials, so that Quaternary floodplains are also nutrient-depleted, having a similar source.
Clay mineralogy is composed of mixed kaolinite (dominant) and gibbsite in all soils, with minor contents of Fe oxides (hematite and goethite), only present in the upland Latossolos (Acrudox) (P1 and P2), controlled by the dynamics of Si and Al in the system.Traces of 2:1 minerals (vermiculite and HIV) were identified in the back-slope (P2) and low-lying soils at the toe-slope/lower part of the landscape (P3 and P4), possibly due to resilication downslope.
Kaolinite of all soils had a high structural disorder, influenced by the weathering degree conditions of this tropical environment, neoformation, and the possible presence of structural Fe, to be confirmed by further studies.
The micromorphological properties indicate a strong composite micro-granular (microped) microstructure in a weak sub-angular blocky structure, with no evidences of illuviation, at the well-drained upland Latossolo (Oxisol) domain.It changes into the lowland "hydromorphic" domain, with redoximorphic degradation features such as Fe-impregnating hypocoatings, clay/Fe depletions (bleached zones, albans), and reduced matrices (low chroma).Hence, the "oxic" (Latosolic) microped structure is not related to changes in clay mineralogy and closely associated with aged soils of well-drained slopes.
The presence of well-preserved, inactive redoximorphic features in the Pseudogleysol, now well-drained, suggests that paleogleying is not reverted to a re-oxidized matrix when the soil parent material has low Fe contents.Destruction or coalescence of microaggregates are associated with this intense Fe removal, corroborating a degradation process leading to the degradation and collapse of the "Latosolic" microstructure and resulting in massive structure development.

Figure 1 .
Figure 1.Location of the Alegre river basin, Espírito Santo State, Brazil.

Figure 2 .
Figure 2. Topographic profile of the topossequence drawn from the digital elevation model and soil profile pictures.

Figure 6 .Figure 7 .
Figure 6.Optical microscope photomicrographs (OMP) in parallel (left) and crossed polarized light (right) of the P1 profile, highlighting: A horizon = sub-angular blocky microstructure with groundmass composed by angular/sub-angular quartz and opaque grains (coarse materials) and brownish yellow micromass with undifferentiated or weak speckled b-fabric; Bw1 horizon = granular microstructure with enaulic relative distribution.The coarse materials of the groundmass are composed of angular/subangular quartz, hematite, and opaque grains, and the micromass is brownish yellow with a circular or crescent striated b-fabric; Bw2 horizon = medium granular and vughy microstructure, with the same coarse materials and micromass of the Bw1 horizon.The presence of large charcoal fragments inter-aggregates and very small fragments inside them is common.Qz = quartz; V = vughy; cpv = complex packing voids; hg = hematite grain; op = opaque grain; ch = channel.Some granular aggregates of Bw (1 and 2) horizon are highlighted with a dashed yellow line.

Table 1 .
Physical and morphological characterization of soils

Table 2 .
Chemical characterization of soils