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4 November 2026

19 Pages

Soil Organic Carbon Fractions and Fertility Across Degraded, Forest, and Restoration Areas in Desertification Hotspots of the Brazilian Semiarid Region

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1
Graduate Program in Agricultural Sciences, Paraíba State University, Campina Grande 58429-500, PB, Brazil
2
Department of Agricultural and Environmental Sciences, Paraíba State University, Lagoa Seca 58117-000, PB, Brazil
3
Academic Unit of Agricultural Sciences, Federal University of Campina Grande, Pombal 58840-000, PB, Brazil
4
Postgraduate Program in Agronomy, Federal University of Paraíba, Areia 58397-000, PB, Brazil
Horticulturae2026, 12(11), 1379;https://doi.org/10.3390/horticulturae12111379
This article belongs to the Special Issue Soil Fertility and Nutrient Management for Sustainable and High-Quality Horticultural Production
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Abstract

Soil organic carbon fractions and fertility attributes may differ among land-use conditions in the Brazilian semiarid region due to environmental heterogeneity, soil properties, and land-use history. This study compared soil attributes in degraded, forest, and restoration areas across four desertification hotspots located in Ceará (CE), Paraíba (PB), Pernambuco (PE), and Piauí (PI), Brazil. Surface soil samples (0–10 cm) were collected from four independent plots per land-use condition within each hotspot and analyzed for particle-size distribution, pH, total organic carbon (TOC), humic fractions, available phosphorus, exchangeable cations, and potential cation exchange capacity (CEC). The magnitude and direction of differences among land-use conditions varied across the investigated hotspots. Forest areas exhibited 12% and 57% higher TOC contents than degraded areas in PB and PI, respectively. In PB, humic acid carbon contents were approximately 85–95% higher in forest and restoration areas than in the degraded area, while humin carbon content in the restoration area was 136% higher than in the degraded area. Available phosphorus was markedly higher in restoration areas in PB and PI. In PI, the restoration area exhibited high Ca2+ and Mg2+ contents and potential CEC, whereas soil pH ranged from approximately 5.1 in the forest area to 8.0 in the degraded area. These findings reveal substantial spatial variation in soil carbon fractions and fertility attributes among land-use categories, potentially associated with differences in soil texture, pedogenic characteristics, and previous land management. The results highlight the importance of considering local edaphic conditions when assessing soil quality across contrasting land-use categories in semiarid environments.

1. Introduction

Soil performs essential functions in agricultural production and the provision of ecosystem services, including nutrient cycling, water regulation, carbon storage, and support for biodiversity [1]. These functions arise from interactions between its mineral and organic components and may be impaired by changes in land use. The conversion of native vegetation to cropland and pasture, particularly when coupled with inadequate management, can intensify erosion, compaction, organic matter loss, and fertility decline, increasing the vulnerability of soils to degradation and desertification [2,3].
These processes are particularly relevant in northeastern Brazil, a region characterized by considerable environmental heterogeneity and a mosaic of native vegetation, cropland, pasture, degraded land, and areas at different stages of restoration [4]. In the Brazilian Semiarid region, irregular rainfall, prolonged water deficits, and high temperatures may interact with human pressures to accelerate soil degradation and reduce the capacity of soils to sustain plant production and ecosystem services. Vegetation removal, overgrazing, intensive land use, and inadequate irrigation practices are among the factors that can intensify these processes [5,6,7].
The vulnerability of soils in the Brazilian Semiarid region is also shaped by their considerable pedological diversity. Differences in parent material, relief, climate, and soil-forming processes generate substantial variation in physical and chemical properties, even across relatively short distances [8]. Constraints such as shallow effective soil depth in Leptosols, marked textural contrasts in Luvisols and Planosols, and susceptibility to salinization in some alluvial soils can limit water and nutrient storage and increase sensitivity to human disturbance [9]. Accordingly, associations between land-use categories and soil quality should be interpreted in the context of both land use and the edaphic characteristics of each site.
In this context, soil organic matter (SOM) is a key component of soil quality and resilience in semiarid environments. In addition to serving as a major carbon reservoir, SOM contributes to aggregate formation and stability, water retention, cation exchange capacity, nutrient availability, and microbial activity [10,11]. Its dynamics reflect the balance between carbon inputs from plant residues, roots, and other organic materials and losses through decomposition, erosion, and related processes. Differences in soil organic carbon concentrations may therefore provide insight into variation among land-use categories, including areas undergoing restoration.
Total organic carbon alone, however, may not fully capture differences in SOM composition. Chemical fractionation into fulvic acid, humic acid, and humin separates carbon into operationally defined fractions with different solubility properties, providing complementary information on its distribution within the soil [12]. Examining these fractions alongside particle-size distribution and chemical attributes may help clarify how vegetation cover, land-use history, and inherent soil properties are associated with carbon retention and nutrient availability. Because the fractions are defined by the extraction method, their concentrations should not be interpreted as direct measures of carbon persistence.
Although the relationship between land use and soil organic carbon has been widely studied, less is known about how total carbon, chemically extracted carbon fractions, and fertility attributes vary together across degraded, forest, and restoration areas under contrasting edaphic conditions in the Brazilian Semiarid region. This gap matters because soils exposed to similar climatic conditions may differ in texture, mineralogy, management history, and capacity to retain organic matter. These differences can influence the soil attributes observed across degraded, forest, and restoration areas, independently of their land-use classification.
Therefore, this study aimed to evaluate how three land-use categories (degraded, forest, and restoration areas) are associated with variation in soil particle-size distribution, total organic carbon, humic fractions, and fertility attributes across four states in the Brazilian Semiarid region.

2. Materials and Methods

2.1. Study Areas and Soil Sampling

The study was conducted in four desertification hotspots in the semiarid region of northeastern Brazil: Irauçuba, Ceará (CE); São João do Cariri, Paraíba (PB); Cabrobó, Pernambuco (PE); and Gilbués, Piauí (PI). Three land-use and vegetation-cover conditions were evaluated in each state: degraded area (DA), forest area (FA), and restoration area (RA). These regions are characterized by different degrees of soil degradation resulting from the interaction between inherent climatic and edaphic constraints and long-term anthropogenic pressures, particularly overgrazing, deforestation, agricultural activities, and mining.
The region is predominantly characterized by a hot semiarid climate (BSh, according to the Köppen classification), with a mean annual temperature of approximately 26 °C and marked spatial variation in annual precipitation. Irauçuba (3°44′46″ S, 39°47′00″ W; 152 m a.s.l.) receives approximately 540 mm year−1, mainly from January to April, and its degradation process is strongly associated with overgrazing. Cabrobó (8°35′22″ S, 39°21′02″ W; approximately 325 m a.s.l.) receives approximately 500 mm year−1 and is characterized by degradation associated with deforestation. São João do Cariri (7°22′55″ S, 36°31′45″ W) receives approximately 400 mm year−1, with degradation mainly associated with overgrazing and vegetation removal for charcoal production. In contrast, Gilbués (9°45′55″ S, 45°21′00″ W; 481 m a.s.l.) receives approximately 900 mm year−1, but has undergone severe degradation associated with deforestation, agricultural expansion, livestock production, and mining. The native vegetation consists predominantly of Caatinga, including areas of seasonally dry forest, and the terrain ranges from undulating to strongly undulating. Additional information on the geographical coordinates, soil classification, native vegetation, main drivers of desertification, restoration initiation years, and restoration practices for each desertification hotspot is provided in Supplementary Table S1.
Within each land-use condition (degraded area, forest area, and restoration area), four independent 50 × 50 m plots (2500 m2 each) were established in each desertification hotspot. A central 40 × 40 m sampling area (1600 m2) was delimited within each plot to minimize edge effects. Three soil subsamples were collected at a depth of 0–10 cm within the central sampling area of each plot. These subsamples were homogenized to form a single composite soil sample per plot, accounting for within-plot spatial variability. The composite samples were kept separate for each plot, without combining soil material from different plots. Thus, four independent composite soil samples were obtained for each combination of desertification hotspot and land-use condition, resulting in 48 composite soil samples obtained from 144 sampling points across the study areas.
Soil profiles were opened at different landscape positions for morphological description and soil classification. Profile description and sampling followed Santos et al. [13], and soils were classified according to the Brazilian Soil Classification System [14]. For the analyses performed in this study, disturbed soil samples were collected from the 0–10 cm layer and placed in sterile bags. Samples were transported to the Soil Laboratory of the State University of Paraíba, Lagoa Seca, PB, Brazil, for particle-size and chemical analyses and soil organic matter fractionation.

2.2. Particle-Size Analysis

Particle-size distribution was determined after chemical dispersion with 1 mol L−1 NaOH, according to Teixeira et al. [15]. The sand fraction (2.000–0.053 mm) was separated by sieving, whereas the clay fraction (<0.002 mm) was determined using the pipette method. Silt content was calculated by difference between the initial sample mass and the combined masses of the sand and clay fractions. Results were expressed as g kg−1 of soil.

2.3. Soil Chemical Analyses

Soil pH was determined in water. Available phosphorus and exchangeable potassium were extracted with Mehlich-1 solution (0.05 mol L−1 HCl + 0.0125 mol L−1 H2SO4). Phosphorus was determined colorimetrically using a UV–Vis spectrophotometer (V-M5, BEL Photonics, Piracicaba, Brazil), and potassium was determined by flame photometry using a flame photometer (Model 910, Analyser, São Paulo, Brazil). Exchangeable calcium, magnesium, and aluminum were extracted with 1 mol L−1 KCl. Potential acidity (H + Al) was extracted using 0.5 mol L−1 calcium acetate solution buffered at pH 7.0 and determined by titration with 0.025 mol L−1 NaOH, using phenolphthalein as an indicator. Calcium and magnesium were determined by atomic absorption spectrometry using an atomic absorption spectrometer (SavantAA, GBC Scientific Equipment, Melbourne, Australia), and aluminum was determined by titration, according to Teixeira et al. [15].
Potential cation exchange capacity (potential CEC, T), representing the soil’s cation exchange capacity at a reference pH of approximately 7.0, was calculated as the sum of exchangeable bases and potential acidity (H + Al):
T = Ca2+ + Mg2+ +K+ + (H + Al)
All cation concentrations were converted to the same unit before calculation.

2.4. Total Organic Carbon and Soil Organic Matter Fractionation

Soil total organic carbon (TOC) was determined by wet oxidation with potassium dichromate, according to Yeomans and Bremner [16].
Soil organic matter was chemically fractionated into fulvic acids, humic acids, and humin, following the procedure described by Benites et al. [17]. The fractions were operationally defined according to their solubility under alkaline and acidic conditions.
For alkaline extraction, 1.0 g of air-dried, sieved soil was placed in a centrifuge tube, and 10 mL of 0.1 mol L−1 NaOH was added. The mixture was allowed to stand for 24 h and then centrifuged at 5000× g for 30 min. The alkaline supernatant was collected and separated from the insoluble residue.
The pH of the alkaline extract was adjusted to 1–2 with 20% H2SO4 to precipitate the humic acid fraction. After centrifugation, the supernatant, corresponding to the fulvic acid fraction, was separated from the humic acid precipitate. The fractions were transferred to 50 mL tubes, and their volumes were adjusted with distilled water. Carbon in each fraction was determined by wet oxidation with potassium dichromate.
The carbon contents of the fulvic acid, humic acid, and humin fractions were expressed as FAC, HAC, and HC, respectively. These fractions represent operationally defined carbon pools obtained through chemical fractionation. Their sum may differ from TOC due to methodological differences in carbon extraction, separation, and quantification.

2.5. Statistical Analysis

For statistical analysis, the study was considered as a 4 × 3 factorial arrangement, comprising four desertification hotspots (CE, PB, PE, and PI) and three land-use systems (DA, FA, and RA), with four replicates per combination. Residual normality and homogeneity of variances were assessed using the Shapiro–Wilk and Bartlett’s tests, respectively, at the 5% significance level. The Shapiro–Wilk test was applied to the residuals of the factorial model, while Bartlett’s test was used to evaluate homogeneity of variances across the 12 combinations of desertification hotspots and land-use systems. The corresponding p-values are presented in Supplementary Table S2. When significant effects were detected (p ≤ 0.05), means were compared using Tukey’s test at the 5% significance level. All statistical analyses were performed using R software, version 4.5.1.
The four states were treated as fixed levels representing distinct desertification hotspots rather than as a random sample of the Brazilian Semiarid region. The State × land-use interaction was used to assess whether associations between land-use categories and soil attributes differed among these hotspots. Given their contrasting climatic, pedological, and management conditions, the results were interpreted as site-specific spatial associations, without implying causal effects of land use or generalizing the findings to the entire Brazilian Semiarid region.

3. Results

3.1. Soil Particle-Size Distribution

ANOVA revealed a significant interaction between Brazilian state and land-use system for all variables evaluated (Table S2), indicating that the differences observed among degraded, forest, and restoration areas depended on the edaphic context of each state.
The distribution of soil particle-size fractions differed among land-use systems and Brazilian states (Figure 1). Overall, the soils were predominantly characterized by a high sand content, although the magnitude of the differences among degraded, forest, and restoration areas varied across states.
Figure 1. Sand (a), silt (b), and clay (c) contents in soils under different land-use systems across different states of Northeastern Brazil. CE, Ceará; PB, Paraíba; PE, Pernambuco; PI, Piauí. Lowercase letters compare land-use systems within each state, whereas uppercase letters compare states within each land-use system according to Tukey’s test (p ≤ 0.05). Error bars represent the standard error of the mean (n = 4).
In Ceará, the highest sand contents were observed in DA and FA, which did not differ significantly from each other, whereas RA exhibited a lower sand content (Figure 1a). In Paraíba, FA had a higher sand content than DA, while RA showed an intermediate value. In Pernambuco, RA exhibited the highest sand content, followed by DA, whereas FA had the lowest value. The most pronounced contrast was observed in Piauí, where FA exhibited a high sand content, whereas RA showed a substantially lower value.
Silt content also responded differently to land-use systems across states (Figure 1b). In Ceará, RA exhibited a higher silt content than DA and FA, which did not differ significantly from each other. In Paraíba, the highest silt content was recorded in DA. In Pernambuco, DA and FA exhibited higher silt contents than RA. In contrast, in Piauí, RA had the highest silt content, followed by DA, whereas FA exhibited the lowest value.
Clay distribution showed a pattern generally opposite to that observed for sand (Figure 1c). In Piauí, RA exhibited the highest clay content, contrasting with the high sand content observed in FA. In Pernambuco, FA had a higher clay content than DA. In Paraíba, no significant differences in clay content were detected among land-use systems, whereas in Ceará, DA exhibited a lower clay content than FA and RA, which did not differ significantly from each other.

3.2. Soil pH

Soil pH also responded differently to land-use systems across states (Figure 2). In Ceará, DA exhibited the highest pH (6.2), followed by FA (5.8) and RA (5.5), with significant differences among the three land-use systems. In Paraíba, pH values remained close to neutral, ranging from 7.0 to 7.2; RA exhibited the highest pH, FA the lowest, and DA an intermediate value. In Pernambuco, FA showed the highest pH (7.4), whereas DA and RA, with values close to 6.9, did not differ significantly from each other. The most pronounced contrast was observed in Piauí, where DA exhibited a pH of 8.0, followed by RA (7.5), whereas FA had a markedly lower value of 5.1. Thus, Piauí exhibited both the greatest variation in soil pH among land-use systems and the most extreme pH values across all areas evaluated.
Figure 2. pH in soils under different land-use systems across different states of Northeastern Brazil. CE, Ceará; PB, Paraíba; PE, Pernambuco; PI, Piauí. Lowercase letters compare land-use systems within each state, whereas uppercase letters compare states within each land-use system according to Tukey’s test (p ≤ 0.05). Error bars represent the standard error of the mean (n = 4).

3.3. Total Organic Carbon and Humic Fractions

Total organic carbon (TOC) was significantly influenced by the interaction between state and land-use system (Figure 3a). In Paraíba and Piauí, the highest TOC contents were observed in FA. In Paraíba, TOC in FA was 12% higher than that recorded in DA, whereas in Piauí, this difference reached 57%. In Ceará, FA exhibited the highest absolute TOC value, although it did not differ significantly from DA. In Pernambuco, no significant differences were observed among land-use systems. Overall, RA exhibited intermediate or lower TOC contents than FA, with differences ranging from 10% to 35%, depending on the state.
Figure 3. Total organic carbon (a), fulvic acid carbon (b), humic acid carbon (c), and humin carbon (d) contents in soils under different land-use systems across different states of Northeastern Brazil. CE, Ceará; PB, Paraíba; PE, Pernambuco; PI, Piauí. Lowercase letters compare land-use systems within each state, whereas uppercase letters compare states within each land-use system according to Tukey’s test (p ≤ 0.05). Error bars represent the standard error of the mean (n = 4).
Carbon associated with the fulvic acid fraction (FAC) also showed state-dependent responses (Figure 3b). In Ceará, FA exhibited the highest FAC content, 30% higher than that in DA, whereas RA showed an intermediate value. In Paraíba, RA exhibited the highest FAC content, 50% higher than those observed in DA and FA, which did not differ significantly from each other. In Pernambuco, DA and FA showed the highest FAC contents, whereas RA exhibited a reduction of 30% compared with DA. In Piauí, FA had the highest FAC content, exceeding those of DA and RA by 50%, with no significant difference between the latter two land-use systems.
For carbon associated with the humic acid fraction (HAC), FA exhibited the highest values in Ceará and Piauí (Figure 3c). In Ceará, HAC content in FA was 65% higher than that in DA and 120% higher than that in RA. In Paraíba, DA exhibited the lowest HAC content, whereas FA and RA showed similar values, with increases of 85–95% compared with DA. In Pernambuco, a contrasting pattern was observed, with higher HAC contents in DA and RA; in RA, HAC was 50% higher than that observed in FA. In Piauí, FA exhibited HAC contents 80% and 35% higher than those in DA and RA, respectively.
Humin carbon (HC) showed patterns distinct from those observed for FAC and HAC (Figure 3d). In Ceará, DA and RA exhibited the highest HC contents, 9.6 and 9.9 g kg−1, respectively, whereas FA showed 7.4 g kg−1. In Paraíba, RA exhibited the highest HC content (13.0 g kg−1), representing increases of 71% and 136% compared with FA and DA, respectively. In Pernambuco, differences among land-use systems were less pronounced, with HC contents of 9.4, 10.1, and 10.6 g kg−1 in DA, FA, and RA, respectively. In Piauí, FA exhibited the highest HC content (18.9 g kg−1), 58% higher than that in DA (12.0 g kg−1) and 64% higher than that in RA (11.5 g kg−1).

3.4. Soil Chemical Properties

Mehlich-1-extractable phosphorus (P) varied markedly among the investigated desertification hotspots and land-use conditions (Figure 4a). In Ceará, the forest area (FA) exhibited a P content 113% higher than that of the degraded area (DA), whereas the restoration area (RA) showed a value 90% higher than that of DA. In Paraíba, P content in RA was approximately 1400% higher than in FA. In Pernambuco, RA exhibited the highest P content, exceeding that of DA by approximately 120%, with FA showing an intermediate value. In Piauí, the contrast was particularly pronounced, with P content in RA more than 6000% higher than in FA.
Figure 4. P (a), K+ (b), Ca2+ (c), and Mg2+ (d) contents in soils under different land-use systems across different states of Northeastern Brazil. CE, Ceará; PB, Paraíba; PE, Pernambuco; PI, Piauí. Lowercase letters compare land-use systems within each state, whereas uppercase letters compare states within each land-use system according to Tukey’s test (p ≤ 0.05). Error bars represent the standard error of the mean (n = 4).
These results indicate substantial variation in Mehlich-1-extractable P among the investigated soils. However, the extraction efficiency of Mehlich-1 depends on soil properties, including texture, mineralogy, and phosphorus retention capacity. Therefore, the measured concentrations should not be considered directly equivalent to plant-available P or assumed to be equally comparable across contrasting soil environments.
Exchangeable K+ content showed contrasting patterns among states (Figure 4b). In Ceará and Pernambuco, FA exhibited the highest K+ contents, differing significantly from DA and RA. In Paraíba, FA also showed the highest K+ content, followed by RA and DA. In contrast, in Piauí, DA exhibited the highest K+ content, exceeding those observed in FA and RA.
For exchangeable Ca2+, no significant differences among land-use systems were observed in Ceará (Figure 4c). In Paraíba, FA exhibited a higher Ca2+ content than DA and RA, which did not differ significantly from each other. In Pernambuco, FA and DA did not differ significantly from each other and showed higher Ca2+ contents than RA. The highest absolute Ca2+ contents across all states were recorded in Piauí, particularly in RA, followed by DA, whereas FA exhibited the lowest value.
Exchangeable Mg2+ content was also affected by the interaction between state and land-use system (Figure 4d). In Ceará, FA exhibited a higher Mg2+ content than DA and RA. In Paraíba, no significant differences in Mg2+ content were observed among land-use systems. In Pernambuco, FA exhibited the highest Mg2+ content, whereas RA showed the lowest. In Piauí, RA exhibited a substantially higher Mg2+ content than those observed in DA and FA.
Potential cation exchange capacity varied widely among states and land-use systems (Figure 5). In Ceará, potential CEC values were relatively low and similar among land-use systems, with the highest value observed in FA and the lowest in DA. In Paraíba, FA exhibited a higher potential CEC than DA and RA. In Pernambuco, DA and FA did not differ significantly and exhibited higher potential CEC values than RA. In Piauí, the greatest contrast in potential CEC among land-use systems was observed, with RA exhibiting the highest value, followed by DA, whereas FA showed the lowest value (Figure 5).
Figure 5. Potential cation exchange capacity (Potential CEC) in soils under different land-use systems across different states of Northeastern Brazil. CE, Ceará; PB, Paraíba; PE, Pernambuco; PI, Piauí. Lowercase letters compare land-use systems within each state, whereas uppercase letters compare states within each land-use system according to Tukey’s test (p ≤ 0.05). Error bars represent the standard error of the mean (n = 4).

4. Discussion

4.1. Particle-Size Distribution, Soil pH, and Edaphic Heterogeneity

The significant interaction between state and land-use system indicates that differences among degraded, forest, and restoration areas cannot be interpreted independently of the pedogenetic context of each region. Although differences in particle-size distribution were detected among land-use systems (Figure 1), soil texture is a relatively stable property primarily determined by parent material, pedogenic processes, and landscape position [18,19]. Therefore, the contrasts observed among DA, FA, and RA likely reflect not only land-use history but also the inherent spatial heterogeneity of the studied areas.
The predominance of the sand fraction (Figure 1a), particularly in Ceará and in some areas of Piauí, is consistent with the occurrence of sandy- to medium-textured soils in different regions of Northeastern Brazil [14]. A high proportion of sand may limit water and nutrient retention and reduce the physicochemical protection of soil organic matter, potentially increasing soil vulnerability to the effects of vegetation-cover removal.
Nevertheless, land management and degradation processes can modify the distribution of soil particles in the surface layer through selective erosion, sediment transport, and deposition. Vegetation-cover removal increases soil exposure to raindrop impact and surface runoff, promoting particle mobilization, particularly of the finer fractions [20]. In this context, differences in silt (Figure 1b) and clay (Figure 1c) contents among land-use systems may reflect both pre-existing edaphic characteristics and the cumulative effects of erosional processes.
The contrast observed in Piauí is particularly noteworthy. The high proportion of sand in FA coincided with lower silt and clay contents, whereas RA showed the opposite pattern (Figure 1). This relationship indicates that differences among land-use systems should not be attributed exclusively to ecological restoration, as vegetation recovery would not be expected to substantially alter soil particle-size distribution over a short period. Instead, differences in parent material, topographic position, sediment deposition, and erosion history are more likely to contribute to this pattern [18,19].
Differences in soil pH further highlight the combined influence of intrinsic soil properties and land-use history. While soils in Paraíba and Pernambuco generally exhibited pH values close to neutrality, Piauí showed the greatest contrast, with pH values of approximately 5.0 in FA and 8.0 in DA (Figure 2). This wide range indicates that the current land-use system alone is insufficient to explain soil reaction, as soil pH reflects the combined influence of parent material, mineral weathering, base-cation leaching, and the composition of the soil exchange complex, as well as past management practices such as fertilization and liming [21,22].
In forest environments, acidification of the surface soil layer may be associated with the production of organic acids during litter decomposition, differential uptake of cations and anions by plants, and leaching of base cations [23,24]. Conversely, higher soil pH values may reflect greater abundance of Ca2+, Mg2+, and other base cations in the exchange complex, as well as potential residual effects of previous management practices. Therefore, the pronounced pH contrast between FA and DA in Piauí (Figure 2) should be interpreted in conjunction with the other soil chemical attributes rather than simply as a consequence of soil degradation.
This interpretation is further supported by the high Ca2+ (Figure 4c) and Mg2+ (Figure 4d) contents and potential CEC (Figure 5) values observed mainly in RA and/or DA in Piauí, in contrast to FA. The concurrence of higher pH and greater abundance of exchangeable bases suggests substantial differences in the composition of the soil exchange complex among these areas. At the same time, the forest area in Piauí exhibited higher TOC (Figure 3a), demonstrating that greater soil organic carbon accumulation did not necessarily correspond to higher pH or greater concentrations of exchangeable bases.
From a functional perspective, variation in soil pH can alter nutrient availability and the surface charge of soil constituents, while also influencing microbial activity and organic matter decomposition and mineralization processes [25,26]. In soils dominated by pH-dependent charges, changes in soil reaction can modify potential CEC and, consequently, cation retention. Therefore, part of the variation in soil fertility observed among states and land-use systems should be interpreted by considering the combined effects of soil pH, particle-size distribution, mineralogy, and organic matter.

4.2. Organic Carbon and Humic Fractions as Indicators of Land-Use Changes

The differences in total organic carbon (TOC) observed among land-use conditions varied considerably across the investigated desertification hotspots (Figure 3a). In Paraíba and Piauí, forest areas exhibited TOC contents 12% and 57% higher than those of degraded areas, respectively. These findings are consistent with the recognized role of vegetation cover in providing organic inputs through litter deposition, root turnover, and rhizodeposition, while reduced soil disturbance may favor carbon retention within soil aggregates and associations with mineral surfaces [27,28,29]. Nevertheless, because the present study was based on a cross-sectional comparison of distinct sampling areas, these differences cannot be directly interpreted as carbon losses caused by degradation or as evidence of carbon recovery following restoration.
In Ceará and Pernambuco, TOC contents did not differ significantly among land-use conditions (Figure 3a), further demonstrating that soil carbon patterns were not uniform across the investigated regions. Soil organic carbon is influenced by several interacting factors, including vegetation characteristics, organic residue inputs, soil texture, mineralogy, moisture availability, microbial activity, and previous land management [30,31]. In particular, sandy soils generally exhibit a lower capacity for organo-mineral carbon protection, which may influence their carbon retention potential [32,33,34].
Although restoration areas generally exhibited lower TOC contents than forest areas, this pattern does not necessarily indicate incomplete carbon recovery or a particular stage of restoration. The sampled areas may have differed in their initial carbon contents, soil-forming factors, and historical management practices. Previous studies have documented changes in soil organic carbon following vegetation restoration [35]; however, determining whether similar processes occurred in the present study would require baseline measurements or repeated sampling over time.
The chemical fractionation of soil organic matter provided complementary information on the distribution of carbon among operationally defined fractions. Fulvic acid carbon (FAC) exhibited contrasting patterns across the investigated hotspots (Figure 3b). In Ceará and Piauí, forest areas exhibited higher FAC contents than degraded areas, whereas the restoration area in Paraíba exhibited FAC contents approximately 50% higher than those observed in degraded and forest areas. Conversely, the restoration area in Pernambuco exhibited lower FAC contents than the degraded area. These differences may be associated with variations in organic matter inputs, soil properties, and the chemical characteristics of organic compounds [36,37,38,39]. However, the higher FAC content observed in the restoration area of Paraíba does not demonstrate that this fraction increased during restoration or responded earlier than TOC.
Humic acid carbon (HAC) also exhibited marked variation among land-use conditions (Figure 3c). In Ceará and Piauí, forest areas showed higher HAC contents than the other land-use categories, whereas in Paraíba, forest and restoration areas exhibited similar HAC contents, approximately 85–95% higher than those recorded in the degraded area. These findings indicate differences in the distribution of chemically extracted organic carbon fractions among the sampled soils. Such differences may reflect the combined influence of vegetation-derived organic inputs, microbial transformations, soil mineralogy, and historical land management [40,41,42]. Nevertheless, the present results do not establish whether the observed differences developed as a consequence of restoration practices.
Humin carbon (HC) exhibited a similarly heterogeneous distribution among land-use conditions (Figure 3d). In Paraíba, the restoration area showed the highest HC content (13.0 g kg−1), corresponding to values 71% and 136% higher than those observed in the forest and degraded areas, respectively. In contrast, the forest area in Piauí exhibited the highest HC content (18.9 g kg−1), exceeding the values observed in degraded and restoration areas by approximately 58% and 64%, respectively. These contrasts reinforce the importance of site-specific soil characteristics in determining the distribution of organic carbon among chemically defined fractions.
Although humin is operationally defined as the fraction remaining insoluble during alkaline extraction, its concentration should not be considered a direct measure of carbon persistence or stabilization [43]. Soil carbon persistence depends on multiple interacting mechanisms, including mineral associations, aggregate protection, microbial accessibility, and environmental conditions [44,45]. Consequently, the higher HC content observed in the restoration area of Paraíba cannot, by itself, demonstrate greater long-term carbon stabilization resulting from restoration.
The contrasting distributions of FAC, HAC, and HC among the investigated areas indicate that the composition of chemically fractionated soil organic matter varies across land-use categories and environmental settings. These patterns should not be interpreted as evidence of a sequential transformation of carbon fractions or as indicators of specific stages of restoration. Instead, they may reflect differences in organic matter inputs, microbial processing, soil texture, mineralogy, and land-use history [46,47].
The distribution of organic carbon among humic fractions is influenced by several factors, including soil type, vegetation, land use and management, climatic conditions, and soil drainage [48]. Accordingly, the proportion of carbon associated with humic substances relative to TOC may vary considerably among soils and environmental conditions, potentially accounting for more than 50% of soil organic carbon [49]. This variability also extends to the distribution of carbon among individual humic fractions. Oliveira Netto et al. [50] reported differences in fulvic acid carbon, humic acid carbon, and humin carbon across cropping systems and soil depths in the Brazilian Cerrado. Similarly, Crespo et al. [51] observed variations in humic carbon fractions under contrasting land-use conditions. Collectively, these findings indicate that the proportion of carbon associated with humic substances relative to TOC is not constant but varies according to soil characteristics and land-use and management practices, providing context for the differences observed among the investigated soils.
Overall, the combined assessment of TOC and chemically extracted carbon fractions provided a more detailed characterization of soil organic matter across the investigated land-use conditions. However, identifying restoration-induced changes, determining the direction of carbon accumulation, or establishing temporal relationships among carbon fractions would require longitudinal monitoring, baseline soil characterization, and additional information on restoration history and management practices.

4.3. Nutrient Availability and Cation Exchange Capacity

Soil chemical attributes exhibited substantial variation among the investigated desertification hotspots and land-use conditions, highlighting the importance of local edaphic characteristics when interpreting soil fertility patterns. This variability was particularly evident for Mehlich-1-extractable phosphorus (P), with differences among land-use conditions reaching approximately 1400% in Paraíba and exceeding 6000% in Piauí (Figure 4a).
The Mehlich-1 method is widely used for routine soil fertility assessment in Brazil and provides an operational estimate of extractable P using an acidic extracting solution. However, the amount of P extracted by this method depends on soil characteristics and the chemical forms in which P occurs. Its interpretation may be influenced by soil texture, mineralogy, P sorption capacity, and the presence of calcium-associated P compounds. Consequently, Mehlich-1-extractable P should not be regarded as a direct measurement of the entire pool of P potentially available for plant uptake.
In highly weathered soils containing substantial amounts of Fe and Al oxides, P availability is strongly influenced by sorption reactions, and Mehlich-1 extraction may not fully represent the dynamic processes controlling P supply to plants. Conversely, in soils containing appreciable amounts of calcium-associated phosphates, the acidic Mehlich-1 solution may dissolve some P compounds that do not necessarily contribute proportionally to plant P uptake under field conditions. These limitations are particularly relevant when comparing soils from geographically distinct desertification hotspots with potentially contrasting mineralogical and chemical properties.
In Ceará, the higher Mehlich-1-extractable P contents observed in the forest area may be associated with differences in nutrient cycling, organic matter inputs, and soil characteristics. Litter deposition, decomposition, and mineralization can contribute to P redistribution within forest ecosystems [52,53].
The particularly high Mehlich-1-extractable P contents observed in restoration areas in Paraíba and Piauí may reflect several factors, including differences in soil mineralogy, parent material, P retention capacity, and previous land-management practices. Residual effects of historical mineral or organic fertilization represent one possible explanation, since previously applied P may remain in soil pools for extended periods [54,55]. Furthermore, the magnitude of the differences in Mehlich-1-extractable P among the sampled areas should not be interpreted as evidence of restoration-induced improvements in P availability. Because this study was based on a cross-sectional comparison, the observed patterns may reflect pre-existing soil differences and historical management influences rather than changes resulting from restoration practices.
Overall, the interpretation of Mehlich-1-extractable P across the investigated desertification hotspots requires consideration of the chemical and mineralogical characteristics of each soil. The use of complementary P extraction methods, soil P fractionation, and plant-based assessments could help clarify the relationship between chemically extractable P and actual P availability in these contrasting semiarid environments.
The pattern observed for K+ was more consistently associated with forest cover, as FA exhibited the highest contents in three of the four states (Figure 4b). Unlike nutrients predominantly incorporated into complex organic structures, K remains largely in ionic form within plant tissues and can be rapidly released following residue senescence and decomposition [56,57]. Therefore, continuous litter inputs and the biological redistribution of K from subsurface to surface soil layers may favor its maintenance in forest soils [58].
The higher K+ content in DA in Piauí (Figure 4b), however, further demonstrates the importance of edaphic characteristics and land-use history. Potassium retention is strongly dependent on soil mineralogy, CEC, and the presence of minerals capable of supplying or retaining this cation. Therefore, high K+ contents in degraded areas do not necessarily indicate improved biogeochemical functioning but may instead reflect differences in parent material or inputs resulting from previous management practices [59].
The patterns observed for Ca2+ (Figure 4c) and Mg2+ (Figure 4d) followed a similar trend. The greater availability of these cations in FA in some states may be associated with vegetation-mediated nutrient cycling, particularly through the return of base cations to the soil via litterfall [60,61]. However, the high Ca2+ and Mg2+ contents observed in RA in Piauí, together with the high potential CEC (Figure 5) and near-neutral to slightly alkaline pH in the same area, indicate strong control by intrinsic soil properties and previous land management.
Ca2+ and Mg2+ are components of the soil exchange complex, and their retention depends on the amount and nature of surface charges associated with soil organic matter and clay minerals [62,63,64]. Thus, mineralogical and textural differences may explain a substantial proportion of the variability among states, particularly in Piauí, where RA simultaneously exhibited a greater proportion of fine soil particles, high Ca2+ and Mg2+ contents, high pH, and high potential CEC.
This relationship becomes more evident when CEC is considered together with particle-size distribution, soil pH, and organic carbon. The higher potential CEC observed in FA in Paraíba and Pernambuco (Figure 5) may be associated with the combined contribution of reactive mineral fractions and soil organic matter to soil surface charge. Soil organic matter has a high density of pH-dependent charges and can contribute substantially to the CEC of tropical soils, particularly when the mineral fraction is dominated by low-activity minerals [65,66]. Consequently, organic matter losses resulting from soil degradation may reduce not only soil carbon stocks but also the capacity of the soil to retain nutrient cations [67].
In Piauí, however, DA and particularly RA exhibited substantially higher potential CEC than FA (Figure 5). This pattern coincided with marked differences in particle-size distribution and soil chemical properties: RA had a lower sand content and greater proportions of fine particles, whereas FA was characterized by a high proportion of sand. In addition, DA and RA exhibited substantially higher pH values than FA. In variable-charge soils, increasing pH promotes the deprotonation of functional groups associated with soil organic matter and mineral surfaces, thereby increasing negative surface charge density and, consequently, cation retention capacity [68,69]. Thus, the combination of a greater proportion of fine particles, higher pH, and mineralogical differences provides a more consistent explanation for the high potential CEC observed in DA and RA in Piauí than organic matter content alone.
This interpretation is further supported by the fact that FA in Piauí exhibited the highest TOC, FAC, HAC, and HC contents while simultaneously showing the lowest pH and a lower potential CEC than DA and RA. Therefore, soil organic carbon content alone did not determine cation retention capacity in this environment. The decoupling between higher TOC under forest and higher potential CEC in degraded and restoration areas represents one of the most relevant findings of this study, highlighting the need to jointly consider soil organic matter, texture, pH, and mineralogy when assessing soil fertility in the Brazilian semiarid region. These marked textural differences may also confound the interpretation of TOC, humic fractions, and nutrient concentrations across land-use categories. Consequently, the observed chemical differences cannot be attributed exclusively to restoration practices, limiting causal inference regarding restoration effects.

5. Conclusions

Soil organic carbon fractions and fertility attributes differed among degraded, forest, and restoration areas across the four investigated desertification hotspots in the Brazilian semiarid region. These differences were strongly dependent on the geographical and edaphic characteristics of the sampled areas, demonstrating that associations between land-use categories and soil properties were not consistent across all regions.
Forest areas exhibited higher total organic carbon contents than degraded areas in Paraíba and Piauí, whereas restoration areas showed comparatively higher contents of specific humic fractions in some locations, particularly in Paraíba. In this state, the restoration area exhibited higher humic acid and humin carbon contents than the degraded area, indicating differences in the distribution of chemically extracted soil organic carbon fractions rather than demonstrating restoration-induced carbon accumulation or stabilization.
Soil fertility attributes, including Mehlich-1-extractable phosphorus, exchangeable cations, pH, and cation exchange capacity, also exhibited contrasting patterns among land-use categories and desertification hotspots. These differences highlight the importance of considering soil texture, pedogenic characteristics, and previous land management when interpreting soil chemical properties in semiarid environments.
The combined assessment of particle-size distribution, total organic carbon, humic fractions, and soil fertility attributes provided a comprehensive characterization of the investigated soils. Future investigations incorporating baseline characterization, repeated soil sampling, and detailed land-management histories are needed to determine the extent to which restoration practices influence soil carbon dynamics and fertility over time.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/horticulturae12111379/s1, Table S1. Geographical coordinates, soil classification, native vegetation, main drivers of desertification, restoration initiation years, restoration practices, and fertilization history in four desertification hotspots of the Brazilian semiarid region [70], Table S2. Two-way analysis of variance (ANOVA) and p-values from the Shapiro–Wilk normality and Bartlett’s homogeneity of variance tests for soil physical and chemical properties across desertification hotspots and land-use systems in the Brazilian semiarid region.

Author Contributions

Conceptualization, J.A.D., J.F.d.B.N. and R.F.P.; methodology, J.A.D., J.F.d.B.N., C.d.S.S., F.S.d.A., V.V.L.d.A., A.C.A.d.S., R.S.M., E.F.d.M., W.E.P., E.d.C.C., A.S.F.A., E.V.d.M., A.P.d.A.P. and R.F.P.; validation, J.F.d.B.N. and R.F.P.; formal analysis, W.E.P., E.F.d.M. and R.F.P.; investigation, J.A.D., J.F.d.B.N., C.d.S.S., F.S.d.A., V.V.L.d.A., A.C.A.d.S., R.S.M., E.F.d.M., W.E.P., E.d.C.C., A.S.F.A., E.V.d.M., A.P.d.A.P. and R.F.P.; resources, J.F.d.B.N.; data curation, J.F.d.B.N. and R.F.P.; writing—original draft preparation, J.A.D. and R.F.P.; writing—review and editing, J.F.d.B.N., C.d.S.S., F.S.d.A., V.V.L.d.A., A.C.A.d.S., R.S.M., E.F.d.M., W.E.P., E.d.C.C., A.S.F.A., E.V.d.M. and A.P.d.A.P.; visualization, R.F.P.; supervision, J.F.d.B.N. and R.F.P.; project administration, J.F.d.B.N.; funding acquisition, J.F.d.B.N. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by Paraiba State University, grant #01/2026, Coordination of Superior Level Staff Improvement—CAPES (Finance Code 001), and Paraíba State Research Foundation—FAPESQ (Grant #09/2023).

Data Availability Statement

The dataset is available upon request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
CECeará
PBParaíba
PEPernambuco
PIPiauí
TOCTotal organic carbon
CECCation exchange capacity
SOMSoil organic matter
DADegraded area
FAForest area
RARestoration area
FACFulvic acid carbon
HACHumic acid carbon
HCHumin carbon
ANOVAAnalysis of variance

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