Pedogenesis in a topo-climosequence in the Agreste region of Pernambuco 1 Pedogênese em uma topoclimossequência no agreste de Pernambuco

The Borborema Plateau is characterized by different stages of relief evolution, which modify the climate and vegetation, and where high-altitude tropical forests can be seen surrounded by caatinga. The aim of this study was to characterize the soils of a topo-climosequence in the Agreste region of the State of Pernambuco, and evaluate the influence of the relief and climate on the pedogenesis. A topo-climosequence was selected, and trenches were opened in the geomorphological features of high-altitude forest (P1), between forest and pediplane (P2) and on a pediplanation surface (P3 and P4). A morphological description and a physical, chemical and micromorphological characterization were carried out. In general, the soils are sandy, with the predominance of a single-grain structure or weak aggregation. Higher values for pH, S, V% and assimilable P were found on the lower parts of the landscape. From the micromorphological analysis, the incipient development of pedogenic structures was detected in the C horizon in P1 and P3, clay translocation in P2 and the degradation of iron micronodules in P4. The P1 and P3 profiles were classified as Neossolos Regolíticos Distróficos espessarênicos (Regolsols), the P2 profile as a Argissolo Amarelo Distrófico típico (Alisol), and P4 as an Planossolo Háplico Eutrófico arênico (Planosol). Soil variation in the landscape was determined by climate, relief and parent material. Micromorphology was efficient in detecting attributes not seen in the field, such as incipient aggregation in the Neossolos Regolíticos (Regosols) and the degradation of iron micronodules in the Planossolo Háplico (Planosol).


INTRODUCTION
Among the factors that influence soil formation, relief and climate are most often considered the main determinants of pedogenesis due to their influence on water flow and chemical weathering.Variations in relief are responsible for a number of changes in the characteristics and attributes of the soil, as they govern both the water dynamics of the landscape and drainage, controlling the intensity of formation processes.Variations in relief determine climatic change, especially in the water regime, which in turn intensifies reactions and processes in the soil (PEDRON; AZEVEDO; DALMOLIN, 2012).As a result, the degree of soil development, as well as the dynamics of exchangeable cations and the rate of weathering, are directly related to the position of the soil in the landscape (MEIRELES et al., 2012).
The district of Jurema, located in the Agreste region of Pernambuco, is inserted in the Borborema Plateau, characterized by different stages of relief evolution, which is reflected in the variations in climate, plant cover and predominant lithostructure (GURGEL et al., 2013;RODAL;BARBOSA;THOMAS, 2008).The topographic variations in the region promote changes in the climate and the occurrence of orographic rainfall with indices greater than 1,000 mm year -1 in the highest parts of the landscape, where the altitude can reach 900 m.Such conditions are responsible for the formation of pockets of moisture where ombrophilous, evergreen, and semi-evergreen forests, known as high-altitude tropical forests, can be seen, surrounded by caatinga vegetation with a semi-arid climate (RODAL; BARBOSA; THOMAS, 2008).
With the exception of the study by Souza et al. (2010), little is known about the high-altitude forest environments in the State of Pernambuco.Given the importance of conserving these environments due to the special conditions of humidity, temperature and vegetation, characterization of the physical, chemical and macro-and micromorphological attributes of the soil, with the aim of understanding the factors and processes active in its genesis, is an important tool for adopting management practices and maintaining the functions of the soil and organisms.
To this effect, the aim of this study was to evaluate soil genesis under the influence of relief and climate in a topo-climosequence in the Agreste region of the State of Pernambuco, Brazil.

MATERIAL AND METHODS
The study area is located in the district of Jurema, in the Agreste region of Pernambuco, inserted in the microregion of Garanhuns (Figure 1).The geomorphological unit is known as the Borborema Plateau, with altitudes ranging from 200 to 900 meters.These present different stages of relief evolution, the result of tectonic influences combined with changes in climate, followed by folding and fracturing, and associated with successive leveling (BRASIL, 1983).The geology of the region is included in the Garanhuns Group, in which the quartzites, granites and gneisses of the Proterozoic period predominate.These rocks are characterized by their coloration, which varies from whitish and yellowish to pale pink, of phanerocrystalline structure, and a mineralogy predominantly composed of quartz, feldspar and biotite.A topo-climosequence was selected for the study, and four trenches were opened in different geomorphological features: high-altitude forest (P1), between forest and pediplane (P2) and on a pediplanation surface (P3 and P4).General information on the areas whose profiles were described is shown in Table 1.The morphological description of the soil profiles was carried out as per Santos et al. (2015), and disturbed and undisturbed soil samples were collected for laboratory analysis.
The air-dried fine-earth fraction (ADFE) was obtained from the collected samples, after drying, declumping and sieving (2 mm mesh), on which the physical and chemical analysis was carried out.Granulometry was quantified by the pipette method.Values were determined for the pH in H 2 O, and the Ca, Mg, Al, Na, K, assimilable P and H+Al content (DONAGEMMA et al., 2011).The assimilable P content was also extracted with a 0.5 mol L -1 Na 2 CO 3 solution at pH 8.5 and quantified by colorimetry from the color intensity of the phosphomolybdic complex as per Olsen et al. (1954).To quantify the levels of total organic carbon (TOC), K 2 Cr 2 O 7 was used as an oxidizing agent in an acid medium (H 2 SO 4 ), titrating with ammonium ferrous sulfate (YEOMANS; BREMNER, Pedogenesis in a topo-climosequence in the Agreste region of Pernambuco  (DONAGEMMA et al., 2011), where the samples of ADFE were solubilized in 1:1 H 2 SO 4 , heated and then filtered.From the filtered extract, Fe 2 O 3 and Al 2 O 3 were quantified by complexometry with EDTA and CDTA respectively, and TiO 2 by the peroxidation of titanium sulfate to persulfate with H 2 O 2 , determined by spectrophotometry.The SiO 2 content was obtained by solubilization of the residue in NaOH and quantified by colorimetry from the color intensity of the silicomolybdic complex.From the results, the ki and kr indices were calculated.The micromorphological characterization was performed on undisturbed samples, impregnated with polyester resin to make up thin slides.The slides were then observed under a petrographic microscope, and a micropedological description made as per Bullock (1985).From the morphological, physical and chemical attributes, the profiles were classified according to the Brazilian System of Soil Classification (SANTOS et al., 2013).

RESULTS AND DISCUSSION
In general, each profile along the topoclimosequence shows a sandy texture and a poorly developed structure.The profiles at the highest points in the landscape, under the influence of a climate with greater rainfall, show low base saturation, whereas on the pediplanation surface, the semi-arid climate favors greater base saturation in P4, determined by the lower leaching rate.The profiles presented a sequence of horizons: Ap1, Ap2, C1 and C2, in the profile located in the high-altitude forest (P1); Ap, BA, Bt1 and 2Bt2 in the profile between forest and pediplane (P2); and on the pediplanation surface: Ap, C1, C2 and C3 in P3, and Ap, E, Bt1, Bt2 and Bt3 in P4.
As for the morphological attributes, each horizon in profile P1 and P3 has a single-grain structure ( The subsurface horizons of P2 have a subangular blocky structure, while a prismatic structure, composed of angular and subangular blocks, was seen in P4. The main factor in the occurrence of a single-grain structure, are the high levels of the sand fraction in the soil, as evidenced by the presence of the sand and loamysand textural classes, together with a predominance of the coarse-sand fraction and consequently, a low level of mineral colloids, which hinders the formation of soil aggregates.The pattern of a single-grain structure or low degree of development, was also seen by Santos et al. (2012) when studying the Neossolos Regolíticos (Regosols) of the semi-arid region of Pernambuco, where there was a predominance of the loamy-sand textural class in all of the profiles under study.For blocky and prismatic structures, the principal ratio is due to the higher clay content in the subsurface.
Each horizon of the profiles under study present colors of a yellow hue (10YR), indicating the presence of goethite, whose formation is favored by the low iron content of the parent material (KÄMPF; CURI, 2000;MEDEIROS et al., 2013).
The moist color in the surface horizons showed a predominant value of 3. The same pattern was also seen in most of the subsurface horizons, with the exception of the 2Bt2 (P2), C3 (P3) and Bt3 (P4) horizons.The predominance of this darkened color, even in the subsurface, is associated with the presence of goethite, which may present yellowish tones despite low levels of organic carbon.
By analyzing the physical attributes of the soil, a sand content greater than 700 g kg -1 was seen in most of the horizons of the profiles under study, except in 2Bt2 in P2, C3 in P3, and Bt1, Bt2 and Bt3 in P4 (Table 3).Low values were seen for the fine sand/coarse sand ratio (FS/CS), indicating the predominance of coarser particles in the sand fraction.In general, the soils have a sandy texture due to the nature of the parent material, acidic rocks of the Garanhuns Group, with a phanerocrystalline structure and the predominance of the minerals quartz and feldspar.In addition, the low rate of weathering of the alterable primary minerals, which is determined by the climate, does not favor the formation of clay minerals.In the P2 profile, a sharp change can be seen in the FS/CS ratio between the Bt1 and 2Bt2 horizons, suggesting the occurrence of lithological discontinuity (NOVAES FILHO et al., 2012).The low values for the silt/clay ratio in the subsurface horizons of P4 may be a reflection of the processes of deposition and accumulation of pre-weathered sediments in the highest parts of the landscape, considering that the current conditions of imperfect drainage and the low rainfall in the area (500-800 mm) are unfavorable to the weathering process.
There was great variation in the chemical attributes of the profiles under study (Table 4).In general, an increase can be seen in the values for pH, sum of bases (S), and assimilable P on the downward slope.
The values for pH ranged from 3.44 in the P1 profile (high-altitude forest), to 7.26 in P4 (pediplanation surface), being directly related to the increase in the value of S for the same direction in the landscape, which varied from 1.33 (P1) to 13.25 (P4).In each profile, the values for Mg were higher than for Ca.This pattern was also seen in other profiles in the State of Pernambuco, formed from gneiss with a coating of quartz (JACOMINE et al., 1972).
The P1, P2 and P3 profiles had a base saturation of less than 50%, a result of their dystrophic character (SANTOS et al., 2013), followed by the S values, which ranged from 0.77 to 3.11, and the aluminum content of 0.4 at 1.1 cmol c .kg -1 .The P4 profile had a base saturation of more than 50%, resulting from its eutrophic character (SANTOS et al., 2013), associated with higher values for Ca, Mg and K when compared to the other profiles.The predominantly sandy texture, which determines a low capacity for cation retention, together with the greater runoff favored by the relief in the higher parts of the landscape, intensifies base leaching, which explains the low Ca, Mg and K content of P1, P2 and P3.The occurrence of dystrophic soils at the highest points, and eutrophic soils at the lowest, was also seen by several authors (CAMPOS et al., 2010, 2011, MEIRELES et al., 2012), when studying the soil to landscape relationship.According to Anjos et al. (1998), this pattern of a greater sum of bases at the lowest points is defined by the soil to landscape relationship, which controls the behavior of the water flow, as well as the processes for the removal and accumulation of cations.
Higher values for Na were found in the subsurface horizons of P4, where the semi-arid climate conditions favor the addition of Na to the soil by capillary rise from the groundwater, with the desorption of other cations in the sortive complex and the consequent increase in Na values (GONÇALVES; MARTINS; RAMOS, 2015).
The levels of assimilable P vary from 0 to 88 mg kg -1 , with the highest values in P4 compared to the other profiles.However, due to the high pH values in this profile, quantifying the P values by the Olsen method is recommended, where the levels varied from 3 to 52 mg kg -1 .In this case, the use of Mehlich-1 solution may overestimate the assimilable P content, considering that the dilute acid solution dissociates the little-soluble forms of phosphate (CORRÊA et al, 2008).
The total organic carbon content (TOC) is influenced by the low deposition of organic material (1) H: horizon (2) PMeh: phosphorous determinad by Mehlich 1.
(3) POls: phosphorous determinad by Olsen (1954) gives it greater structural stability and resistance to weathering, with a consequently lower dissolution rate, thereby influencing the Si/Al ratio (LIU et al., 2016).
According to the SiBCS [Brazilian System of Soil Classification] (SANTOS et al., 2013), the P1 and P3 profiles present a moderate surface A horizon with the absence of subsurface diagnostic horizons, characterized by the weak action of pedogenic processes.They have a base saturation (V%) of less than 50% and a predominantly sandy texture down to a depth of over 100 cm, basically composed of quartz and feldspar.Together, these attributes classify the profiles as Neossolos Regolítico Distrófico típico (Regosols).
The P2 profile has a B/A textural ratio equal to 2.35, characterizing a significant increase in the clay content in the subsurface, giving rise to the subsurface textural diagnostic B horizon.The surface horizon was classified as moderate A; in addition, the profile displays a predominant yellowish color (hue 10 YR) and low base saturation.Despite the presence of high-activity clay in the Bt horizon, the dystrophic character allows it to be classified as a Argissolo Amarelo Distrófico típico (Alisol).on the surface of P3 and P4; however for P1 and P2, the forest results in a greater contribution; despite this, TOC levels were lower than 10 g kg -1 in each profile, decreasing with depth, except for horizon C2 in P1, which displayed an increase in TOC content.This result for C2 in P1 can be explained by the loamysand texture throughout the profile, which helps the plant root system to develop to greater depths, with the translocation of organic material.
As for the weathering complex, all the profiles displayed a low Fe 2 O 3 content (Table 5), due to the nature of the parent material that has low levels of this element.Despite the silt/clay ratio indicating an advanced stage of weathering, values for the ki index (silica/aluminum ratio) were greater than 2.25, even reaching 3.89 in P4; however, this is due to the nature of the parent material and the low weathering determined by the climate factor, which favors the highest levels of SiO 2 through the weathering of the feldspars present in the sand fraction and the lesser removal of silica.The greater values for SiO 2 present in the solution after weathering of the primary minerals explain why high activity clay was seen in all the profiles.Furthermore, the quartz present in the parent material The P4 profile showed abrupt textural change and yellowish colors with low subsurface saturation, reflecting its low permeability due to the imperfect drainage, thereby characterizing the planic subsurface diagnostic B horizon.This horizon presents high-activity clay, indicating the presence of type 2:1 clay minerals.In addition, the profile shows a high base saturation and a sandy texture from the surface to the start of the planic B horizon, which occurs at a depth of 50 cm.These attributes together classify the profile as an Planossolo Háplico Eutrófico arênico (Planosol).
With the micromorphological analysis, it was possible to find typical features from the action of different pedogenic processes.In general, there was a predominance of quartz grains in the coarse fraction, coated in clay and organic matter.Alteration of the mineral feldspar was also found (Figure 2), in its turn, releasing silica, which explains the higher SiO 2 content of the weathering complex and consequently the high values of the ki index.
The Neossolos Regolíticos (Regosols) (P1 and P3), in spite of the non-aggregated single-grain structure seen in the field evaluation, show evidence of the incipient action of pedality in the micromorphological analysis, where a granular microstructure with the development of a few blocks was seen.In both profiles, organization of the subsurface pores was of the impacted type with a few fissures.
In the P1 profile, coating of the coarse fraction by clay particles and organic colloids was seen (Figure 3), which in turn act by uniting the primary particles, explaining the occurrence of an incipient blocky structure.In addition, these coatings of organic material in the subsurface may explain the increase in the TOC content of the subsurface relative to the overlying horizons, where its mobilization may have been facilitated by the predominantly thick texture.
The C2 horizon in the P3 profile displayed features similar to P1, with incipient development of a blocky structure (Figure 4), which is due to the presence of biological features (Figure 5) that aid in cementing and arranging the particles on a microscopic scale.
in the quartz can be seen with coating and filling by the organic matter.Another pedofeature was the occurrence of degraded iron micronodules (Figure 9), which is associated with the drainage conditions of the profile.
Pedogenesis along the topo-climosequence is strongly influenced by the felsic parent material and its interaction with the climate, determining low rates of weathering, which can be evidenced by the high amount of feldspars, contributing to the predominance of a very sandy texture.This texture, in turn, affords a slight capacity for cation retention, which together with the predominance of coarse sand, intensifies the leaching process.Variations in relief are responsible for the drainage conditions and the In the 2Bt2 horizon of the P2 profile, there is a predominance of quartz grains, distributed in a dense mass of fine material (porphyritic pattern) and organized in a granular microstructure and in blocks.The main pedofeatures seen were the abundant clay coating and dense clay filling of the cavitary porosity (waxiness) (Figure 6 and Figure 7).
In Bt3 in the P4 profile, there was also the dominance of quartz grains with a porphyric distribution pattern.In this horizon, the formation of angular and subangular blocks can be seen between the macropores, while microporosity is responsible for the formation of microfissures that give rise to the prismatic structure (Figure 8).Alterations Pedogenesis in a topo-climosequence in the Agreste region of Pernambuco surface runoff, altering the chemical attributes of the soil and the action of pedogenic processes, favored not only by removal and accumulation but also by cycles of wetting and drying.In addition, the climate factor, which favors the occurrence of greater rainfall in the higher parts of the landscape, intensifies leaching and surface runoff, with subsequent base accumulation in the pediplane.
Among the more active specific pedogenic processes, most important are eluviation/illuviation and elutriation, in the formation of the textural B horizon in P2; lessivage and leucinization, favoring the formation of the E horizon; and ferrolysis in the formation of the planic B horizon and E horizon in P4.The P1 and P3 profiles are characterized by the low-intensity action of the pedogenic processes, showing no subsurface diagnostic horizon;  Formation of the P2 profile is due to the action of the pedogenic processes of eluviation/illuviation and elutriation.In the first, translocation of clay particles takes place, with water as the transport agent, a process facilitated by the sandy texture, with the predominance of coarse sand determined by the parent material, and the subsequent accumulation of these clay particles in the subsurface (FANNING;FANNING, 1989).From the micromorphological evaluation of the 2Bt2 horizon, clay coating and filling were found in the pores, confirming the occurrence of translocation.In the second process, a position in the relief with a greater slope and a weak aggregation of the surface horizon facilitates the action

Figure 1 -
Figure 1 -Location of the study area in the Agreste region of Pernambuco

F
i g u r e 9 -Textural pedological feature of degraded iron micronodules (G).Bt3 horizon, P4 profile however, as seen in the micromorphological analysis, incipient action of the process of pedality can be seen.

Table 1 -
General information on the topo-climosequence in the district of Jurema in the State of Pernambuco

Table 2 )
, just like the surface horizons of the P2 and P4 profiles.

Table 2 -
Morphological attributes of soils of a topo-climosequence in the district of Jurema in the State of Pernambuco

Table 3 -
Physical attributes of the soils of a topo-climosequence in the district of Jurema in the State of Pernambuco Pedogenesis in a topo-climosequence in the Agreste region of Pernambuco

Table 4 -
Chemical attributes of the soils of a topo-climosequence in the district of Jurema in State of Pernambuco

Table 5 -
Total silicon, iron, aluminum and titanium content by sulfuric attack