Micromorphological Features of Carbonate Soils Threatened by Desertification in Northeastern Brazil

Abstract

Desertification is a major environmental problem in drylands, resulting from the interaction between climatic and anthropogenic factors and involving processes such as vegetation loss and soil erosion. In northeastern Brazil, long-term land-use pressure has intensified the vulnerability of semi-arid landscapes, leading to severe degradation and reduced environmental resilience. Within this context, the Salitre River Basin (Bahia State) represents a particularly relevant area for investigating desertification, as it combines a history of intense human pressure and soil degradation with arid climatic conditions recently recognized through climatological assessments. This study investigates the micromorphological organization, hillslope dynamics, and environmental degradation of carbonate soils of the Salitre river basin, aiming to reconstruct pedogeomorphological evolution and identify indicators of desertification. A toposequence-based approach was applied using ten soil profiles distributed across sectors with contrasting erosion intensity. Soil horizons were characterized through field descriptions, physical and chemical analyses, and micromorphological observations of thin sections. Soils are predominantly eutrophic, carbonate-rich, shallow Calcisols, with very high CaCO₃ contents (500–900 g kg⁻¹), alkaline pH, and low total organic carbon. The spatial organization of profiles results from strong lithological and geomorphological controls, with well-developed horizons in stable sectors, whereas truncated and homogeneous profiles occur in areas affected by severe erosion. Micromorphological features, including planar and moldic voids, Fe–Mn nodules, calcite coatings, and needle calcite infillings, indicate active carbonate dissolution/redistribution processes under increasing seasonality and aridity (calcification). The coexistence of inherited dissolution and recent precipitation features suggests polyphasic pedogenesis linked to Holocene climatic oscillations. The preservation of well-developed needle-fiber calcite is consistent with prolonged water-deficit conditions, although its precise chronological significance remains unconstrained in the absence of direct dating. At landscape scale, these processes are associated with hillslope retreat and dense networks of linear erosion, defining a scenario of severe land degradation.

1. Introduction

Desertification is recognized as one of the most severe environmental problems of the present time, with its areas of occurrence expanding across different regions of the globe [1]. Although multiple definitions have been proposed, the most widely adopted is that of the United Nations Convention to Combat Desertification, which defines the phenomenon as land degradation in arid, semi-arid, and dry sub-humid regions, resulting from both climatic and anthropogenic factors [2]. This concept has been extensively used in several studies [3,4,5,6,7,8,9,10,11,12,13]. Yet, the concept remains complex, as it encompasses multiple degradation processes, often irreversible in nature [4,14], reinforcing the need for integrated approaches that consider physical, biological, and socio-economic dimensions [15,16]. The interplay between climatic constraints and human activities, such as deforestation and inadequate land management, has been identified as the main driver of the process [17,18,19].

In Brazil, the semi-arid Northeast is considered the most vulnerable region to desertification, with 59% of its area classified as moderately susceptible, 35% highly susceptible, and only 6% at low susceptibility to desertification [11]. The interplay between climatic constraints and human activities, such as deforestation and inadequate land management, has been identified as the main drivers of the process. Areas of extreme degradation have been identified as Desertification Nuclei [20,21], equivalent to the internationally recognized “desertification hotspots” [22,23]. Soil erosion, in particular, is regarded as the most recurrent and irreversible indicator of desertification, affecting soil fertility, water retention capacity, and productivity [24,25,26].

In the semi-arid region of Bahia, the Salitre River Basin exemplifies a highly degraded landscape, characterized by intense soil erosion, loss of vegetation cover, and declining rainfall, with an estimated removal of approximately 450,000 m³ of soil over 2000 hectares [27]. Land degradation in this basin is marked by extremely high soil loss rates, averaging 71 Mg ha⁻¹ yr⁻¹—up to 20 times greater than soil formation, and is primarily driven by anthropogenic activities [28]. In addition, soils developed over colluvium, calcareous tuffs, and calcretes exhibit calcic horizons and inherent chemical and physical fragilities, which further intensify degradation processes and reduce ecosystem resilience [29].

At the international level, carbonate-rich soils and pedogenic carbonates have received considerable attention in arid and semi-arid regions because their forms, distribution, and degree of development provide important information on soil-forming processes and environmental conditions [30,31]. Carbonate dissolution, redistribution, and precipitation are strongly controlled by soil-water dynamics, biological activity, and climatic conditions, making pedogenic carbonates valuable indicators of both contemporary pedogenesis and past environmental changes [31,32,33,34]. Micromorphological analysis is particularly useful in this context because it allows the recognition of calcitic pedofeatures and their spatial relationships within the soil matrix, providing evidence of successive phases of carbonate mobilization and precipitation that may not be identified through macromorphological observations alone [32,35]. Studies conducted in different environmental settings have consequently demonstrated the potential of micromorphology to reconstruct the evolution of carbonate-rich soils and to distinguish inherited features from those associated with more recent pedogenic conditions.

Despite these advances, the relationship between microscale evidence of carbonate pedogenesis and hillslope-scale degradation remains less explored, particularly in landscapes undergoing intense erosion and desertification. Pedogenic carbonate features have been extensively used as indicators of soil-forming conditions and paleoenvironmental change, whereas their integration with soil-profile truncation, sediment redistribution, and contemporary hillslope dynamics has received comparatively less attention. Bridging these scales is particularly important in severely degraded drylands, where the present organization of soil profiles may reflect both long-term pedogenic processes and more recent erosional reworking.

Within this broader research context, the Salitre River Basin provides a particularly suitable setting for addressing this question because carbonate-rich soils occur along hillslopes affected by contrasting degrees of erosion and severe land degradation. Previous studies in the basin have characterized these soils through macromorphological, physical, and chemical attributes, but their micromorphological organization and its relationship with hillslope dynamics remain poorly understood. Soil micromorphology, by allowing the identification of micro-features and arrangements that precede or are not readily visible through field-scale observations, provides a means of linking pedogenetic dynamics to landscape evolution in arid environments [36].

Therefore, the novelty of this study lies in integrating micromorphological evidence with soil morphology, physical and chemical properties, erosion intensity, and landscape position along a toposequence. This multiscale approach allows carbonate pedogenic features observed at the microscopic scale to be related to soil-profile organization, truncation, and hillslope dynamics. Accordingly, this study aims to identify micromorphological indicators capable of explaining the severe soil degradation observed in the Salitre Basin and to relate them to the recently recognized aridity indices in Brazil, thereby contributing to a broader understanding of the interactions among carbonate pedogenesis, erosion, landscape evolution, and desertification in drylands.

2. Materials and Methods

2.1. Study Area

The Salitre River Basin is located in the semi-arid region of Bahia state, northeastern Brazil, and forms part of the middle course of the São Francisco River sub-basin (Figure 1). It covers an area of 14,452 km², extending from its headwaters in the Chapada Diamantina (Morro do Chapéu) to its confluence with the São Francisco River at Juazeiro municipality [37].

The study area is situated in the middle course, within a valley-floor depression between the rural communities of Salgadinho and Abreus, in the Campo Formoso municipality. This sector is characterized by low population density and a landscape marked by exposed soils and widespread erosional features [38,39].

The relief comprises two main geomorphological units: marginal mountainous areas developed over metasedimentary rocks of the Chapada Diamantina Group, and a central flattened area known as the carbonate plateau, dissected by the fluvial incision of the Salitre River [40,41].

Geologically, the basin lies on the northern portion of the São Francisco Craton, encompassing Archean, Proterozoic, and Tertiary–Quaternary units [42,43]. The study area is dominated by the Caatinga Formation, composed of carbonate substrates, mainly calcretes (petrocalcic materials) formed through pedogenetic processes and watertable fluctuations [41,44,45,46]. These materials give rise to poorly developed soils, predominantly eutrophic Haplic Cambisols, Litholic Neosols, and Planosols [47], generally shallow, calcium-rich, and with incipient pedogenesis, strongly influenced by the lithological substrate [29,48,49,50].

The climate is classified as hot semi-arid (BSh, Köppen–Geiger), with mean annual rainfall ranging between 300 and 500 mm, concentrated mainly between November and March, with the highest rainfall generally occurring from December to February, highly irregular in distribution, and interspersed with long dry periods. Average annual temperature is about 27 °C, while potential evapotranspiration exceeds 1800 mm year⁻¹, resulting in the absence of water surplus [51,52,53,54]. The natural vegetation is predominantly hyperxerophilous Caatinga, a deciduous thorny shrub-tree formation, with high floristic diversity recorded in the region [48,55]. The hydrography is characterized by intermittency: most streams are ephemeral and dry up during the annual drought, with only a few perennial segments, such as in the Pacuí River, while the Salitre River itself presents very low average discharge, often reaching zero flow for extended periods [37,50,56].

The Salitre River Basin exhibits an advanced stage of land degradation driven by intense linear erosion processes, expressed by a high density and connectivity of rills and gullies across the landscape. According to the mapping performed [27], a total of 734 linear erosion features (LEFs) occur within an area of 20.78 km², forming a complex dendritic network with a cumulative length of 145.53 km. These features range from 14 m to 1788 m in length, with a mean of 199 m, indicating both localized and highly developed erosional systems. Approximately 71% of the mapped features are shorter than 200 m, while only 3% exceed 800 m, reflecting a predominance of small but numerous erosive channels [27]. As a consequence, the landscape is profoundly degraded (Figure 2), with accelerated soil loss that severely constrains agricultural activities and compromises the livelihoods of local populations. In addition, intense sediment transfer promotes significant siltation of the river channel and contributes to the advanced degradation of the Caatinga vegetation cover.

2.2. Sampling and Analysis

The sampling strategy was designed to encompass soils representative of sectors with contrasting incidence of linear erosional processes along a toposequence established on a flat valley-bottom surface extending toward the central channel of the Salitre River. The lateral organization of soil horizons was interpreted based on the vertical analysis of 10 soil profiles distributed along this sequence, allowing the identification of pedological variations associated with topographic position and degree of erosional dissection.

Based on the morphological analysis of these profiles, three distinct sectors were identified. differentiated by the organization of the soil cover in relation to the vertical distribution of horizons (Figure 3). Sector 1, located in the upper portion of the toposequence on a surface with an average slope of approximately 2%, comprises profiles with a well-developed sequence of horizons Ak–Bk–Cα (profiles P1 and P2). This sector corresponds to areas with the lowest incidence of concentrated erosion, showing a low density of linear erosional scars such as rills and gullies, although signs of sheet erosion are present.

Sector 2, corresponding to slope breaks with an average slope of approximately 20%, is characterized by the absence of Ak and Bk horizons, with profiles composed exclusively of Ck and Cα horizons (profiles P3, P4, and P9). This sector is strongly affected by linear erosion, with frequent occurrence of gullies organized in dendritic networks, associated with exposed roots, toppled shrubs, and intense removal of the soil surface.

Sector 3, located in the lower portion of the sequence, exhibits greater variability. Profiles either preserve remnants of Ak and Bk horizons or are composed solely of Ck and Cα horizons (profiles P5, P6, P7, P8, and P10). Similarly to Sector 2, this sector shows a high incidence of concentrated erosion features, with well-developed gully systems, exposed root systems and unstable vegetation cover.

Based on this framework, undisturbed samples were collected along the identified horizons throughout the sequence, summing up 19 samples in Kubiena boxes. These samples were used to prepare thin sections [57]. The thin sections were analyzed using a Zeiss Axioscope (Carl Zeiss Microscopy GmbH, Jena, Germany) petrographic microscope equipped with an integrated digital camera. The micromorphological descriptions followed concepts presented in the book “Guidelines for Analysis and Description of Soil and Regolith Thin Sections” [58,59]. Considering the sampled horizons, the following number of thin sections were described: Ak (P1, P2, and P5 profiles), Bk (P1, P2, P5 and P10 profiles), Ck and its subhorizons (P1 to P10 profiles), and Cα (P1, P6, P8 and P9 profiles).

In addition to micromorphological analyses, soil horizons were characterized in terms of their physical and chemical properties [29]. Soil morphological description and sampling were performed according to Anjos et al. [60], whereas horizon nomenclature and soil classification followed the World Reference Base for Soil Resources [61]. Particle-size analysis was performed by the pipette method [62], using sodium hexametaphosphate as a dispersant due to the carbonate-rich nature of the samples. Soil pH was determined potentiometrically in water and 1 mol L⁻¹ KCl, using a 1:2.5 soil-to-solution ratio. Total organic carbon (TOC) was determined by the Walkley–Black wet oxidation method, whereas CaCO₃ equivalent was determined by HCl attack followed by NaOH titration [63]. The pH difference (ΔpH) was calculated as pH(KCl)−pH(H₂O). Exchangeable Al³⁺, Ca²⁺, and Mg²⁺ were extracted with 1 mol L⁻¹ KCl, whereas K⁺ and Na⁺ were extracted with Mehlich-1 solution. The sum of bases (S) was calculated as the sum of exchangeable Ca²⁺, Mg²⁺, K⁺, and Na⁺. Potential cation exchange capacity (PCEC) was calculated as S + (H⁺ + Al³⁺), and base saturation (BS%) as (S/PCEC) × 100. Analyses were conducted at the IGC/UFMG and LASO/IMA laboratories. Statistical analysis was based on descriptive statistics, with arithmetic means and standard deviations calculated for the analyzed physical and chemical properties, grouped and presented by soil horizon.

3. Results

3.1. General Aspects and Soil Properties Along the Toposequence

The physical and chemical properties of the soil horizons along the toposequence are presented in

Table 1. Mean values and standard deviation (sd) of physical and chemical properties of soil horizons along the toposequence.

Horizon		Thickness	Gravel	Cs	Fs	Silt	Clay	pH H2O	pH KCl	ΔpH	Ca2+	Mg2+	K+	Na+	SB	Al3+	H+	PCEC	TArg	BS	P	CAEq	TOC
Ak	mean	4.00	43.67	343.67	163.33	213.00	295.33	8.30	7.37	−0.93	15.77	1.70	0.73	0.23	18.45	0.10	0.67	19.13	64.57	96.00	14.17	87.67	8.31
	sd	0.00	11.56	118.67	68.67	36.48	48.58	0.22	0.09	0.31	3.79	0.22	0.23	0.01	3.84	0.00	0.05	3.81	6.22	0.82	14.23	9.53	0.59
Bk	mean	54.88	77.50	305.50	169.38	190.75	334.63	8.31	7.04	−1.26	15.41	1.50	0.16	0.27	17.34	0.10	0.74	18.09	54.19	95.50	1.09	124.13	4.12
	sd	18.21	34.71	101.50	49.84	76.93	25.66	0.45	0.52	0.16	4.07	0.40	0.14	0.15	4.16	0.00	0.22	3.99	11.82	2.51	1.14	57.00	1.73
Ck	mean	100.83	244.25	223.00	132.67	378.33	266.00	8.57	7.55	−1.03	12.64	1.03	0.10	0.38	14.13	0.10	0.61	14.78	56.42	95.50	0.92	692.17	3.79
	sd	53.72	135.92	65.57	28.28	75.93	43.84	0.22	0.12	0.24	3.89	0.25	0.06	0.12	4.06	0.00	0.03	4.07	15.72	1.38	0.58	96.19	2.95
Cα	mean	53.83	332.67	146.33	131.83	560.33	161.33	8.62	7.73	−0.90	11.80	1.03	0.06	0.55	13.44	0.10	0.56	14.07	97.70	95.17	0.65	837.17	0.75
	sd	14.57	228.89	113.39	65.92	162.85	65.01	0.35	0.20	0.40	4.53	0.49	0.04	0.23	4.76	0.00	0.06	4.75	43.78	2.23	0.52	280.20	1.23

Horizon—soil horizon, Thickness—horizon thickness (cm), Gravel—coarse fragments (>2 mm) (g kg⁻¹), Cs—coarse sand (g kg⁻¹), Fs—fine sand (g kg⁻¹), Silt—silt fraction (g kg⁻¹), Clay—clay fraction (g kg⁻¹), pH H₂O—soil pH in water, pH KCl—soil pH in KCl solution, ΔpH—difference between pH H₂O and pH KCl, Ca²⁺—exchangeable calcium (cmolc kg⁻¹), Mg²⁺—exchangeable magnesium (cmolc kg⁻¹), K⁺—exchangeable potassium (cmolc kg⁻¹), Na⁺—exchangeable sodium (cmolc kg⁻¹), SB—sum of bases (cmolc kg⁻¹), Al³⁺—exchangeable aluminum (cmolc kg⁻¹), H⁺—potential acidity (cmolc kg⁻¹), PCEC—potential cation exchange capacity at pH 7 (cmolc kg⁻¹), TArg—cation exchange capacity of the clay fraction (cmolc kg⁻¹), BS—base saturation (%), P—available phosphorus (mg kg⁻¹), CAEq—calcium carbonate equivalent (g kg⁻¹), TOC—total organic carbon (g kg⁻¹).

. The Ak horizon occurs continuously in Sector 1, is absent in Sector 2, and appears discontinuously in Sector 3. It is a shallow horizon, with a mean thickness of 4 cm, displaying colors ranging from dark red to dark yellowish brown, and a sandy clay loam to clay loam texture, with weak structure composed of small to very small granular and subangular blocky aggregates. The material is predominantly loose, containing numerous rounded black nodules and rare carbonate fragments. Chemically (Table 1) it shows alkaline pH (8.30 ± 0.22), negative ΔpH (−0.93 ± 0.31) and a high potential cation exchange capacity at pH 7 (PCEC) (19.13 ± 3.81 cmolc kg⁻¹). The particle-size distribution is dominated by coarse sand (343.67 ± 118.67 g kg⁻¹), followed by clay (295.33 ± 48.58 g kg⁻¹) and silt (213.00 ± 36.48 g kg⁻¹). Available phosphorus reaches the highest values in the Ak horizon (14.17 ± 14.23 mg kg⁻¹), and TOC is also highest in this horizon (8.31 ± 0.59 g kg⁻¹).

The Bk horizon shows the same lateral distribution pattern as the Ak horizon, occurring continuously in Sector 1, absent in Sector 2, and discontinuous in Sector 3. It has a mean thickness of 54.88 ± 18.21 cm and displays moderate to strong subangular blocky structure. In Sector 1, although continuous, the horizon exhibits lateral variability, particularly in color. In the initial segment of the toposequence, it shows predominantly yellowish-brown colors, whereas toward the distal part of this sector, near the slope break, reddish to yellowish-red colors prevail, features that are also evident in satellite imagery. In terms of structure and other attributes, these lateral variants are similar, differing mainly by a higher quartz content in the reddish portions. It contains abundant black nodules and rare vertical features, with an increasing expression of carbonate-related features with depth. In physical terms, clay contents increase (334.63 ± 25.66 g kg⁻¹), as do gravels (77.50 ± 34.71 g kg⁻¹), while coarse sand decreases relative to the surface horizon. Soil pH remains alkaline (8.31 ± 0.45), with more negative ΔpH values (−1.26 ± 0.16). The PCEC remains high (18.09 ± 3.99 cmolc kg⁻¹), whereas available phosphorus decreases sharply (1.09 ± 1.14 mg kg⁻¹) and TOC reduces to 4.12 ± 1.73 g kg⁻¹ (Table 1).

The Ck horizon occurs throughout the entire toposequence, but its position within the soil profiles varies among sectors. In Sector 1, it occurs between the underlying Cα horizon and the overlying Bk horizon, forming part of a more complete and vertically differentiated soil profile. In Sectors 2 and 3, the Ck horizon may also overlie the Cα horizon, but erosion has locally removed the A and B horizons, leaving the Ck horizon directly exposed at the soil surface. Morphologically, it is characterized by abundant calcium carbonate efflorescences, nodules and concretions. The particle-size distribution is dominated by silt (378.33 ± 75.93 g kg⁻¹), followed by clay (266.00 ± 43.84 g kg⁻¹) and reduced coarse sand contents (Table 1). Soil pH remains alkaline (8.57 ± 0.22), and the PCEC is 14.78 ± 4.07 cmolc kg⁻¹. Calcium carbonate equivalent increases markedly (692.17 ± 96.19 g kg⁻¹), while TOC remains low (3.79 ± 2.95 g kg⁻¹).

The horizon occurring at the base of the profiles raises the most important question regarding horizon designation. In Sector 1, this material is cemented, hard, and coherent, whereas in Sectors 2 and 3 it is non-cemented, pulverulent, and friable. At first glance, these contrasting properties could support their designation as Ckm in Sector 1 and Ck in Sectors 2 and 3. However, their genetic relationship with the present soil profiles requires a different interpretation. Although these materials are carbonate-rich, their carbonates are not products of the current pedogenic cycle recorded by the overlying horizons. Instead, they represent the parent materials from which the present soils developed. These materials correspond to ancient paleosols and calcretes of the Caatinga Formation, formed under markedly arid climatic conditions during the Pleistocene. Thus, although their carbonates originated from earlier processes involving the transformation, redistribution, and precipitation of carbonate derived from limestone, these processes predate the development of the present soils. Within the current pedogenic system, they must therefore be interpreted as inherited carbonate-rich parent materials rather than as horizons containing secondary carbonates formed during ongoing pedogenesis. This genetic distinction is fundamental to their designation. Accordingly, we use Cα for these basal horizons following the WRB nomenclature, emphasizing that their carbonates are primary relative to the present soil-forming cycle and that these horizons represent the parent material of the contemporary soils.

The Cα horizon exhibits a predominantly light-colored matrix, silt loam texture, and massive structure, locally tending toward subangular to angular blocky structure. Granulometrically, the Cα horizon it is characterized by high silt content (560.33 ± 162.85 g kg⁻¹), medium clay content (161.33 ± 65.01 g kg⁻¹), and significant gravel fraction (332.67 ± 228.89 g kg⁻¹), with sand fractions distributed between coarse (146.33 ± 113.39 g kg⁻¹) and fine (131.83 ± 65.92 g kg⁻¹). Soil pH remains alkaline (8.62 ± 0.35 in H₂O; 7.73 ± 0.20 in KCl), with ΔpH values of −0.90 ± 0.40. Exchangeable Ca²⁺ is dominant (11.80 ± 4.53 cmolc kg⁻¹), while Mg²⁺ (1.03 ± 0.49 cmolc kg⁻¹), K⁺ (0.06 ± 0.04 cmolc kg⁻¹) and Na⁺ (0.55 ± 0.23 cmolc kg⁻¹) occur in lower concentrations, resulting in a sum of bases of 13.44 ± 4.76 cmolc kg⁻¹ and the PCEC of 14.07 ± 4.75 cmolc kg⁻¹. Base saturation is high (95.17 ± 2.23%).

Calcium carbonate equivalent reaches very high values (837.17 ± 280.20 g kg⁻¹), while available phosphorus (0.65 ± 0.52 mg kg⁻¹) and total organic carbon (0.75 ± 1.23 g kg⁻¹) are the lowest.

3.2. Soil Classification and Taxonomic Definition of Toposequence Sectors

According to the WRB classification, all soils along the toposequence are classified within the Calcisols, reflecting the taxonomic precedence of the diagnostic carbonate accumulation, expressed by the occurrence of calcic horizons (k). Nevertheless, important morphological differences in profile organization occur among the three sectors, allowing different types of Calcisols to be distinguished (P1 and P2—cambic Calcisols).

In Sector 1, the preservation of a B horizon showing incipient pedogenic development differentiates these more developed profiles from those occurring in the other sectors and supports the use of the Cambic qualifier. In Sectors 2 and 3, erosion has progressively removed the A and B horizons, resulting in truncated profiles in which carbonate-rich Ck horizons commonly occur close to or directly at the mineral soil surface. Their high carbonate contents, with CaCO₃ equivalent ≥ 50% within 100 cm of the mineral soil surface, meet the criteria for the Hypercalcic qualifier (P3 to P10—hypercalcic Calcisols).

3.3. Micromorphology

The micromorphological description of each horizon is presented in

Table 2. Micromorphological description of soil horizons.

Profile/ Horizon	Microstructure	Groundmass			Organic Compounds	Pedofeatures
		c/f Relation Distribution	Coarse Material	Micromass
P1 Ak	Poorly accommodated granular microstructure associated with moderately accommodated subangular blocky microstructure, with complex packing voids, planar voids, and channel voids	Open enaulic to double-spaced porphyric	Carbonate fragments with corroded edges, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand and rare shell fragments	Yellowish-brown, carbonate composition, speckled; crystallitic b-fabric	Humified organic matter impregnating the micromass; root fragments; plant tissue residues; organic punctuations and few organic nodules	Mn-impregnations and hypocoatings; typical Fe–Mn nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings, with crystallitic b-fabric; loose continuous infillings
P1 Bk	Poorly accommodated granular microstructure associated with poorly accommodated subangular blocky microstructure, with complex packing voids, planar voids, and channel voids	Open enaulic to double-spaced porphyric	Carbonate fragments with corroded edges, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand	Yellowish-brown, carbonate composition, speckled; undifferentiated b-fabric, with portions of crystallitic b-fabric	Plant tissue residues, root fragments, and organic punctuations	Mn-impregnations; typical Fe–Mn nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings, with crystallitic b-fabric; loose continuous infillings
P1 Cα	Massive microstructure, with fissures and moldic voids	Double-spaced porphyric	Quartz grains, subrounded to rounded, fine to medium sand; chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand	Pale yellowish, carbonate composition (micritic calcite and sparitic veins), with crystallitic b-fabric.	-	Mn impregnations in dendritic forms; concentric calcite coatings around quartz crystals
P2 Ak	Moderate accommodated granular microstructure associated with moderately accommodated subangular blocky microstructure, with complex packing voids, planar voids, and some channel voids	Open enaulic to single-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; rare feldspar grains, subrounded, medium to fine sand	Yellowish-brown, carbonate composition, speckled; crystallitic b-fabric	Humified organic matter impregnating the micromass; root fragments; organic punctuations and few organic nodules	Mn-impregnations; typical Fe–Mn nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings, with crystallitic b-fabric; loose continuous infillings
P2 Bik2	Poorly accommodated granular microstructure with compound and complex packing voids	Closed enaulic	Carbonate fragments, subrounded, medium to coarse sand; abundant quartz grains, subrounded to angular, fine to medium sand; chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand	Reddish, carbonate composition, speckled; undifferentiated b-fabric, with portions with crystallitic b-fabric	Root fragments; plant tissue residues	Typical Fe–Mn nodules, black to reddish-black, with internally undifferentiated b-fabric; calcite needle coatings with crystallitic b-fabric; abundant loos continuous infillings
P3 Ck1	Moderately accommodated subangular blocky microstructure, with complex packing voids and planar voids	Double-spaced porphyric	Carbonate fragments, subrounded, with corroded edges, medium to coarse sand; quartz grains, subrounded, fine to medium sand; chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand; and rare shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Root fragments	Mn-impregnations and hypocoatings; typical Fe–Mn nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings, with crystallitic b-fabric; loose continuous infillings
P3 Ck2	Moderately to poorly accommodated subangular blocky microstructure, with complex packing voids and planar voids	Double-spaced porphyric	Carbonate fragments, subrounded to rounded, with corroded edges, medium to coarse sand; quartz grains, subrounded, fine to medium sand; and occasional chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	-	Fe–Mn typical nodules with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric
P3 Ck3	Moderately to poorly accommodated subangular blocky microstructure, with complex packing voids and some portions with planar microstructure	Double-spaced porphyric	Carbonate fragments, subrounded, with corroded edges, medium to coarse sand; quartz grains, subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	-	Mn-impregnations; calcite needles coatings and infillings, with crystallitic b-fabric
P4 Ck1	Poorly accommodated subangular blocky microstructure, with complex packing voids and planar voids	Single-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; occasional chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Plant tissue and fecal pellets	Fe–Mn typical nodules with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric
P4 Ck2	Moderately to poorly accommodated subangular blocky microstructure, with complex packing voids and some portions with planar microstructure	Single-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; rare feldspar grains, subrounded, medium sand	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Plant tissue and root fragment	Fe–Mn typical nodules with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric
P5 Ak	Moderately accommodated subangular blocky microstructure, with complex packing voids	Double-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; feldspar grains, subrounded, medium to fine sand	Brown-yellowish, carbonate composition, speckled, crystallitic b-fabric	Humified organic matter impregnating the micromass; root fragments; organic punctuations	Typical Fe–Mn nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings, with crystallitic b-fabric; loose continuous infillings
P5 Bk	Moderately accommodated subangular blocky microstructure, with complex packing voids, some channels and cavities	Double-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; occasional chert fragments composed of microcrystalline quartz, angular to subrounded, fine sand	Yellowish-brown, carbonate composition, speckled; undifferentiated and crystallitic b-fabric	Root fragments; organic punctuations	Typical Fe–Mn nodules, black, with internally undifferentiated b-fabric, calcite needles coatings and infillings, with crystallitic b-fabric; loose continuous infillings
P5 Ck	Moderately accommodated subangular blocky microstructure, with complex packing voids	Single-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; quartz grains, subrounded, fine to medium sand; rare feldspar grains, subrounded, fine sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled; undifferentiated and crystallitic b-fabric	Root fragments	Fe–Mn typical nodules with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric; loose continuous infillings
P6 Cα	Massive to fissural microstructure, with straight and curve planar voids and vughs	Double-spaced porphyric	Carbonate fragments, subrounded to rounded, medium to coarse sand; chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand	Pale yellowish-gray, carbonate composition (micritic calcite and sparitic veins), with crystallitic b-fabric.	-	Mn impregnations in dendritic forms; concentric calcite coatings around quartz crystals
P8 Ck1	Poorly accommodated subangular blocky microstructure, with complex packing voids	Single-spaced porphyric	Carbonate fragments, subrounded, with corroded edges, medium to coarse sand; quartz grains, subrounded, fine to medium sand; chert fragments composed of microcrystalline quartz, angular to subrounded, medium sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Plant tissue	Mn-impregnations, Fe–Mn typical nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric
P8 Ck2	Poorly accommodated subangular blocky microstructure, with complex packing voids and planar voids	Single-spaced porphyric	Carbonate fragments, subrounded, with corroded edges, medium to coarse sand; quartz grains, subrounded, fine sand; and occasional chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand; rare mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Plant tissue and fecal pellets	Fe–Mn typical nodules with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric
P8 Cα	Massive to fissual microstructure, with straight and curve planar voids	Double to single-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand	Pale yellowish-gray, carbonate composition (micritic calcite), with crystallitic b-fabric	-	Mn impregnations in dendritic forms; concentric calcite coatings around quartz crystals
P9 Ck1	Poorly accommodated subangular blocky microstructure, with complex packing voids and planar voids	Single-spaced porphyric	Carbonate fragments, subrounded, with corroded edges, medium to coarse sand; quartz grains, subrounded to rounded, fine to medium sand; feldspar grains, subrounded, medium sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Plant tissue and fecal pellets	Mn-impregnations, Fe–Mn typical nodules, both with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric, loose continuous infillings
P9 Ck2	Poorly accommodated subangular blocky microstructure, with complex packing voids and planar voids	Single-spaced porphyric	Carbonate fragments, subrounded to rounded, with corroded edges, medium to coarse sand; quartz grains, subrounded, fine sand; occasional chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand; mollusc shell fragments	Yellowish-brown, carbonate composition, speckled, with crystallitic b-fabric	Plant tissue and fecal pellets	Fe–Mn typical nodules with black color and internally undifferentiated b-fabric; calcite needles coatings and infillings with crystallitic b-fabric
P9 Cα	Massive to fissural microstructure, with straight and curve planar voids and vughs (moldic voids)	Double-spaced porphyric	Carbonate fragments, subrounded, medium to coarse sand; quartz grains, subrounded, fine sand	Pale yellowish-gray, carbonate composition (micritic calcite), with crystallitic b-fabric.	Root fragments	Mn impregnations in dendritic forms; concentric calcite coatings around quartz crystals
P10 Bkl	Moderately accommodated subangular blocky microstructure, with complex packing voids, some vughs and channels	Double-spaced porphyric	Carbonate fragments, subrounded, medium sand; and occasional chert fragments composed of microcrystalline quartz, angular to subrounded, fine to medium sand	Yellowish-brown, carbonate composition, speckled; undifferentiated and crystallitic b-fabric	Root fragments; organic punctuations	Typical Fe–Mn nodules, black, with internally undifferentiated b-fabric, dense, continuous calcitic infillings

, with representative photomicrographs shown in Figure 4, Figure 5, Figure 6 and Figure 7.

The Ak horizon exhibits a composite microstructure, characterized by poorly accommodated granular aggregates associated with moderately accommodated subangular blocky units (Figure 4). The pore system is dominated by complex packing voids and planar voids, with subordinate channel voids. The c/f related distribution ranges from open enaulic to double-spaced porphyric. The micromass is carbonate-rich, yellowish-brown, with a speckled pattern and predominantly crystallitic b-fabric. The coarse fraction consists mainly of carbonate fragments and subrounded quartz grains, with rare feldspar grains and gastropods shell fragments. Organic components include humified organic matter impregnating the micromass, root fragments, plant tissue residues, organic punctuations, and organic nodules. Pedofeatures are represented by Mn impregnations and hypocoatings, typical Fe–Mn nodules with undifferentiated b-fabric, calcite needle coatings and infillings with crystallitic b-fabric, and loose continuous infillings.

The Bk horizon exhibits a composite microstructure, defined by the association of granular aggregates and subangular blocky units, with lateral variations that reflect the macroscopic color differences observed along the toposequence (Figure 5). These differences are confirmed at the micromorphological level by distinct groundmass coloration. The yellowish-brown variant presents granular microstructure associated with subangular blocky units, whereas the reddish variant is dominated by a more developed granular microstructure, with less evident blocky organization. Porosity is dominated by compound and complex packing voids, with additional planar voids, channels, and occasional cavities. The c/f related distribution ranges from open to closed enaulic, locally transitioning to double-spaced porphyric.

The micromass is carbonate-rich, with a speckled pattern, and shows contrasting b-fabric according to color: it is predominantly undifferentiated in the reddish domains and clearly crystallitic in the yellowish-brown domains. Coarse material is similar in both variants, consisting of carbonate fragments and quartz, with occasional chert and rare feldspar grains. However, the reddish Bik horizons show a markedly higher quartz content, including grains ranging from subrounded to angular. Pedofeatures are broadly similar between variants, being dominated by Fe–Mn nodules and calcitic features, including calcite needle coatings and infillings with crystallitic b-fabric, as well as loose continuous infillings. The main distinction lies in the more frequent occurrence of Mn impregnations in the yellowish-brown Bik horizons.

The Ck horizon is dominated by subangular blocky microstructure, ranging from moderately to poorly accommodated, with frequent complex packing voids and planar voids, and locally with portions exhibiting planar microstructure (Figure 6). The c/f related distribution is predominantly porphyric, varying from single- to double-spaced. The micromass is consistently carbonate-rich, with yellowish-brown coloration and a speckled pattern, and is characterized by well-developed crystallitic b-fabric, locally associated with undifferentiated domains.

Coarse material is similar across all Ck horizons, consisting mainly of carbonate fragments with corroded edges and subrounded quartz grains, with recurrent occurrence of microcrystalline chert. Rare feldspar grains and gastropods shell fragments are also observed. Pedofeatures are consistently dominated by calcitic features, including calcite needle coatings and infillings with crystallitic b-fabric, as well as loose continuous infillings. Fe–Mn nodules with internally undifferentiated b-fabric are frequent, and Mn impregnations and hypocoatings occur locally. Overall, the micromorphological organization of the Ck horizons is relatively homogeneous along the sequence, with only minor variations in porosity and pedofeature abundance.

The Cα horizons are characterized by predominantly massive microstructure, locally transitioning to fissural and vughy types, with the development of planar voids, vughs, and moldic voids (Figure 7). Two distinct structural conditions are observed. In the cemented variant (P1 and P2 profiles), the material is more cohesive, with fewer fractures and a more continuous massive organization. In contrast, the non-cemented Cα horizons (P3 to P10 profiles) are more fractured and pulverulent, with a greater expression of fissures and voids. The c/f related distribution is consistently porphyric, ranging from double- to locally single-spaced. The micromass is carbonate-rich, with pale yellow to yellowish-gray colors, dominated by micritic calcite, with frequent sparitic calcite nucleations forming small veins and localized domains. The b-fabric is consistently crystallitic. Coarse material is compositionally similar across all profiles, consisting of carbonate fragments, quartz, and microcrystalline chert. Quartz occurs both as individual grains and as aggregated clusters, indicating its origin as siliciclastic rock fragments embedded within the carbonate matrix. Rare feldspar grains are also observed.

Pedofeatures are uniform across the Cα horizons and are dominated by calcitic and manganiferous features. Mn impregnations commonly occur in dendritic forms, while calcite is expressed as concentric coatings around quartz grains. These features are consistently associated with the carbonate groundmass and reflect pervasive mineral redistribution within the horizon.

4. Discussion

Soil micromorphology in the Salitre River Basin demonstrates that pedogenetic evolution is influenced by the carbonate substrate of the Caatinga Formation, particularly through its effects on the internal organization of soil profiles and the pedogeomorphological dynamics of the landscape. Combined with Quaternary climatic oscillations, this lithological control has contributed to the development of the present-day arid framework and the high environmental vulnerability of the region. In this setting, erosion and soil loss play a central role in landscape transformation and desertification [2,4,64], while micromorphology provides a means of identifying the imprint of pedogenic processes and associated environmental changes at the microscale [31,35].

Our results reveal a clear lateral differentiation of the soil cover along the toposequence, expressed by contrasts in horizon morphology among sectors. Sector 1 preserves Cambic Calcisols, whereas Hypercalcic Calcisols predominate in the other sectors. This taxonomic differentiation reflects an important change in profile organization: the absence of a B horizon in the Hypercalcic Calcisols is interpreted as the result of its removal by erosion rather than as evidence of intrinsically weaker pedogenic development. Thus, at the hillslope scale, spatial variations in erosion intensity control the preservation or truncation of diagnostic horizons and, consequently, present-day soil classification. At the profile scale, this process is expressed by differences in horizon preservation, thickness, and physicochemical properties, while at the microscale, it is recorded by changes in soil structure and pedofeatures associated with carbonate dissolution, redistribution, and precipitation. The integration of these features across spatial scales provides a coherent record of soil–landscape degradation and represents one of the main contributions of this study.

The Caatinga Formation overlies the Irecê Basin within the São Francisco Craton and developed on Neoproterozoic carbonate substrates of the Salitre Formation [65,66]. Its origin was initially interpreted as sedimentary, either lacustrine or palustrine [67,68], and was later reinterpreted as resulting from in situ dissolution and precipitation processes [69]. Suguio et al. [44] redefined the unit as being composed of calcretes, an interpretation subsequently reinforced by studies highlighting its polygenetic origin involving pedogenetic and diagenetic processes under dry, semi-arid conditions [45,70,71]. More recent studies further support this complexity, indicating post-depositional modification controlled by both pedogenetic and structural factors [46,72,73].

The Caatinga calcretes display marked facies variability [46]. The Muddy Micrite facies (Cta) is characterized by a laminated micritic matrix with subordinate siliciclastic input, whereas the brecciated facies (Ctb and Ctc) contain abundant intraclasts, intense fracturing, and evaporitic pseudomorphs comparable to those described in polygenetic calcretes elsewhere [35]. The Ctc facies differs in its higher siliciclastic content and frequent silicification, while the Wackestone facies (Ctd) consists predominantly of friable micrite. The Calcretized Sediments and Soils facies (Cte), in turn, records stronger pedogenetic modification through nodules, pisoliths, and infilling-related features. This facies diversity reflects the polygenetic character of the unit and produces pronounced lithological heterogeneity along the studied toposequence, consistent with the variability recognized in other calcrete systems [35,74,75].

This lithological heterogeneity has a direct pedogeomorphological expression along the toposequence. In Sector 1, more indurated calcretes, likely corresponding to the Ctb and Ctc facies, favor the preservation of surface and subsurface horizons (Ak and Bk), allowing greater pedogenetic expression and the development of more organized microstructures. In contrast, Sectors 2 and 3 are dominated by friable calcretes associated with the Ctd facies. In these areas, drainage incision enhances material removal because of the low cohesion of the substrate, resulting in the progressive loss of surface horizons and exposure of the Ck and Cα horizons. This preferential erosion of less resistant materials reinforces the role of lithology in landscape organization [76].

Therefore, the transition from Ak–Bk–Ck/Cα profiles in the more stable sector to Ck–Cα-dominated profiles in strongly eroded sectors should not be interpreted merely as a taxonomic change. Rather, it represents a progressive erosional simplification of the soil mantle. The present distribution of soils along the hillslope thus records different degrees of preservation of an originally more developed pedological system, establishing a direct connection between landscape-scale erosion and the internal organization of the Salitre soil cover.

From a geochemical perspective, the soils exhibit strong carbonate inheritance, expressed by high CaCO₃ contents, high base saturation, and alkaline pH. This signature is consistent with limited leaching and restricted chemical weathering under dry conditions, which favor carbonate preservation and accumulation during pedogenesis [31,33,77].

At the microscale, pedogenesis begins with calcrete fragmentation associated with the expansion of fracture networks and selective carbonate dissolution. Straight and curved planar voids, commonly associated with moldic voids, indicate the combined action of physical weathering and chemical dissolution, particularly at contacts between the carbonate matrix and more resistant constituents [78]. Dissolution and reprecipitation of pedogenic carbonates may also respond dynamically to seasonal variations in soil-water conditions [34]. Groundmass transformation proceeds from these zones of weakness, isolating carbonate nuclei and promoting structural and color changes. The persistence of a micritic calcitic groundmass with dominant crystallitic b-fabric reflects strong lithological inheritance and a residual character [35], while progressive fragmentation promotes the development of angular to subangular blocky microstructures.

Bioturbation contributes to the structural organization of these soils. Granular aggregates associated with biogenic pores, including channels and cavities, are particularly abundant in surface horizons but also occur in the Ck horizon, indicating that biological activity extends into saprolitic levels and contributes to material redistribution and structural development [79,80,81,82,83,84,85,86,87].

The coarse fraction includes carbonate fragments, quartz, siliciclastic fragments, chert, subordinate feldspars, and microcrystalline silica, further evidencing the polygenetic nature of the calcrete-forming system [46]. Shell fragments also occur and may record episodes of greater water availability in ephemeral environments, consistent with evidence of wetter phases documented in northeastern Brazil during the Quaternary [88,89,90].

Fe–Mn pedofeatures provide additional evidence of fluctuations in soil-water and redox conditions. In Cα horizons, manganese occurs as dendrites, indicating rapid precipitation from Mn²⁺-rich solutions [91,92]. These dendrites are progressively fragmented and redistributed in pedogenized horizons and may evolve into Fe–Mn nodules. Iron exhibits similar behavior and may contribute to hematite formation, particularly in coarser and better-drained materials [93,94,95].

Needle-fiber calcite coatings represent typical pedogenic calcification features associated with precipitation under conditions of high evapotranspiration and limited water availability [35,77]. Their formation may involve both biomineralization and physicochemical precipitation [96,97]. Together with other carbonate pedofeatures, they provide evidence of carbonate redistribution and precipitation within the soil system.

An important result of the micromorphological analysis is therefore not the occurrence of individual carbonate pedofeatures per se, but their coexistence and spatial relationships within soils subjected to contrasting degrees of erosion. Dissolution features record phases of carbonate mobilization, whereas coatings, infillings, and needle-fiber calcite indicate subsequent carbonate precipitation [31,34,35]. Their coexistence indicates that the present soils preserve superimposed pedogenic signals rather than products associated with a single environmental condition.

This combination of dissolution, redistribution, and precipitation is consistent with polyphasic soil evolution marked by alternation between wetter and drier conditions [34]. Geochronological evidence indicates that the regional climate experienced prolonged dry periods interspersed with shorter humid episodes [89,90,98], associated with shifts in the Intertropical Convergence Zone. Independent paleoenvironmental records support this climatic variability [88,99,100,101].

More specifically, U-series dating of travertines and water-table speleothems from semi-arid northeastern Brazil documents discrete episodes of enhanced groundwater recharge and wetter conditions during the Late Quaternary, contrasting with the predominantly dry conditions that characterize these environments today [89]. In northern Bahia, geochronological and paleoenvironmental studies of superficial carbonates from the Salitre and Jacaré valleys have also identified multiple generations of carbonate formation, reinforcing the polygenetic character of these deposits and their potential to record successive environmental conditions [72]. For the Late Holocene, lacustrine records with radiocarbon-based chronologies indicate pronounced hydroclimatic variability rather than a simple monotonic trend toward aridification, including prolonged dry phases alternating with wetter intervals [102,103].

This distinction between inherited and more recent signals is particularly relevant for interpreting aridification. The micromorphological record does not simply indicate that carbonate pedogenesis occurred under dry conditions; rather, it shows that phases of dissolution and pedogenesis were subsequently overprinted by carbonate precipitation features compatible with a strong water deficit [31,35,77]. However, because no direct geochronological ages were obtained for the pedogenic carbonate features analyzed in the Salitre profiles, their specific temporal attribution to the Late Holocene cannot be directly demonstrated. We therefore interpret the coexistence of dissolution features and needle-fiber calcite as pedological evidence consistent with the regional record of alternating humid and dry phases, while the preservation of calcification features suggests prolonged water-deficit conditions. Their precise age remains a hypothesis that requires direct geochronological or isotopic testing. Consequently, current degradation should not be inferred from individual pedofeatures alone, but from their association with profile truncation, reduced horizon development, and increasing erosion along the toposequence.

In Sectors 2 and 3, intense erosion has resulted in the removal of surface horizons and the development of a dense network of linear erosional features [29], representing a major expression of desertification in drylands [2,13,17,18,19]. This process is associated with declining productivity, vegetation loss, and progressive soil degradation [4,14,26]. Soil erosion is particularly relevant as an indicator of desertification [5,26] because the loss of surface soil directly compromises fertility and water-retention capacity [24,25].

In the Salitre Basin, these processes reflect the interaction of natural and anthropogenic factors. The regional climate is characterized by low annual precipitation (~400 mm), high rainfall variability, and prolonged dry periods [54,104], while intense rainfall events increase erosivity [105,106]. At the microscale, the carbonate pedofeatures identified in the studied soils primarily record carbonate mobilization and precipitation associated with soil-water and physicochemical conditions and are therefore interpreted mainly as evidence of the natural pedological response to water limitation rather than as direct indicators of anthropogenic disturbance [31,34,35]. Socioeconomic vulnerability and intensive land use, particularly sisal cultivation and extensive grazing, further exacerbate landscape degradation [38,107,108]. These anthropogenic pressures are superimposed on a naturally vulnerable climatic and pedological setting and may intensify vegetation loss, soil exposure, structural degradation, and erosion. Studies in Brazilian drylands, for example, show that overgrazing contributes to the deterioration of soil physical and biological properties, whereas grazing exclusion can promote their recovery [19]. Recent climatological assessments identifying arid climatic conditions in northern Bahia reinforce the significance of the regional water deficit [64].

The evidence obtained here allows this regional degradation to be interpreted at a finer pedological scale. Along the toposequence, increasing erosion is accompanied by horizon loss and progressive simplification of the soil cover, while micromorphology records structural changes and carbonate redistribution within the remaining horizons. These two sets of evidence are complementary: profile truncation records the geomorphological expression of degradation, whereas micromorphological features record the internal pedogenic response of carbonate soils to changes in water availability and landscape exposure. Although variations in biogenic pores and granular aggregates may also reflect changes in vegetation cover, grazing pressure, and associated surface-soil conditions, these anthropogenic effects cannot be unequivocally distinguished from climatic controls at the microscale with the present dataset.

The micromorphological evidence provides an additional mechanistic basis for this interpretation. The increasing development of planar fracture networks and the progressive disruption of the carbonate groundmass indicate greater structural discontinuity, potentially facilitating water penetration and material detachment. Likewise, the transition from crystallitic to more undifferentiated b-fabrics, together with reduced aggregation, is consistent with increasing structural disorganization toward Sectors 2 and 3. These features occur alongside pedofeatures recording carbonate dissolution, redistribution, and reprecipitation, indicating that carbonate dynamics and structural reorganization accompanied the progressive truncation of the soil mantle. Although shear strength and aggregate stability were not directly measured, the observed micromorphological changes provide a plausible physical mechanism linking internal soil reorganization to the increasing horizon truncation and linear erosion observed along the toposequence.

Bioturbation provides an additional link between soil organization and degradation. Biological activity contributes to aggregation and structural development [81,85,87], whereas its reduction may increase soil susceptibility to structural degradation and erosion. Accordingly, the lower expression of biogenic pores and granular aggregates under more degraded conditions may represent an integrated response to water limitation, vegetation loss, and land-use pressure rather than an exclusively climatic signal. Micromorphology therefore helps distinguish relatively stable areas, characterized by more organized structures, from degraded areas dominated by disaggregation and carbonate precipitation. This pattern is consistent with carbonate systems in which secondary carbonate precipitation is an important pedogenic process [109,110,111,112,113].

The absence of isotopic data currently limits our ability to determine whether the carbonate observed in the micritic groundmass and secondary pedofeatures was inherited directly from the carbonate parent material, redistributed within the soil profile, or incorporated from external sources. Consequently, although the micromorphological relationships provide evidence of carbonate dissolution, mobilization, and reprecipitation, they cannot independently establish the source of the carbonate involved in successive pedogenic generations. Future stable-isotope analyses (δ¹³C and δ¹⁸O) comparing the carbonate substrate, micritic groundmass, and distinct secondary carbonate pedofeatures could help discriminate among inherited, redistributed, and newly precipitated carbonate. Such analyses would provide an independent geochemical test of the genetic relationships inferred from micromorphology and would strengthen the reconstruction of carbonate dynamics within the soil profiles. Direct dating of pedogenic carbonate generations would also be necessary to test the proposed temporal relationship between the micromorphological features identified here and the regional Late Holocene hydroclimatic record.

Finally, degradation in the Salitre Basin may operate as a self-reinforcing process: vegetation loss increases soil exposure and erosion, which in turn restrict vegetation recovery and further reduce landscape resilience [4,18,19]. The significance of the present results lies in showing how this broader degradation process is recorded simultaneously at the hillslope, soil-profile, and micromorphological scales.

This study therefore advances beyond the use of erosion features, carbonate pedofeatures, or soil properties as isolated indicators of desertification. By integrating micromorphological evidence with soil-profile organization, substrate characteristics, landscape position, and erosion intensity along the toposequence, we demonstrate that degradation can be interpreted as a multiscale pedogeomorphological process. In carbonate drylands, the combined occurrence of profile truncation, structural reorganization, and carbonate redistribution provides a framework for distinguishing inherited pedogenic signals from those associated with ongoing landscape degradation. In this framework, micromorphological evidence of carbonate dynamics primarily records the natural pedological response to water availability, whereas the broader desertification process emerges from the interaction of this natural vulnerability with anthropogenic pressure at the landscape scale. This approach extends the significance of the Salitre Basin beyond a regional case study and provides a conceptual framework that may be tested in other carbonate drylands affected by increasing aridity, erosion, and anthropogenic pressure.

5. Conclusions

Soil development and preservation along the Salitre River Basin toposequence are strongly conditioned by the carbonate substrate and erosion intensity. Sector 1 preserves more complete profiles classified as Cambic Calcisols, whereas the more intensely eroded sectors are dominated by truncated Hypercalcic Calcisols. The absence of B horizons in these sectors is interpreted as a consequence of erosional removal rather than intrinsically weaker pedogenic development, demonstrating a clear relationship between hillslope dynamics, horizon preservation, and present-day soil classification. Micromorphological features record different components of carbonate-soil evolution: planar and moldic voids indicate fracturing and dissolution, whereas calcite coatings, infillings, and needle-fiber calcite record carbonate redistribution and precipitation. Their coexistence indicates superimposed pedogenic processes associated with changing soil-water conditions, while the persistence of calcification features is consistent with prolonged water deficit. However, without direct dating or isotopic analyses, these features cannot be assigned to specific climatic periods or used independently to establish a temporal trend of aridification. The relationship between soil-profile organization and micromorphological features provides the strongest evidence of degradation along the toposequence, as increasing erosion toward sectors 2 and 3 is accompanied by horizon truncation and greater exposure of carbonate-rich parent materials, while micromorphological changes indicate structural reorganization within the remaining soil horizons. Thus, profile truncation records the geomorphological expression of degradation, whereas micromorphology records the internal pedogenic response of carbonate soils to changes in water availability and landscape exposure. The main contribution of this study is therefore the integration of evidence across hillslope, soil-profile, and microscopic scales. In the Salitre Basin, desertification emerges from the interaction between the natural vulnerability of carbonate soils, climatic water deficit, erosion, and anthropogenic pressure. Micromorphology alone does not provide an unequivocal indicator of desertification, but, when integrated with soil-profile truncation, substrate characteristics, landscape position, and erosion intensity, it provides an effective framework for understanding soil–landscape degradation in carbonate drylands.