Genesis and micropedology of soils at Serra do Divisor and Moa river floodplain, northwestern Acre, Brazilian Amazonia

The northwestern part of the Acre State (Brazil) possesses singular soils in Brazilian Amazonia, but have been very little studied. This study aimed to discuss the genesis and some micropedological aspects of the soils from Serra do Divisor and adjacent floodplain soils of the Moa river, to enhance the knowledge on their formation. A toposequence of soils ranging from the uppermost part of sub-Andean Serra do Divisor to the Alluvial soils of Moa river floodplain was studied, regarding chemical, physical, mineralogical, and micromorphological attributes. The parent material of the Serra do Divisor is basically quartzose sandstone, and the soils along the toposequence were classified as Typic Haplorthods (P1), Spodic Quartzipsamment (P2), Lithic Quartzipsamment (P3), and Lithic Quartzipsamment (P4). Along the Moa river floodplain, we also identified and collected, Typic Udifluvent (P5), Typic Kandiudult (P6), Typic Kandiudalf (P7), and Arenic Plinthic Kandiudult (P8). The Serra do Divisor soils have very low fertility, high acidity, and low cation exchange capacities, presenting a coarse sandy texture, even shallow pedons. The X-ray diffraction analysis of these soils indicates the predominance of kaolinite, with traces of quartz and gibbsite. The shallow mountain Podzols on sandstone have an expressive accumulation of organic material in surface horizons, with evidence of ferrihydrite and imogolite in the subsurface. At the Moa river floodplain, all soils are originated from recent sediments (Cenozoic), which have a geological source upstream. Varying sedimentary layers are key aspects influencing soil genesis. Those soils have evidence of 2:1 clays with hydroxyl-Al interlayers in subsurface horizons. The Serra do Divisor steep landforms and the coarse texture of the soils promote good drainage and favor leaching and chemical impoverishment. Kaolinite and gibbsite were formed by severe leaching and there are evidences of in situ neoformation of gibbsite by extreme Si losses. All studied soils have some peculiarities such as high accumulation of organic material or 2:1 clay minerals. Most investigated soils were affected by colluvial, reworking, mass movements or a strong variation on sedimentation.


INTRODUCTION
The northwestern of the Acre State (Brazil) possesses unique, poorly studied pedoenvironments in the Brazilian Amazonia. The region is directly associated with the presence of first Andean folded/faulted elevations, under a neotectonic regime. Some peculiar aspects are observed in the formation of these soils, with strong contrasts with lowland Amazon soils (Carvalho et al., 1977). The main physiographic feature in this region is the Serra do Divisor mountain range, reaching 700 m a.s.l., consisting predominantly of Cretaceous sandstones (Moura and Wanderley, 1938;Carvalho et al., 1977).
The Amazonian tropical rainforest is the main vegetation type of this region, developing on sandy soils under an equatorial climatic regime. The combination of humid tropical conditions and a sandy, nutrient-poor substrates could favor the development of illuviation process leading to organic subsurface layers and podzolization, with downward migration of Al/Fe compounds, complexed by organic matter, and subsequent accumulation of amorphous constituent, forming spodic horizon (Lundström et al., 2000;Schaetzl and Anderson, 2005;Soil Survey Staff, 2014a).
Podzolization can be explained by two major processes: (1) formation and downward transport of the organometallic complexes with Al and Fe; (2) weathering of silicates followed by Al and Si translocation, as inorganic forms, or imogolite type materials (Farmer et al., 1980;Anderson et al., 1982;Lundström et al., 2000). Although theories involving eluvial and illuvial organic compounds and aluminosilicates through adsorption, precipitation, and microbial degradation are partly contradictory (e.g., Malcolm and McCracken, 1968;Farmer et al., 1980;Anderson et al., 1982;Buurman and van Reeuwijk, 1984;Little, 1986;Lundström et al., 2000), some processes can act simultaneously. Biogeochemical interactions are consensual, as well as the formation of organic compounds, with a key role in the genesis of the spodic horizon (Lundström et al., 2000).
Amazon Podzols (Spodosols) have been traditionally described in classical studies, which revealed their chemical, mineralogical, and micromorphological attributes, and the possible genesis (Klinge, 1965;Sombroek, 1966;Lucas et al., 1984;Bravard and Righi, 1990;Dubroeucq et al., 1991;Andrade et al., 1997;McClain et al., 1997;Mafra et al., 2002;Patel-Sorrentin et al., 2007;Mendonça et al., 2014). Also, studies in French Guiana and Brazil have shown that Podzols can be formed from various parent materials by the transformation of an initial clayey Oxisol cover Dubroeucq et al., 1991;Andrade et al., 1997); in which Podzols are considered a final degradation stage of the tropical soil cover under a dominant humid equatorial climate Mafra et al., 2002). Other studies indicate a sequence of soil development ranging from Podzol to Histosols, with thick peat layers in the bottom valleys (Dubroeucq and Volkoff, 1998;Bonifacio et al., 2006), often affected by frequent flooding.
While most Podzols studied in Amazonia are close to the rivers and under hydromorphic conditions of the bottomlands, the Serra do Divisor sandy soils are found on elevations and steep slopes, representing very contrasting pedo-environments to date, the only previous study of soils from the Serra do Divisor and Moa river floodplain is the RADAMBRASIL Project (Carvalho et al., 1977). For most Amazon floodplain soils, some characteristics are well known, like the richer-nutrient sediments afforded by the Andean/sub-Andean influence and source, which renew the chemical status by annual floods (Sombroek, 2000;Schaefer et al., 2017). Nevertheless, in the Acre basin (Upper Amazon) influence the extent of sub-Andean has not been investigated, and soil formation remains poorly studied (Schaefer et al., 2017).
In the present study, we examine representative in situ soils of Serra do Divisor, as well as Alluvial (allochthonous) soils of the Moa river floodplain, to enhance the knowledge on the genesis of these low mountain Podzols and floodplain soil of the Moa river basin.

Study area
The study area is located in the northwestern part of the Acre State (Brazil), between 73° 40' 20" W / 7° 26' 20" S and 73° 39' 10" W /7° 27' 18" S ( Figure 1). It is part of the National Park of Serra do Divisor, at the westernmost part of Brazil, near the border with Peru. Koppen's Af climates predominate (tropical), without well defined dry seasons (Alvares et al., 2013), under a typical udic moisture regime. The annual mean temperature is 25.5 °C; July is the coldest month with mean temperatures of 24.1 °C, and the warmest month is January with 26.0 °C (Climate-Data.org, 2020). The mean annual rainfall ranges from 2,500 to 2,750 mm (IBGE, 1994).
The geology of Serra do Divisor is composed of distinct stratigraphic units, with a basement of metamorphic and igneous Pre-cambrian to Paleozoic rocks of limited outcrops. Overlying, folded Cretaceous sandstones (Km: Moa formation; Kra: Rio Azul formation; Kd: Divisor formation) are widespread (Carvalho et al., 1977) and represent the parental materials of all soils studied. These sedimentary rocks (Km, Kra, Kd) are composed mostly of quartz-rich sediments, forming in situ, colluvial deposits, and unconsolidated sediments (Qai) (Carvalho et al., 1977;IBGE, 1994). The tectonic evolution of this area indicates a Cretaceous basin open to the Pacific, where mature sandy sediments eroded from the Brazilian continental landmass were deposited (Rossetti et al., 2005). The Andean elevation started in the mid-tertiary and drastically changed the landscape, causing the inversion of the sedimentary basin and drainage network, from the original pacific outlet to the present-day Atlantic, in the Late Cenozoic (Plio-Pleistocene) (Carvalho et al., 1977). At the foot slopes of the Serra do Divisor mountains, Cenozoic sediments are widespread, forming in a regional scale a gentle, dissected relief, at about 200 m a.s.l., formed by Plio-Pleistocene deposits (Solimões formation -TQs) and Holocene alluvial sediments (Qai) of the Moa and Juruá river ( Figure 1). Locally, the Solimões formation encompasses mostly argillites and siltites of continental origin, deposited in a floodplain-lacustrine-swampy environment with local gyppsite veins and calcareous concretions are present (Carvalho et al., 1977;Latrubesse et al., 2010). The Moa river crosses the Serra do Divisor mountain through a series of faulted lines, forming a canyon, reaching the Juruá floodplain across the dissected lowlands ( Figure 1).
Overall, the northwestern part of Acre is covered by typical lowland Tropical Forest, characterized by the Open and Dense physiognomies. It is one of the richest floristic regions worldwide in terms of palm species (Silveira and Daly, 1997). However, in the Serra do Divisor, a type of Submontane Dense Forest is present where unusual highland species occur (Carvalho et al., 1977). In addition, a typical vegetation of the eastern low Andean elevation, regionally named Ceja Forest, is also present, which is characterized by small trees, ferns, shrubs, orchids, mosses, epiphytes and, notably, by abundant Bromeliaceae (Lathrap, 1970).

Soil sampling
Eight representative soils of two main landforms of the Serra do Divisor National Park were studied (Table 1) Soil pits were dug manually, followed by a morphological description (Santos et al., 2005). Samples of genetic horizons and important horizon boundaries (undisturbed) were collected and submitted to chemical, physical, mineralogical, and micromorphological analysis. The soils were classified according to the Soil Taxonomy (Soil Survey Staff, 2014a) and Brazilian Soil Classification System (Santos et al., 2018).

Analysis
Soil samples were air-dried and passed through a 2 mm sieve, to obtain air-dried fine earth (ADFE). Soil colours (dry and moist) were obtained using the Munsel Colour Chart. Particle size analysis was based on wet sieving, dispersion, and sedimentation, followed by siphoning of the <0.002 mm fraction (Ruiz, 2005).
All routine analytical chemical and physical measurements were obtained using standard procedures (Sparks et al., 1996;Claessen, 1997). The following properties were determined: pH(H 2 O) and pH(KCl) in KCl 1 mol L -1 with a soil:solution ratio of 1:2.5; available P, exchangeable Na and K extracted with Mehlich-1 (P was determined spectrophotometrically; Na and K by flame emission photometry); exchangeable Ca and Mg by atomic absorption spectroscopy and exchangeable Al by titration after extraction with KCl 1 mol L -1 and potential acidity (H+Al) by titration after extraction with calcium acetate 0.5 mol L -1 at pH 7.0. We also evaluated P adsorption by soils through measuring the P remaining (Prem) in a CaCl 2 10 mmol L -1 solution, with P 60 mg L -1 , is put in shaken with a soil sample (Novais and Smyth, 1999;Alvarez et al., 2000;Donagemma et al., 2008). The total organic carbon (TOC) of ADFE was determined by titration of K 2 Cr 2 O 7 with 0.2 mol L -1 Fe (NH 4 ) 2 (SO 4 ) 2 .6H 2 O after wet oxidation treatment (Yeomans and Bremner, 1988).
The mineralogy of clay fraction was determined for all horizons with X-ray diffraction (XRD) analysis, using monochromated CuKα radiation on oriented samples. Were selected samples of the diagnostic horizons to identify the 2:1 clay minerals. The selected samples were saturated with potassium (KCl 1 mol L -1 ) and magnesium (MgCl 2 1 mol L -1 ), submitted to heating (350 °C for the samples saturated with K + ) and treated with glycerol 10 % (for Mg-saturated samples). The diffractograms were interpreted according to Chen (1977). Amorphous Fe and Al were extracted with ammonium oxalate (McKeague and Day, 1966) and the free iron oxides with dithionite-citrate-bicarbonate (Mehra and Jackson, 1960). To extract the organic bounds forms, we used pH 10.0 sodium pyrophosphate (Soil Survey Staff, 2014b). Iron, Al, and Si were determined by atomic absorption spectrometry.
Soil humic substances were chemically fractionated by the alkali and acid solubility, according to Swift (1996), adapted by Mendonça and Matos (2005).
The micromorphology of the diagnostic horizons of P1, P2, P5, and P6 was investigated in thin sections (Table 2), following the recommendations of Stoops et al. (2018), using a petrographic microscope Olimpus CH30. Semi-Quantitative elementary analysis and microchemical maps were obtained after stoichiometric normalization by ZAF procedures using electron microscopy and EDS analyses (LEO Zeis -430i model operated in 84 eV).

Soils of Serra do Divisor
All soils studied at Serra do Divisor are sandy with high amounts of coarse and fine sand and less than 10 % of clay and 11 % of silt (Table 3), being derived from Cretaceous weathered sandstone. They are nutrient-poor, extremely dystrophic, with very low cation exchange capacity (CEC), high Al saturation and high amounts of total organic carbon (TOC) ( Table 3). All soils have an expressive accumulation of organic material in the surface horizons, reaching 306.6 g kg -1 in the O horizon of P3 (Table 4). Nutrient contents decrease with depth in all soils, along with reduced TOC levels and increase in pH (Table 4).
The physical properties of soils from Serra do Divisor, with a sandy texture, associated with high rainfall, favor the migration of organic compounds, as confirmed by the Rev Bras Cienc Solo 2020;44:e0200038 subsurface TOC data (Table 3), forming Al-rich organic coating on quartz grains ( Figure 2). In the Spodosol (P1) and Entisol (P2), TOC values increase from 0.35 to 0.70 m depths and from 0.70 to 0.90 m depths, respectively, indicating active podzolization (Table 4). For P1, macromorphological features showed a subangular blocky structure for the Bhs horizon (Table 2).
All soils showed kaolinite mineralogy (major reflections at 7.22 Å and 3.58 Å), with traces of quartz and gibbsite (Table 5 and Figure 3).
For P1, the high amounts of Al extracted by ammonium oxalate and sodium pyrophosphate and the high Fe o /Fe d ratio (Table 5) in the Bhs are associated with high TOC, which indicate the accumulation of poorly crystalline illuvial organometallic complexes formed in the spodic horizon (Figura 2). In P1, the Bs and Bhs horizons had the highest value of Fe o /Fe d ratio, nearing 1.0 (Table 5), suggesting the presence of ferrihydrite (Schwertmann et al., 1986). Another poorly crystalline mineral occurring in this soil is imogolite, indicated by the Al o -Al p difference or/and the Al o + ½ Fe o >0.5 ratio (Ugolini and Dahlgren, 1991;Soil Survey Staff, 2014a,b).
In the C2 horizon of P2, the accumulation of illuvial organometallic complexes is similar to Bs horizon of P1, but insufficient to define a spodic horizon, due to low Al o , Fe o , and TOC values. The point-source microanalysis of C2 horizon in P2 and the chemistry and mineralogy properties reveal a predominantly kaolinitic composition of the micromass with Fe oxide and fine coatings of organic matter with Al phases on subangular to subrounded quartz grains ( Figure 4). The K amounts in the groundmass are much lower than in the inner feldspar crystals, due to the high mobility and lixiviation of this cation ( Figure 4). In this layer, we also verified the presence of gibbsite by XRD (Figure 3), suggesting deep weathering.
The P3 and P4 present similar values to P2 of Al and Fe extracted by oxalate and DCB. However, P4 has higher levels of Fe d indicating more crystalline Fe phases and the  presence of Al-goethite by XRD, also suggested by the yellowish color. The Al-goethite was indicated by the dithionite-citrate-bicarbonate extraction (Al d and Fe d , Table 5), and also by a peak shift to higher angles in XRD of the goethite (Schwertmann and Cornell, 2000).
Regarding the humic fractions, there is a small increase the fulvic acids fraction (FA) in the subsurface horizons of P1, P2, and P3 (Table 6). The P1 has accumulation of FA and humic acid fractions (HA) in the Bhs horizon (Table 6).
The P1 and P2 showed clear podzolized horizons (Bhs and C2, respectively) with many illuvial features, like organic coatings (or organs) (Figure 2 and Table 7) (Brewer, 1973;Paton, 1978). The backscattered electrons images of P1 (Bhs horizon) and P2 (C2 horizon) ( Figure 2) revealed the macroporosity with the prevalence of subangular/subrounded quartz grains, which favor the downward migration of the organic metal complexes in the soils. The Bhs horizon of P1 has a greater relative micromass proportion, which is related to its well-developed macrostructure. These pedo-environments present local bridged-grain microstructure, biologic channels, organic compound coatings, nodules, pellety, and partially decomposed organic fragments (Table 7), with clear evidence of bioturbation. Ferruginous concretions also occur but in low amounts, due to the coarse texture and the low content of ferruginous material in the sandstone substrate (Table 7). In the C2 horizon of P2, we found OM illuvial features ( Figure 4 and Table 7), associated with Al forms and, in a lesser amount, Fe forms; and fine K-feldspar grains.

Moa river floodplain soils
The soils of the Moa river floodplain are considerably richer in nutrients than soils on sandstones of Serra do Divisor. Enhanced microbial activity results in organic matter mineralization, as indicated by the low levels of organic carbon and the small amounts of OM at the topsoil for all soils (Table 3).
Except for P5, which is closer to the Serra do Divisor ( Figure 1)   (1) Extracted with Mehlich-1 (P was determined spectrophotometrically; and Na and K by flame emission photometry).
(2) Extracted with KCl 1 mol L -1 and determined by atomic absorption spectroscopy.
(4) Determined by titration after extraction with calcium acetate 0.5 mol L -1 at pH 7.0. Holocene Alluvial sediments (Table 1). A pedological discontinuity from 1.00 to 1.10 m in P5, is indicated by increasing clay content, which results in higher levels of Na, Ca 2+ , Al 3+ , H+Al, and TOC, and a decrease in Prem (Tables 3 and 4) and the absence of illuvial features. Also, figure 5 shows differences in c/f (coarse/fine) relation distribution for C1 and 2C4 horizons of P5, enaulic and chitonic, respectively.
The P6 has an increment of clay with depth, higher than 1.7 times the clay content of the overlying eluvial horizon (Table 3), and with some coatings of oriented clay on the surface of peds (Table 7), suggesting argilluviation. The P6 had a base saturation lower than 34 %, decreasing with depth. The P7 is the only eutrophic soil (BS ≥50 %), with an argillic horizon and abrupt textural change. The argillic horizon has a four-fold increment of clay content in relation to the overlying horizon (Table 3). The loamy and silty substrates of Solimões formation favor well developed aggregates (peds), as present in P6 and P7, and a subangular blocky structure (Table 2).
In P8, the sedimentary discontinuity in the subsurface, indicated by abrupt differences in particle size, distinct color layers, and the absence of illuvial features, are important properties, which are related to changing fluvial deposition. At the Btv1 horizon, there is an abrupt increase in clay accompanied by a slight increase in organic C (Tables 3 and 4). This is related to a sandy material overlaying clayey sediment, with higher organic C content, typical of meandering rivers. Plinthite occurrence was confirmed by laboratory test, which shown iron concretions remained firm when moist and became irreversibly hardened after exposed to the atmosphere and repeated wetting and drying (Soil Survey Staff, 2014a), and confirmed by pedogenic analyses (Table 7). Micromorphology indicates an incipient pedological development and association with hydromorphic features, like massive reducing (bleached) zones, intense polychromy, rare biological channels, absence of illuvial features, depletion zones by ferrolysis and gleization, and also a collapsing structure with the destruction of clay and clogging of pores with dispersed material (Table 7). All these features point a prolonged hydromorphism.
The diagnostic horizons of P6, P7, and P8 present high Fe d and low Fe o /Fe d ratios suggesting the presence of goethite in the clay fraction, confirmed by the XRD analyses (Table 5). In P8, the Fe o /Fe d ratio close to 1.0 in eluvial horizons (2E2, 3E3, and 4E4) can indicate the presence of ferrihydrite. These soils are more yellowish, although the oxidation and reduction process causes mottled zones in subsurface horizons (Table 7) and can be corroborated by the multicolored layers (Table 3).
The alluvial soils of Moa river have a higher variation of TOC values and humic substances with depth, following variable energies of sediment deposition. We did not find any micropedological evidence of carbon illuviation so that an in situ origin for the humic substances is inferred.
The backscattered electrons images of P5 reveal that 2C4 is richer in fine fraction (clay + silt) than C1, due to the sediment discontinuity ( Figure 5). In P6 backscattered electrons images of Bt1 horizon, we identified well-developed peds ( Figure 5), which is in agreement with increasing clay content (Table 3). The illuvial features described by micropedological observation are stress cutans (Table 7). We also detected the presence of zircon grains in P6, as indicated by the Zr and Si presence in the backscattered electrons images (Figure 5c). Figure 6 shows higher amounts of K + in the inner K-feldspar grain, compared with the edge, and greater Na amount at the edge showing an intergrowth nature of Na-Plagioclase and K-Feldspar. The groundmass analysis indicates some Si losses ( Figure 6). This is also observed in the Si/Al ratio, higher in the K-feldspar grain, reducing in the groundmass. Iron contents are higher in the micromass, with ferruginization of K-Feldspar grains at the edge, as hypocoatings, and along fissures, as Fe-oxides infillings ( Figure 6).

Podzolized soils of the Serra do Divisor
In the altitudinal sequence of Serra do Divisor, the migration of humic substances forming Fe and Al complexes is characteristic. Besides the sandstone geological substrate, which provides the coarser sandy texture, the high precipitation in this region also favors the podzolization process.
Our findings show nutrient-poor, dystrophic soils, with very low cation exchange capacity (CEC), high Al saturation, and low pH values on Serra do Divisor, which in agreement with previous results for other sandy soils studied in the Amazon (Lucas et al., 1993;Mafra et al., 2002;Gomes et al., 2007;Nascimento et al., 2008;Fritsch et al., 2009). However, the Serra do Divisor soils have some peculiar aspects, which result in much higher natural organic carbon accumulation than in other sandy soils of the Amazonia lowlands. In Serra do Divisor, this organic matter accumulation is due to the very low  Organic matter (OM) associated with high Al levels have low biodegradation and high resistance to chemical oxidation (Mendonça, 1995). The predominance of H+Al, and particularly Al 3+ in the exchange complex of the soils from Serra do Divisor, contributes to OM accumulation (Table 4), due to decreasing microbiological activity and accumulation of non-decomposed OM (Sieffermann et al., 1987). The strong nutrient depletion with depth in all soils, along with reduced TOC levels (Table 4), indicates the importance of surface OM for nutrient cycling in these chemically poor environments. Due to the sharp reduction of all nutrients with depth, Al 3+ occupies most of the exchange complex in these sandy soils, with high organic matter mobility. In most soils, dissolved Al 3+ can be leached from the topsoil (with pH <5 in O and A horizons; Table 4) to the subsoil (with an increase in pH with depth; Table 4) during podzolization. Aluminum solubility increases sharply below pH 5 (McLean, 1976), and only precipitates as poorly crystalline Al hydroxides (e.g., imogolite) in the subsurface horizons with pH close to 5 (Table 4 and Figure 3).
Iron preferentially precipitates as ferrihydrite in soils with high organic carbon contents, in relation to goethite, hematite or lepidocrocite (Schwertmann et al., 1986). This is in agreement with the presence of ferrihydrite in the Bs and Bhs horizons at P1 at the highest location of Serra do Divisor.
The mountainous relief and high rainfall at Serra do Divisor enhance erosion and mass movement, which renews the landscape and exposes the weathered quartzrich sediments, also nutrient-poor. These soils show high surface organic matter accumulation, whereas the coarse texture increases drainage and leaching, leaving  kaolinite and gibbsite as detrital minerals despite the quartzose substrate. This is consistent with studies from elsewhere in the Amazon (Bravard and Righi, 1988). However, the co-existence of gibbsite and kaolinite in the clay fraction in P1, with high TOC and low Prem (Table 4 and 5) in a predominantly quartz-sandy soil, indicates a possible in situ neoformation of gibbsite, through biodegradation of organic matter complexed with Al (Volkoff et al., 1984;Bravard and Righi, 1988). The biogeochemical cycling of gibbsite is a possible mechanism in Podzols and the accumulation in deep horizons is not attributed to illuvial processes but to differential neoformation between horizons with high and low organic matter contents (Vasquez, 1981). The crystallization of gibbsite in surface horizons is inhibited by the presence of organic matter (Wilson, 1969) and interaction with the organic matter may cause its re-dissolution to form highly stable organo-metallic complexes which are immobile due to their high content of complexed Al (Vasquez, 1981). Righi and DeConinck (1977) detected gibbsite in spodic horizons, suggesting in situ crystallization. The low Prem values at Bhs in P1 (Table 4) indicate the poorly crystalline Al/Fe phases (Table 5), with high P adsorption capacity (Novais and Smyth, 1999;Donagemma et al., 2008;Broggi et al., 2010).
The presence of imogolite was expected only in Bhs of P1, by the second one ratio, but this evidence must only be comproved by spectroscopy techniques. The imogolite genesis can be associated with Al-Si precipitated in the spodic horizons coming from O and E horizons, besides the participation of other minerals and humus accumulation can also be associated (Lundström et al., 2000). Normally, the imogolite is typical of Bs and C horizons of some Spodosols (Wada, 1989;Ugolini and Dahlgren, 1991). Some of the Al translocation occurs as "proto-imogolite" which might explain the presence of imogolite in the Bs horizons (Farmer and Lumsdon, 2001).

Moa river floodplain soils
The studied Alluvial soils were less influenced by carbonate-rich sediments (from the Ramon and Divisor Formations) upstream of Moa river or to rich clay of Solimões formation (Carvalho et al., 1977;Latrubesse et al., 2010), compared with the local Cretaceous sandstone. Hence, as evidenced by P5, the local erosion of the prominent Serra do Divisor leads to the deposition of sandy sediments, and formation of soils with high fine sand contents and with little contribution of the underlying Solimões formation.
Typic Kandiudults is the dominating soil developed on the Solimões Formation (Carvalho et al., 1977) and is represented in by P6, with the typical argillic horizon, and clay illuviation process. The lower nutrient availability with depth reflects the importance of vegetation in nutrient cycling and maintenance of higher bases saturation in surface horizons. However, clay illuviation and accumulation in depth in P7, is associated with high Ca 2+ , forming an eutrophic argillic horizon. This high Ca 2+ values can be explained by the influence of calcareous concretions in the Solimões formation in the local Cenozoic basin, as commonly found in Acre (Carvalho et al., 1977;Latrubesse et al., 2010). These results indicate a highly dynamic and complex environment, where deposition of alluvial or, and, colluvial material gives origin to soils with strong lithological variations. Our study shows significant changes in particle size distribution or mineralogy within a soil profile, like abrupt textural contacts, contrasting sand sizes, soil color, and micromorphological features, which indicate discontinuities (Soil Survey Staff, 2014).
The presence of plinthite in P8 (Btv1 horizon) indicates imperfect drainage and variations of water table depth, representing a limited zone for root and pedofauna development. The plinthite appears in a small amount in Btv1 horizon, without form a continuous phase, but indicates a plinthization process (Eze et al., 2014). This characteristic suggests an initial pedogenic process and changing drainage conditions. Rev Bras Cienc Solo 2020;44:e0200038 The mineralogical aspects of the Moa river floodplain soils have some similarities to other soils from the Acre State. Gama (1986) studying soils developed on Plio-Pleistocenic sediments, elsewhere in Acre found chloritized minerals, mica, kaolinite, and quartz in the silt and clay fractions. Martins (1993) studying similar soils found kaolinite, montmorillonite, smectite with hydroxyl-Al interlayers, and muscovite. Mixing of materials, distinct depositional events and reworking of materials are possible explanations for the co-existence of such minerals in these soils.
All soils studied at the Moa river floodplain have clear redoximorphic processes, associated with water table variation, evidenced by the multicolored soil layers (Table 3). At the Moa river floodplain, all soils have layers with higher clay content and much lower Prem values than the upper layers, which we interpret as discontinuities and attribute the low Prem to the occurrence of Al and Fe oxides (gibbsite and goethite), probably with a detrital origin. The photomicrographs confirm the discontinuities, represented by differences in micromass content and c/f relative distribution ( Table 7). The presence of zircon grains in P6 ( Figure 5) indicates the mature detritic material of possible weathered quartz-rich Cretaceous sandstone, which influenced the fluvio-lacustrine deposits of Solimões formation (Kronberg et al., 1989;Horbe et al., 2019).
The dissolution features on feldspar grains suggest ongoing chemical weathering ( Figure 6). The groundmass has no genetic relation to the K-feldspar, indicating a mixed sediment contribution (Solimões Formation). Dessilification process and enrichment of Fe hydroxides were detected in the P6 groundmass.

CONCLUSIONS
The highland Serra do Divisor soils have peculiar aspects with high natural organic carbon accumulation, in contrast with the low carbon contents of the adjacent Moa river floodplain soils. The Podzols have an accumulation of organic material in the surface horizons, attributed to the low nutrient status and high Al 3+ levels and low decomposition. Kaolinite, quartz, and gibbsite coexist, and there is evidence of ferrihydrite and imogolite presence in these soils.
The coarse sandy nature of Serra do Divisor soils promote good drainage and leaching, forming detrital kaolinite and gibbsite but in situ neoformation of gibbsite, from the degradation of OM-Al compounds, is also possible.
At the Moa river floodplain, all soils are originated from Holocene sediments. Many contrasting soil processes were detected, like clay illuviation, sediment discontinuity, plinthization, and redoximorphism as dominants. These soils are generally richer in nutrients than those from the Serra do Divisor, have high Al 3+ levels, and mixed mineralogy, with 2:1 clays, hydroxyl-Al interlayered smectite and kaolinite.