Pedon Characterization in the Caatinga Biome Area in Chapada Apodi, RN, Brazil

The Apodi Plateau is a Brazilian with important agricultural activity, which is located between the states of Ceará and Rio Grande do Norte, in an area of the Caatinga biome. Therefore, this work was developed to characterize the morphological, physical, chemical and mineralogical attributes of soil profiles of this region. For this purpose, trenches were open in three areas in the Tabuleiro Grande Settlement Project, in the municipality of Apodi, RN, Brazil. The profiles were described in the field and samples of the horizons were collected for physical, chemical and mineralogical analyzes. The profiles were classified up to the fourth categorical level and correlated with Soil Taxonomy.According to the Brazilian Soil Classification System, the soils studied were Vertissolo Háplico Órtico chernossólico (Mollisols), Chernossolo Rêndzico Petrocálcico típico (Mollisols) e Vertissolo Háplico Órtico chernossólico (Vertisols).

According to Sekhar, Naidu, Ramprakash, and Balaguravaiah (2019), soil is non-renewable and finite natural resource, undergoing degradation at preoccupying levels due to indiscriminate use and lack of adequate management. For its best management are necessary Soil Survey investigations that point its limitations and potentialities. For this, the first step is soil characterization and classification. In this context, there is no sufficient information about soils occurring at the Apodi Plateau. Hence, the present study aims to characterize morphological, physical, chemical and mineralogical attributes, and classify soils in Caatinga biome area of the Apodi Plateau, Rio Grande do Norte, Brazil.

Method
The area under study belongs to the Tabuleiro Grande Settlement Project, located in the municipality of Apodi, Midwest of the state of Rio Grande do Norte (5°24′33.33″S and 37°46′40″W), with medium elevation of 109 meters. It is inserted in the Chapada do Apodi microregion, predominantly represented by the limestones of the Jandaíra Formation at the top and the sandstones of the Açu Formation, located at the base (Pessoa Neto et al., 2007).
The predominant climate of the region is of the BSh type (dry semiarid, with annual potential evapotranspiration greater than annual precipitation), according to the Köppen classification. Average annual rainfall ranges from 380 to 760 mm, with a general average of 600 mm; the average annual air temperature is 28° C and the average annual relative humidity is approximately 65% (Alvarez, Stape, Sentelhas, Gonçalves, & Spavorek, 2013). The local vegetation is of the hyperxerophilous Caatinga type, associated with secondary forest formations with varied herbaceous substrate, whose native plant diversity is represented basically by Caesalpinia bracteosa Tul., Aspidorperma pyrifolium Mart., Croton argyrophylloides Mull. Arg., Croton blanchetianus Baill, Combretum leprosum Mart., Mimosa caesalpiniifolia Benth, and Auxemma oncocalyx (Fr. All.) Baill.
The definition of the studied areas and the opening of profiles were based on reconnaissance expeditions throughout the area of the Tabuleiro Grande Settlement Project and the observation of the most representative pedoforms of the region. Thus, three locations for trench opening were selected (Table 1).
The morphological descriptions and sample collection of the profiles were performed according to the recommendations of Santos, Lemos, Santos, Ker, Anjos, and Shimizu (2015). The soil samples collected in each horizon were air dried, grinded and sieved with a 2 mm mesh opening, obtaining the air-dried fine earth (ADFE) to perform the physical, chemical and mineralogical analyzes in each horizon of profiles.
The nomenclature of the diagnostic horizons and the taxonomic classification of soils up to the 4th categorical level were made according to the Brazilian Soil Classification System (Santos et al., 2018). Physical and chemical analyzes were performed with three repetitions, according to methods described by (Teixeira, Donagema, Fontana, & Teixeira, 2017). In addition to the particle size analysis by the pipette method, the following characteristics were determined: pH in water; electrical conductivity (EC), measured in soil-water solution with conductivity meter; available phosphorus extracted by Mehlich-1 solution and determined by spectrophotometry; exchangeable potassium and sodium cations, extracted with Mehlich-1 solution, and calcium and magnesium extracted with KCl solution 1 mol L -1 ; exchangeable aluminum (Al 3+ ) extracted by the KCl solution 1 mol L -1 ; potential acidity (H + Al) extracted with calcium acetate solution 0.5 mol L -1 . Total organic carbon (TOC) was determined using the method proposed by Yeomans & Bremner (1988), determined by the oxidation of organic matter by potassium dichromate. From the analyzes performed the following indexes were obtained: cation exchange capacity at pH 7.0 (T); base saturation (V), exchangeable aluminum saturation (m) and exchangeable sodium percentage (ESP).
For the mineralogical characterization, the ADFE was used from the diagnostic horizons of each soil profile, which were ground to powder. The identification of the mineral phases was performed by X-ray diffraction (XRD), using a SHIMADZU diffractometer, model XRD-6100, using Cu kα1 emission. The source potential was 40 kV and the current 30 mA. A sweep rate with a step of 0.02° every second was applied. The sweep range (2θ) was from 5 to 70°. The identification of the peaks was performed with the aid of the program 'Raio X v. 1.0.0.37', and the minerals were identified according to Chen (1977).
The obtained data were analyzed by Pearson linear correlation between the physical and chemical attributes of the evaluated profiles. In addition, multivariate analysis was used as a tool for soil profile distinction, and the Principal Component Analysis (PCA) procedure was applied using STATISTICA 7.0 software.
When analyzing the morphological descriptions, a strong structure degree is observed in all profiles except P2 (MDlk), which has a moderate structure degree ( Table 2). The structure of P2 is of granular type along the profile with sizes ranging from small to very small. The profiles P1 (VXo) and P3 (VXo) presented similarities in the structure type, with granular structure in the superficial horizon contrasting with blocks and prisms in the subsurface horizons. This type of feature, typical of the Vertisols, is called self-granulation, caused due to the presence of cracks in the profile. The cracks were observed in the subsurface horizons of profile P1 (VXo), measuring up to 3.0 cm wide, and in the horizons A and Bv1 of profile P2 (MDlk), with 2.0 cm width. Similar results were obtained by Lima (2014), regarding the degree and type of structure in the surface and subsurface horizons in Haplic Vertisols inserted in calcareous regions in the municipalities of Juazeiro and Terra Nova in the state of Bahia.Profiles P1 (VXo) and P3 (VXo) showed abundant friction surfaces (slickensides) on horizons Bv1 and Bv2 (P1) and horizon Bv2 (P3), demonstrating the strong expandability of the clay material of these profiles. The color of the profiles varied between dark brown on the surface horizons and yellowish brown on the subsurface horizons (Table 2). In the Cr horizons of the P2 (MDlk) and P3 (VXo) profiles, light gray (P2) and yellow (P3) coloration were observed. The dark color in the Vertisols is due to the presence of organic constituents, mainly in the superficial horizons (Dudal & Eswaran, 1988). In addition to these constituents, the 2:1 clay mineral smectite also influences the dark coloration of these soils.
Soil consistency (Table 2) in P1 (VXo) and P3 (VXo) profiles is predominantly very hard, when dry, and varies from friable, firm to extremely firm, when wet. The fact that the consistency is plastic and sticky when wet can be explained by the presence of expansive 2:1 clays, typic of Vertisols. P2 (MDlk) has a soft consistency when dry, very friable when wet, and slightly plastic and sticky when wet. Similar results were obtained by Melo et al. (2017) in relation to the surface and subsurface horizon consistency of a Rendzic Chernosol in the state of Rio Grande do Norte, derived from material of limestone origin.
Clay and silt contents (Table 3) in Vertisols ranged from 76 to 273 g kg -1 and 114 to 677 g kg -1 , respectively. Sand contents ranged from 226 to 665 g kg -1 and, in general, the fine sand fraction predominated over coarse sand, which is a positive aspect for moisture retention. The clay content was considered low for Vertisols (P1 and P3), which may be due to the climate of the region, which caused less intense weathering and provided larger amounts of sand. However, clay contents ranging from 547 to 710 g kg -1 were observed in Haplic Vertisols of the island of Fernando de Noronha, located in areas of quaternary deposits composed of calcareous, psammitic and pellitic sediments (Marques et al., 2014).  Different soil textural classes were observed in soil profiles and horizons (Table 3). The sandy-clay-loam texture predominated in surface horizons of profiles P1 (VXo) and P3 (VXo). It was also observed the sandy-loam texture in the subsurface horizon of P1, in addition to the clay-loam and loam in subsurface horizons in P3. In the P2 profile (MDlk), the silt-loam texture was observed in the superficial horizon and the clay-loam in the Crk horizon. In this profile, the silt fraction predominated, which indicates little weathering activity. The silty-loam texture and high silt contents were observed in the surface horizon of a Rendzic Chernosol by Melo et al. (2017). The silt/clay ratio, which indicate the degree of soil development, ranged from 0.52 to 1.44 in the P1 (VXo) and P3 (VXo) profiles, with higher values in subsurface horizons. In the P2 profile (MDlk), this ratio was higher in surface and decreased with depth.
Regarding chemical attributes (Table 4), the three profiles (P1, P2 and P3) presented values close to pH in water, ranging from 7.77 to 8.27 in surface and subsurface horizons. These values identify the parent material of these soils, limestone, and characterize more alkaline soils. The low EC values in the studied profiles, between 0.11 and 0.26 dS m -1 (Table 4), indicate that the salt levels in the soil solution are not high.
The TOC values ranged from 22.5 to 26.2 g kg -1 in all horizons of the three soil profiles (    Corrêa et al. (2003) in Vertisols originated from calciferous clay and sandstone in the Lowlands of Sousa, PB, Brazil. Meanwhile, the low Na + and K + contents in all horizons of the three soil profiles reflect the low amount of these elements in the parent material.
The P content in profile 2 (MDlk) ranged from 0.83 to 1.29 mg kg -1 (Table 4), which is considered low compared to typical soils of this class, which generally have high fertility and between 3 and 20 mg kg -1 of P. The P1 (VXo) and P3 (VXo) profiles also presented low P values (1.31 to 3.19 mg kg -1 ), determined by the ion exchange resin method. In this sense, Silveira, Bezerra, De Sá, and Valadares (2006) report low P extraction by the resin method in different soils of the semiarid region of Paraíba and Pernambuco states, including Vertisols.
Significant linear correlations were observed between the physical and chemical attributes of the studied soil profiles (Table 5). Among them is the positive relationship between Coarse Sand (CS) and K + (0.93). This indicates that the weathering process is not very pronounced in the studied area, since the high K + contents are directly related to areas with a sandy texture, which determines the gradual release of this nutrient to the soil system. The same behavior was observed for Ca 2+ , whose correlation with CS is 0.79. The significant correlation between Mg 2+ and Fine Sand (FS) can be interpreted in the same way as that of K + and Ca 2+ with sand. The three elements mentioned are those basically made available to the soil system by the process of hydrolysis and breakage of primary minerals present in the parent material exposed to the pedogenetic process. The other correlations between the attributes confirmed the expected behavior for these soils.   The ordering diagram resulting from the PCA (Figure 1) presents distinct groups between the horizons of the evaluated soil profiles. According to SiBCS (Santos et al., 2018) there are two soil classes diagnosed in the studied area, and the distinction between them is confirmed by the PCA. In this respect, the horizons of Vertisol (P1 and P3) form a single group in the ordering diagram, except for horizon A of profile P3, which differs from the others. The other group formed includes the horizons of the P2 profile, which is classified as Chernosol. This grouping confirms that the physical and chemical attributes of these two soil classes are distinct, with a cumulative total variance of 72.36% (Table 6).

Figure 1.
Note. Grou igure 2) and t most influenc tributes that m en this result i her than that of uential attribut ESP, being P2 the only one that presented higher levels of sodium throughout the profile and consequent higher risk of sodicity. The formation of Group 3 (red color) was more influenced by the attributes P, T, FS, Mg 2+ and V. This shows that the chemical attributes were more important for distinguishing this group from the others. It can also be observed in the P2 and P3 profiles the highest P, and Mg 2+ content, higher T value, and V values close to 100%, which determine eutrophic character. The FS also stands out in the distinction for presenting higher values in P1 and P3 when compared to P2. These findings help in the interpretation of the landscape, as it is possible to separate the most influential physical and chemical attributes in each soil class studied. This is fundamental for evaluations of practical implications in Vertisols and Chernosols in the Apodi Plateau region, as these attributes become indicators of these soils, making possible a faster and more accurate diagnosis of the attributes that may be limiting for agricultural production or that can potentiate such activity.
X-ray diffractometry (XRD) (Figure 3) showed that the mineralogical composition in the ADFE of the diagnostic horizons of the soils studied is mainly quartz, followed by the illite mineral. There is also the occurrence of calcite and montmorillonite. In studies of J. C. A. Mota, Assis Júnior, Amaro Filho, Ilho, Romero, F. O. B. Mota, and Libardi (2007), in the Apodi Plateau, quartz minerals predominate over other constituents, which is due to its being extremely resistant to weathering.
The illite, a 2:1 clay mineral originated from the mica transformation, was identified in all evaluated horizons of the three profiles. This is due to the calcareous parent material in which micaceous minerals are common (Kampf & Curi, 2003). The presence of illite was also observed in a study by Oliveira et al. (1998) in soils developed from limestone of the Bambuí Group (MG, Brazil), in addition to small amounts of other expansive clay minerals.
The horizons Ak and Bv2 of profiles P2 and P3, respectively, presented in common the calcite (Figures 3A and  3B). The occurrence of this mineral is due to the nature of the parent material, limestone, which is formed from the calcite mineral. Calcite was also found by Lemos, Curi, Marques, and Sobrinho (1997) on the fine sand fraction of soil profiles of the Apodi Plateau.
In addition to the minerals already mentioned, montmorillonite ( Figure 3C), a partially expansive 2:1 mineral, was found on horizon Bv2 of profile P3. This indicates a soil with less weathered minerals due to the climate of the region. This same clay mineral was identified by Lima et al. (2015) in the clay fraction of a Vertisol originated from clay shales and limestones in the Santo Amaro region, BA, Brazil. The formation of more