Introduction

The soil is the product of the combinations of factors and formation processes determined by local conditions (Dias et al. 2016). The intensity of these factors and processes promotes progressive transformations in the parent material, which over time are expressed on the morphological, chemical, physical, and mineralogical properties of the soil (Anjos et al. 2007; Miguel et al. 2013; Silva et al. 2020). One of the main factors that influence the diversity and characteristics of Amazonian soils is the rapid weathering of the mineral material by the intense climate of the region, characterized by an extreme water regime and high temperatures (Dalamerlinda et al. 2017). Therefore, identifying the interaction between these factors and their role in soil formation is necessary (Campos et al. 2012a; Silva et al. 2020).

Soil position in the landscape reveals its formation history (Campos et al. 2011; Rodrigues et al. 2016; Capoane et al. 2017). Thus, the landscape reflects the soil underneath it, which, in turn, determines landscape features and patterns (Minasny and Mcbratney 2006). In general, the soil inherits the characteristics of its parent material. Nevertheless, soil attributes might variate within a local scale as a reflection of rock diversity (Silva et al. 2020) and differences along the toposequence. These determine the specific soil formation processes that occur, and consequently, soil development (Campos et al. 2012a; Braga et al. 2019; Tunçay et al. 2020). Finally, the relief also acts directly on the factors that influence soil characteristics, such as water dynamics and pedogenic processes, and is, therefore, determinant for the variability of soil attributes (Bockheim et al. 2005; Vasconcelos et al. 2012; Li et al. 2015).

To better understand soil distribution in the landscape, a good comprehension of the concepts behind the soil–landscape relationship is crucial for the interpretation of the spatiotemporal variability of soil attributes and the visualization of dynamic processes such as the transport of water, solutes, and sediments (Sommer 2006; Campos et al. 2011). Despite the marked representation of the Middle Madeira River, the predominant physiographic environments are floodplains, native fields, and transition areas, dominated by Cambissolos, Argissolos, and Entisols. These variations might have a local scale, since in this exuberant landscape, different parent materials originate variations in soil classes and their respective mineralogical, morphological, chemical, and physical properties (Campos et al. 2012a; b).

Several authors have studied the direct effect of the parent material composition on soil formation. Montanari et al. (2010) verified higher contents of goethite and kaolinite in a concave landform compared to linear and convex landforms. On the other hand, the authors also confirmed higher hematite contents in linear landforms. In a toposequence of Oxisols, Curi and Franzmeier (1984) found higher gibbsite concentration in the higher landscape positions. In the lower landscape positions, the authors reported the predominance of kaolinite associated with goethite of low mean crystal diameter. These results reinforce the importance of characterizing soil mineralogy to comprehend the relationship between landscape and parent material. The latter is especially valid for Fe and Al oxides, influenced by the environmental conditions and characterized by their long-term persistence in the soil (Kämpf and Curi 2000).

The diffusion of geotechnologies to elaborate pedological maps in semi-detailed or detailed scales (Lacerda and Barbosa 2012) using soil distribution models that consider the triple interaction between landscape, soil, and parent material (Campos et al. 2007) represents an alternative to be applied. However, implementing these techniques in the Amazon is a big challenge given the territorial extension of the region. Nevertheless, previous studies indicate various characteristics and peculiarities of the Amazonian landscape and have revealed new directions for research regarding the soil–landscape relationship.

It is, therefore, essential to know the soil within the soil–landscape context, to better define its use suitability and limitations (Pedron et al. 2004; Carvalho Filho et al. 2010; Silva et al. 2020). This information can contribute to the strategic planning of land use, mitigating the risks of environmental impacts, especially within the State of Amazonas, which is globally significant. The present research aimed to study the soil–landscape relationship in a sandstone-gneiss topolithosequence and the factors determining soil diversification in the State of Amazonas, Brazil.

Materials and methods

Characterization of the physical environment

The study area is located 150 km from the beginning of the Transamazonic Highway, municipality of Manicoré, State of Amazonas (08°08′12.6″ S and 61°50′06.5″ W). It belongs to the hydrographic basin of Amazonas (Fig. 1). The collection of soil samples occurred between April and May, during the autumn, which is regionally known as the Amazonian winter. According to the Köppen classification (Alvares et al. 2013), the regional climate is Am or rainy tropical (monsoon rains) and has a short dry period. The temperature varies from 25 to 27 °C, and the average annual precipitation is 2.500 mm. The rainy season begins in October and extends until June, with the relative air humidity ranging between 85 and 90%.

Fig. 1
figure 1

Location of the profiles studied in the municipality of Manicoré in the state of Amazonas, Brazil

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The relief characterizes by the presence of plateaus in the highest parts. These have flat topographic surfaces and borders marked by aligned hills and ridges. On the other hand, pediplains occupy the lower areas (CPRM 2001). Regarding the geology, the study area extends over Rondonian Granites characterized by the presence of muscovite, biotite, adamellites, and granodiorites of intrusive cratogenic origin, in the form of stocks and batholiths (Brasil 1978; Melo et al. 2018). According to the ZEE (2008) and Campos et al. (2011), the regional soils are Oxisols. The vegetation in the region is dense, dominated by medium and large trees with a height of up to 50 m (Fig. 2).

Fig. 2
figure 2

Images of the collection environment. A–Savanna vegetation; B–Forest vegetation; C–Cerrado vegetation

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The study extended along a transect of 9253 m, from the top of the relief towards the deposition foothill. Measurements of the altitudes were carried out along this transect to make the altimetric profile. Hillside segments were identified based on the model of Dalrymple et al. (1968) using the variation of the terrain slope (Fig. 3).

Fig. 3
figure 3

Schematic profile of the terrain topography and soils in the sandstone-gneiss topolithosequence in the Amazonas state, Brazil. Cambissolo Háplico (CXvd), Neossolo Regolítico (RRd), Neossolo Quartzarênico (RQo), Plintosolo Pétrico (FFc) and Agissolo Amarelo (PAd) in the Amazonas state, Brazil

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Five profiles were selected and opened, considering topographic and vegetation variations along the toposequence (Fig. 3). Soil characterization was as follows: P1 = top (08°08′46.0″ S and 61°49′25.1″ W); P2 = colluvial footslope (08°08′41.8″ S and 61°49′33.9″ W); P3 = convex creep slope (08°08′18.3″ S and 61°49′56.3″ W); P4 = transport foothill (08°07′59.6″ S and 61°50′17.4″ W); and P5 = deposition foothill (08°07′59.6″ S and 61°50′17.4″ W). The identification, morphological description, and sampling of the horizons within the soil profiles followed the methodology proposed by Santos et al. (2018). Soil classification followed the criteria established by the Brazilian Soil Classification System (Santos et al. 2018), the World Reference Base of Soils (IUSS Working Group WRB 2015), and Soil Taxonomy (2014).

Particle size analysis

The pipette method was used to determine particle size in soil samples through chemical and mechanical dispersion with a 0.1 mol L−1 NaOH solution and low rotation mechanical stirring for 16 h, respectively (Donagema et al. 2017). The clay fraction was separated by gravitational sedimentation, the coarse and fine sand fraction by sieving, and the silt fraction was calculated by difference.

Chemical analyses

The total iron content (Fet) of the clay fraction was determined by selectively dissolving the soil in sulfuric acid (H2SO4) in a 1:1 ratio. The latter is a classic procedure in the taxonomy of Brazilian soils in which tropical climate conditions induce Fe concentration in the clay fraction (Donagema et al. 2011). Crystalline free iron (Fed) was extracted with sodium dithionite–citrate–bicarbonate (DCB) at 25 °C for 16 h and determined according to Mehra and Jackson (1960). Finally, poorly crystalline iron oxides (Feo) were extracted with ammonium oxalic acid and quantified according to the methodology proposed by McKeague and Day (1966).

Soil pH was determined in water and KCl (1.0 mol L−1), the latter with a KCl to soil solution ratio of 1:2.5 (Donagema et al. 2017). The exchangeable cations Ca2+, Mg2+, and Al3+ were extracted with 1.0 mol L−1 KCl and measured by atomic absorption spectroscopy. The hydrogen ion and Al3+ were extracted with 0.5 mol L−1 calcium acetate at pH 7.0 and determined by titration (0.025 mol L−1 NaOH) (Donagema et al. 2017). Organic carbon was determined through the wet oxidation method (Walkley and Black, 1934).

The anion exchange resin (AER) was used to extract and quantify bioavailable particulate phosphorus (Pbp). The principle of the AER is the continuous P removal from the solution through exchange with bicarbonate from the resin, creating a concentration gradient that forces P release from the surface of the colloids until an electrochemical equilibrium between the soil and the AER is reached (Skogley and Dobermann 1996).

Bioavailable particulate P extraction followed the methodology developed by Kroth (1998). For this, 0.5 g of soil (1 mm) was added in 15 mL falcon tubes containing 10 mL of distilled water and a sheet of AER saturated with 0.5 mol L−1 NaHCO3. These tubes were stirred for 16 h on an end-over-end stirrer (33 rpm) followed by the sheet removal and washing in distilled water and subsequent dilution in 10 mL of 0.5 mol L−1 HCl. The tubes remained uncapped for 90 min and then closed and stirred again for 30 min in a horizontal stirrer. Subsequently, a 3 mL aliquot was taken from the extract to determine P content (Murphy and Riley 1962). The sum of bases (SB), the cation exchange capacity (CEC), and base (V) and aluminum (m) saturation were calculated based on the results of the chemical analyses.

Clay mineralogical analysis

The powder X-ray diffraction method served to characterize the minerals of the clay fraction: hematite (Hm), goethite (Gt), kaolinite (Kt), and gibbsite (Gb). The latter required concentrating the iron oxides by boiling the clay fraction in NaOH (Norrish and Taylor 1961) followed by derrification (Mehra and Jackson 1960). Sample diffraction had a scanning speed of 1° 2θ min−1 using a Mini-Flex Rigaku II (20 mA, 30 kV) equipped with Cu Kα radiation. The Hm/(Gt + Hm) ratio was estimated by comparing the Hm/(Gt + Hm) peak areas with the proportions obtained from the standard Gt–Hm mixtures. The percentage of Hm and Gt in the clay fraction was expressed as the difference between free (Fed) and poorly crystalline iron (Feo).

The peak areas of Gb (002) and Kt (001) reflections were used to calculate the Kt/(Kt + Gb) ratio. The calculus of the content of isomorphic substitution of iron by aluminum in Gt used the equation mol mol−1 = 1730–572 c (Schulze 1984). The content of isomorphic substitution of iron by aluminum in Hm was calculated through the equation mol mol−1 = 3098.8–615.12 a0 (Schwertmann et al. 1989). The specific surface area (SSA) of Gt was estimated by the formula SSA (Gt) = (1049/MCD100−5)) (m2 g−1) (Schulze and Schwertmann 1984), MCD100 = MCD110 × 0.42 nm (Kämpf 1981), and the SSA of Hm was estimated by the formula SSA (Hm) = 2 × (r + h) × d (m2 g−1) (Schwertmann and Kämpf 1985). The mean crystal diameter (MCD) of Hm and Gt was calculated from the width at half-height (WHH) and the position of mineral reflections using the Scherrer equation (Klug and Alexander 1974).

Results

Soil distribution in the landscape

The area had a predominately plain physiognomy with minimal relief variations, as typical of Amazonian environments where a gradual field to forest transition occurs. The lowest position of the terrain characterizes by seasonal flooding, and the dominant vegetation consists of small shrub species. Dalrymple et al. (1968) established hypothetical hillside units, which may be partially absent or repeated. In this study, five hillside segments were identified and mapped as a representative toposequence of the region (Fig. 3).

The first three landscape segments–top, colluvial footslope, and convex creep slope–are close to each other and range from 120 to 125 m of elevation. According to Silva et al. (2020), the steep slope of the landscape favors the movement and transport of sediments and water, conditioning the instability of soil attributes and, therefore, originating different soil classes within the same area. The transport foothill represents the third segment. It has 119 m of altitude and might have formed by relief microvariations of the previous section. Its residual erosional character extends from the edges of the lower third until the fifth segment. The fifth segment or deposition foothill varies from 114 to 116 m of altitude, has different topographic characteristics, and can be considered a stable geomorphic environment.

The identification of five different soil classes along the studied segments of the landscape indicates the presence of more than one parent material during pedogenesis. The diverse sandstone-gneiss rocks from which these soils originated suggest the parent material as the principal source of soil attribute differentiation. Knowing the different parent materials in a landscape helps understand the variability of soil attributes, confirming that the geology explains the local relief and the soil patterns (Marques Jr and Lepsch 2000; Campos 2012c).

Morphological and textural attributes

According to the genesis and classification of the soils, all the profiles had a moderate A surface diagnostic horizon (Table 1). Given the topographic position of the top segment (P1), its soil had an incipient B diagnostic subsurface horizon, with no evidence of a predominant pedogenic process. The P1 soil was classified as CAMBISSOLO HAPLICO Ta Distrófico típico ((Ochric, Arenic) Dystric Leptic Cambisol).

Table 1 Morphological and granulometric attributes of a sandstone-gneiss topolithosequence in the Amazonas state, Brazil
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Soils of the colluvial footslope (P2) and convex creep slope (P3) were poorly evolved, formed by mineral or organic material, with less than 20 cm of thickness, and lacking a diagnostic B horizon. In the P2 segment, the soil was classified as NEOSSOLO REGOLÍTICO ((Ochric, Arenic, Dystric) Cambic, Lithic, Leptosol), given its lytic contact (depth > 50 cm) and the occurrence of an A horizon over a C or Cr horizon. The soil of the P2 segment had altered primary minerals, with gravels derived from the ACr horizon (12–35 cm) and the presence of semi-weathered rock fragments (Santos et al. 2013). In the P3 segment, the soil was classified as NEOSSOLO QUARTZRÊNICO ((Ochric, Arenic) Dystric Regosol) with no lytic contact within the first 50 cm of depth and an A to C sequence of horizons. The texture was sand to loamy sand in all horizons up to a depth of 150 cm, mainly consisting of quartz with coarse and fine sand fractions of whitish colors (Santos et al. 2018).

Seasonal variations of the water table had a marked influence on the soil of the transport foothill (P4), which, therefore, presented small and contrasting reddish mottles in the subsurface horizons. In association with these mottles, plinthite and petroplinthite were also present in amounts varying from common to abundant, which numerically corresponds to 15–40% of the soil volume. These attributes indicate the occurrence of the pedogenic process of plinthization and represent the diagnostic character of the plinthic horizon, according to the Brazilian Soil Classification System (SiBCS) (Santos et al. 2013). From the attributes and horizons identified, the profile was classified, at the suborder level, as PLINTOSSOLO PÉTRICO Concrecionário típico ((Ochric, Dystric, Loamy) Petric, Plinthosol).

Finally, the soil profile opened in the deposition foothill (P5) was classified as ARGISSOLO AMARELO Distrófico abrúptico (Ochric, Hyperdystric, Clayic) Chromic, Abruptic, Acrisol), presenting a textural B horizon dominated by colors with hues 10 YR and 7.5 YR in the first 100 cm. In this soil, the slope was determinant for the selective removal of clay, contributing to the formation of a textural gradient. In summary, the mineralogical, physical, and chemical attributes of the soils along the topolithosequence accurately elucidated the characteristics of the sandstone-gneiss rocks from which they originated.

Soil color in the surface horizons ranged from very dark grayish brown to dark brown (Table 1). However, subsurface horizons had a marked differentiation, varying from yellowish-brown to very pale brown. The grayish tones of most horizons are due to the low concentration of iron oxides and the predominance in the sand fraction of light-colored minerals, such as kaolinite and quartz (Santos et al. 2018). The results confirm this pattern and the predominance of goethite in all the soils, including the Plinthosol with higher hematite contents (Table 3).

Considering the A horizon, the structure of the P1 and P2 profiles was weak, small, granular, with a sandy texture conditioned by the sandstone, which, in turn, obstructs particle bonding (Santos et al. 2018). On the other hand, the gneiss-derived soils had a higher structural variation, with the P3 classified as weak, small to medium, granular, the P4 as a moderate, medium to large, granular, and the P5 as weak, small to large, granular. The increased variability of the A horizon in the P3, P4, and P5 soils revealed strong landscape influence with a low subsurface structural variation. A possible explanation relies on the dynamism or sediment transport and movement in the convex creep slope, transport foothill, and deposition foothill. The steep slope of the landscape contributes to differentiate soil structure, especially in the superficial horizons (Bernini et al. 2013).

According to Silva et al. (2001), the topographic position conditions soil drainage and is, therefore, strongly related to the type of structure. Moderately to well-drained soils tend to have granular or angular and subangular blocky structures, while poorly to imperfectly drained soils generally exhibit a standard structure in polygonal prisms. Relief conditions also influence several soil attributes since the slope creates complex transport patterns of water and solutes, which act on soil profile development (Campos et al. 2012a).

Except for the Neossolos, which had a sandy constitution, all profiles had surface horizons with a sandy loam texture (Table 1). The sandstone parent material from the Palmeiral Formation and the Nova Campo Verde Complex explains the dominance of the sand fraction in these soils. However, attention should be given to the increased clay content in the Argissolo within the Bt1–Bt2–Btf horizon. As verified by Campos et al. (2011) and confirmed in the present study, the Argissolo and the Plintossolo, given their position in the landscape, might have undergone a rejuvenation process, with probable clay loss from their surface horizons and consequent abrupt textural change.

Along the toposequence, morphological differences between the soil profiles indicated the effect of diverse pedogenic factors and processes and confirmed the soil–landscape relationship. According to Ribeiro et al. (2012), the constitution and formation conditions of the soil determine its morphology. Therefore, soil morphology represents a good indicator of previous pedogenic processes and environmental conditions and allows inferring and interpreting soil and plant responses to management practices.

Regarding particle size composition, sand predominated over the other fractions, ranging from 207 to 891 g kg−1 (Table 1). The high quartz contents of the parent material (sandstone) might explain these results. High total sand contents were also reported by Schiavo et al. (2010), who characterized and classified soils developed from sandstones of the Aquidauana Formation.

Silt contents in the A horizon ranged from 53 to 306 g kg−1, increasing with soil depth (Table 1). Young soils, which are still undergoing formation processes, tend to have low silt contents. Since the studied soils formed from alluvial sediments, particle selection may have occurred, which, in turn, explains the presence of lithogenic (more resistant to changes) and pedogenic materials (with a higher degree of crystallinity). Different from the sand fraction, the clay fraction increased gradually with soil depth and ranged from 48 to 558 g kg−1. Nevertheless, the latter cannot be attributed to the occurrence of the pedogenic process of eluviation and illuviation and might be more likely given by selective removal from surface horizons (sandier), with clay accumulation in the subsurface.

The Neossolos presented higher proportions of coarse fractions (> 2 mm) with consolidated ferruginous material (plinthite and petroplinthite). Decreased levels of the water table might have enhanced the drainage, as suggested by the presence, within the first 48 cm of the soil, of a more reddish hue (2.5 YR) than in the other profiles (Table 1). The predominance of coarse sand fractions combined with the remarkable presence of gravels indicates that the weathering processes in this soil did not promote a significant fragmentation of these fractions (Lima et al. 2008).

In the transport foothill (P4), clay contents ranged from 95 to 99 g kg−1 in the A, AB, and BA horizons, reaching values of up to 548 g kg−1, 558 g kg−1, and 567 g kg−1 in the Bt1, Bt2, and Btf horizons, respectively. The marked textural differentiation between the A and Bt1 horizons characterizes an abrupt textural change with a significant increase of clay compared to the overlying horizons (Table 1).

At the top (P1), colluvial footslope (P2) and convex creep slope (P3), the silt-to-clay ratio (S/C) ranged from 1.43–1.50, 1.19–1.43, and 0.94–1.86, respectively (Table 1), associated with the lower development of these soils. The transport foothill (P4) had the lowest S/C ratio, with values between 0.7 and 0.8. On the contrary, the deposition foothill presented the highest ratios, ranging from 1.49 in the surface horizons to 0.41 in the deeper ones. According to Campos et al. (2011), higher S/C values are due to clay loss or slight increases in the silt fraction, suggesting that minor relief variations provide relative losses or gains and, probably not given by changes in the parent material.

Clay and silt contents tended to increase towards the youngest geomorphic surfaces, this is, from the top (P1) towards the transport (P4) and deposition foothills (P5). The opposite direction of this pattern compared to that of the total sand reflects the recent sedimentary nature of these fractions, since the soils of these geomorphic environments are closely related to the parent material. Similar results were reported by Campos et al. (2012b) when studying the soil-geomorphic surface relationships of a floodplain-upland toposequence in the region of Humaitá, Amazonas, Brazil.

Variations of chemical attributes

The values of pH ranged from 4.17 to 5.48 (H2O) and 3.81 to 5.70 (KCl), especially in the top (P1) and deposition foothill (P5), presenting the highest acidity content in the surface horizon. According to Campos et al. (2012b), the high regional precipitation contributes significantly to base leaching, increasing soil pH. The balance of negative net charges, expressed by ΔpH in the B horizon, showed the highest values at the top (P1), which corresponds with the trend of poorly evolved soils in the youngest surface (Table 2).

Table 2 Soil chemical attributes in a sandstone-gneiss topolithosequence in the Amazonas state, Brazil
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The bases Ca2+, Mg2+, and K+ ranged from 0.20 to 1.0 cmolc kg−1 (Table 2). In general, these elements had only slight variations along the toposequence, given their reduced levels in the minerals of sandstone-gneiss and the regional climate conditions, which favor advanced soil weathering. Moreover, the predominance of oxidic minerals generated positive charges, which, allied with the sandier texture, provided the movement or loss of these cations to subsurface horizons. This pattern, however, does not apply to K, which remained at higher concentrations near the soil surface. According to Neves et al. (1961) and Prietzel et al. (2020), this is a characteristic attributed to the low diffusion power electrostatically adsorbed to negative charges of organic matter or the formation of sphere complexes external to the solid phase.

Exchangeable aluminum (Al3+) content had no significant variation along the topolithosequence, with low values recorded in all profiles, mainly in the A horizon. In addition, the potential acidity (H+ and Al3+) values were high and increased in depth along the topolithosequence (Table 2). Possibly, the intense rainy season of the region and the unimpeded drainage were determining climate factors for edaphic acidity as soil depth increased (Campos et al. 2012).

Along the topolithosequence, organic carbon (OC) contents were higher in the surface horizons because of the concentration of organic matter from the decomposition of native vegetation residues (Santos et al. 2012). Cation exchange capacity (CEC) ranged from high to very high (12–28 cmolc dm−3). The same trend occurred for the sum of bases (SB), which was higher in the A horizon because of the levels of OC and the low clay content and activity.

Due to the low activity of the clay fraction, all segments were classified as dystrophic (Table 2), which is the result of base depletion (Ca2+, Mg2+, and K+). Other authors have reported similar results in the Amazonian region (Campos et al. 2012a; Santos et al. 2012; Martins et al. 2006). The geomorphic surface, that is, the landscape did not influence the variation of the SB. It is, therefore, inferred that the soil inherited the chemical poverty from the gneiss and sandstone rocks, revealing the parent material as the determinant formation factor for the dystrophic character of the soils along the topolithosequence. These results differ from those reported by Campos et al. (2010), who verified a higher degree of soil development and dominance of dystrophic soils at the top positions of the landscape.

Soils along the topolithosequence were Álico, with aluminum saturation (m) ranging from 51 to 91%. These values resemble those reported by Martins et al. (2006) for soils in a field to forest transition in the region of Humaitá, Amazonas, Brazil. This is an inherent characteristic of the sandstone-gneiss, which has by nature an acidic character. As mentioned, the Al3+ richness justifies the formation of gibbsite in the Neossolos (P2 and P3).

Available phosphorus (P) contents did not vary along the topolithosequence and were on average 2.0 mg kg−1, decreasing with soil depth. According to Silva et al. (2006), P in the deeper horizons remains stable due to its low mobility. The strong affinity of iron oxides, especially goethite, with P also promotes its retention (Pinto et al. 2013; Rotta et al. 2015). The high isomorphic substitution (IS) and SSA of Gt in goethite-rich soils, like those in the present study, also potentialize P adsorption. However, available P in the A horizon of the top (P1), transport (P4), and deposition (P5) foothills were 4.6, 3.1, and 5.8 mg kg−1, respectively, representing the highest values in the topolithosequence. The latter could be associated with the accumulation of OC as litter on the soil surface horizon. The OC acts as a physical barrier, inhibiting the direct contact of P with the active sites, therefore, enabling P occurrence in the available soil fraction (Fink et al. 2014).

Fe content and mineralogical attributes

The contents of total Fe (Fet, 9–68 g kg–1), Fe extracted from crystalline oxides (Fed, 3–52 g kg–1), and poorly crystalline Fe oxides (Feo, 1.8–8.2 g kg –1) were low (Table 3) (Kämpf and Curi 2000). Lima et al. (2006) and Aquino et al. (2016) reported similar results for Amazonian environments. The low Fet is typical of the naturally poor levels of Fe minerals in the sandstone and gneiss rocks from which the soils derived and frame these soils as hypoferric (Fet ≤ 80 g kg–1) according to the current taxonomic system of Brazilian soils–SiBCS (Santos et al. 2018). The levels of Fed were higher than those of Feo, especially in the most weathered soils (PAd) and in those with the presence of plinthite, a Fe-rich clay agglomerate, in the diagnostic B horizon of FF. Although lower than Fed, high Feo contents marked the soils CXvd_AB (7.4 g kg–1), PAd_Btf (7.7 g kg–1), and PAd_Bf (8.2 g kg–1). The sandy texture and high levels of organic matter in the superficial horizons of these soils explain their Feo contents. These characteristics can promote Fe loss, inhibiting the crystallization of Fe oxides, this is, the formation of poorly crystalline compounds (Schwertmann and Taylor 1989; Anjos et al. 2007; Miguel et al. 2013).

Table 3 Crystallographic attributes of soil minerals of a sandstone-gneiss topolithosequence in the Amazonas state, Brazil
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The predominance of Fed culminated in low Feo/Fed values (≤ 0.5), mainly for the B horizon of the FF and PAd due to the presence of plinthite. Higher Feo/Fed values might reflect the occurrence of the poorly crystalline Fe oxide ferrihydrite in the system, mainly in RQo_Cr and CXvd_Bi, indicating water stagnation in the profile of these soils. The crystalline Fe forms (hematite and goethite) found as plinthite or within pretro-plinthite might dissolve, giving place to a new generation of poorly crystalline Fe oxides, such as ferrihydrite, for instance (Coelho and Vidal-Torrado 2003; Miguel et al. 2013).

The Fed/Fet ratio indicated different degrees of pedogenesis along the topolithosequence (Table 3). In general, the lower the Fed/Fet ratio, the higher the proportion of Fe released through the weathering of lithogenic minerals for the neoformation of pedogenic Fe oxides such as goethite and hematite, which are the main constituents of tropical soils (Silva et al. 2020). More than 50% of the Fet was represented by Fed, which associated with the reduction of the Fed/Fet ratio indicated higher pedogenesis following the sequence PAd_Bf > PAd_Btf > FFc_BAf > FFc_Bf > CXvd_AB > CXvd_Bi > RRd_ACr > RQo_Cr. Increased Fed values are associated with good vertical drainage and a marked chemical transformation in soils favorable to the crystallinity of pedogenic Fe oxides such as goethite and hematite (Kämpf and Curi 2000; Lima et al. 2006; Cornell and Schwertmann 2003; Camelo et al. 2018; Poggere et al. 2018; Silva et al. 2020). Finally, Fe contents, although low, proved to be sensible environmental pedoindicators, as attributed by Silva et al. (2020) when evaluating the influence of the parent material and the cause–effect relationship of landscape dissection on soil genesis and the mineralogical variability of the clay fraction in basalt-sandstone Brazilian environments.

The minerals kaolinite (Kt), gibbsite (Gb), hematite (Hm), and goethite (Gt) were all present in the clay fraction of the diagnostic and transitional B horizons (Table 3). Along the toposequence, the Fe oxides Hm and Gt followed the order: transport foothill (9–41 g kg−1) > deposition foothill (5–39 g kg−1) > top (5–24 g kg−1) > colluvial footslope and convex creep slope (1–5 g kg−1). Among the iron oxides, Gt was present in the clay fraction of all the studied soils, which is given by the iron poverty of the sandstone-gneiss sediments (Correa et al. 2008; Santos et al. 2010; Silva et al. 2020; Brito et al. 2021). This lithological condition might have favored Gt formation (Barrón and Torrent 2002; Silva et al. 2020), as suggested by the low Hm contents, with a maximum value of 9 g kg−1 in the topolithosequence.

The crystallographic parameters MCD, WHH, SSA, and IS reflected the peculiarities of the topolithosequence (Table 3). The mean crystal diameter (MCD) in the Gt110 and Gt111 peaks varied from 10 to 28 nm and 9 to 21 nm, respectively. These values were lower than those detected for the Hm111 and Hm012 peaks, which were 12–25 nm and 30–89 nm, respectively. Overall, MCD values revealed the persistence of crystalline Hm over Gt, as commonly reported for tropical soils (Inda Jr and Kämpf 2005; Barbieri et al. 2014). The latter was evident in the transport foothill (P4), where Hm012 peaks were well-defined and narrow (Fig. 4). The climatic conditions of the Amazonian region, especially precipitation and temperature, combined with the naturally high contents of OM, the acid pH (Table 2), and the intense microbial activity, increase Al presence in the goethite structure and, in turn, decreases its crystallinity. Except for the top segment (P1), the isomorphic substitution (IS) along the topolithosequence surpassed 0.33 mol mol−1, indicating the occurrence of markedly weathered, non-hydromorphic, and acid soils (high Al activity) (Table 2).

Fig. 4
figure 4

X-ray diffractograms of the clay fraction for hematite (Hm) and goethite (Gt) in the profiles of Cambissolo Háplico (CXvd-AB; CXvd-Bi), Neossolo Regolítico (RRd-ACr), Neossolo Quartzarênico (RQo-Cr), Plintosolo Pétrico (FFc-Bf; FFc-BAf) and Argissolo Amarelo (PAd-Bt; PAd-Btf) in the Amazonas state, Brazil

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These conditions justify the persistence of Gt with high Fe substitution (r = 0.065) by Al (r = 0.053), which promoted the contraction of the unit cell and, consequently, increased the specific surface area (SSA) that ranged between 82 and 234 m2 g−1 in Gt and 41–89 m2 g−1 in Hm. These results are consistent with previous research in Brazilian soils (Melo et al. 2001; Correa et al. 2008) since the higher IS and SSA values in Gt reflect its higher structural capacity to accommodate Al compared to Hm (Schwertmann and Taylor 1989; Carvalho Filho et al. 2015).

Iron oxides concentrated in the soils of each landscape segment following the order: transport foothill (P4) > deposition foothill (P5) > top (P1) > convex creep slope (P3) and colluvial footslope (P2) (Table 3 and Fig. 4). The highest levels of Fe oxides, especially Hm, in the Plintossolo (P4), are given by the increased iron contents in the BAf and Bf horizons, characterized by the presence of ferruginous concretions, as typical of Amazonian Plintossolos (Campos et al. 2012). Even in poorly developed soils such as the RQo_Cr, marked by the occurrence of lytic contact (weak rock or outcropping saprolite), well-defined peaks of Gt formed (Fig. 4). This fact was already expected since goethite is the first Fe oxide that forms in the initial horizons close to the rock during the early stages of pedogenesis (Curi and Franzmeier 1984; Silva et al. 2020).

The Kt/(Kt + Gb) ratio indicated Gb as an important mineralogical constituent of the studied soils, except for the top and the deposition foothill (P5) (Table 2 and Fig. 5). The low Kt contents or even its absence in the transport foothill (P4) might reflect the disruption of Kt nucleation probably due to increased contents of iron oxides as previously reported by Gidhin et al. (2006). Consequently, Kt had higher width at half-height (WHH) (0.483–0.703 nm) than Gb crystals (0.273–0.307 nm). This result indicates that Gb crystals are much more crystalline than Kt, which is characteristic of highly weathered soils (Ghidin et al. 2006; Camargo et al. 2008), where low Si concentrations favor the formation of Gb crystals in the soil (Hsu 1989).

Fig. 5
figure 5

Pattern of X-ray diffractograms of the clay fraction for kaolinite (Kt) and gibbsite (Gb) in the profiles of Cambissolo Háplico (CXvd-AB; CXvd-Bi), Neossolo Regolítico (RRd-ACr), Neossolo Quartzarênico (RQo-Cr), Plintosolo Pétrico (FFc-Bf; FFc-BAf) and Argissolo Amarelo (PAd-Bt; PAd-Btf) in the Amazonas state, Brazil

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The proportions of Kt, as expressed by the [Kt/Kt + Gb > 75] ratio, and the low contents of Fe oxides indicate low pedogenic intensity or predominance of morphogenesis over pedogenesis in the Neossolos (Scarciglia et al. 2005). Thus, geomorphological characteristics such as the more rugged and dissected topography of the landscape segments characterized by the presence of Neossolos (Fig. 3) favored the partial removal of basic cations and silicon, which combine in the soil to form Kt (Kämpf et al. 2009). Additionally, part of the Si and Al removed from the soil through weathering in the top segment of the landscape could have been transported and accumulated in the lower landscape positions, increasing the [Kt/Kt + Gb > 60] ratio in the Argissolo (P5). These results are supported by previous research (Curi and Franzmeier 1984; Ghidin et al. 2006; Campos et al. 2007; Silva et al. 2020), which considered the landscape a passive factor of soil formation and retribution of pedogenic minerals.

Final considerations

Differentiating the landscape into segments and identifying the parent material were efficient to understand the variation of soil attributes along the studied transect. Soil classes depend on the topography and the parent material, thus varying with the physical, chemical, morphological, and mineralogical properties. Mineralogically, the clay fraction is composed of kaolinite, goethite, hematite, and gibbsite, with goethite being the predominant iron oxide. Additionally, as confirmed by soil mineralogy, the chemical properties of the cations of exchangeable bases from sediments originated from these soils were naturally poor in bases and, therefore, not related to the removal process of the system, as widely recognized in the Amazonian region.

Independently of the surface, the sand fraction dominated over the other fractions. The alluvial nature of the parent material explains this result, with the highest values occurring in the Neossolos and increasing with the depth of the soil profile.

Conclusions

Lithological contrast and landscape variations determined the different soil types as well as morphological, physical, chemical, and mineralogical attributes in the topolithosequence. The relief and the parent material, sandstone-gneiss, were the main factors influencing pedogenesis. Goethite was the predominant Fe oxide in all the soils, reflecting the low total iron content of the parent material. The predominance of the sand fraction in all the studied profiles reflected the alluvial nature of the parent material, with the highest values occurring in the convex creep slope. Knowing the geomorphic surfaces and the parent material was effective for understanding the variation of the soil attributes along the landscape.