EFFECTS OF DEGRADATION ON SOIL ATTRIBUTES UNDER CAATINGA IN THE BRAZILIAN SEMI-ARID

ABSTRACT Anthropic activities in their various aspects have promoted soil degradation in the Brazilian semi-arid region (SAB). As a result, significant losses in productivity and in the ability of soils to fulfill their ecological functions have been reported. The present study investigated the effects of degradation on soil attributes and properties under dense (CAD) and sparse (CAE) shrubby Caatinga in Campina Grande, PB, Brazil. Samples from the 0-20 cm layer of soil were investigated via physical (particle size distribution and soil density), chemical (acidity, electrical conductivity, macronutrients, soil organic matter) and microbiological attributes (microbial biomass carbon (C-BMS), basal respiration of the soil (RBS) and metabolic quotient (qCO2) Data were submitted to the Mann-Whitney Test and Principal Component Analysis (PCA). Anthropic actions on the CAE promoted the exposure of the saprolitic layer on the surface. This layer has imperfect drainage, low levels of nutrients and organic matter and high sodicity, which contributes to the slow regeneration of vegetation. Carbon stock and microbial activity are significantly lower in CAE compared to CAD. Degradation resulted in losses of supporting ecosystem services (nutrient cycling and primary production) and regulation (erosion control and climate regulation). The results can be used to understand the dynamics of landscapes of low complexity (high degradation) in the SAB and serve as a framework to find strategies to restore the productive capacity of extensive degraded and/or desertified areas in the SAB.


INTRODUCTION
The Brazilian Semi-arid (SAB), a region with 1,182,697 km 2 (Sudene, 2017), represents almost 14% of the national territory and 76% of the Northeast region. It is a region traditionally subject to droughts, where erosive processes have been intensifi ed due to extensive livestock farming, rudimentary agriculture, and vegetal extractivism (CGEE, 2016).
Studies evaluating the eff ects of land use change on C (carbon) and N (nitrogen) dynamics (Althoff et al., 2018), energy and water fl ows (Silva et al., 2017), nutrient cycling (Moura et al., 2016), microbial abundance (Neves et al., 2021), physical and chemical soil quality (Mota et al., 2014) and microclimate ( Silva et al., 2021) have been conducted in diff erent SAB regions. Numerous studies have also shown the eff ects of degradation on the soil attributes of the SAB. Most of these studies have assessed these changes in the fi rst soil layers (Mora and Lázaro, 2014;Araújo Filho et al., 2017;Althoff et al., 2018) and, to a lesser extent, in depths greater than 40 cm and/or in subsurface pedogenetic horizons (Neves et al., 2021;Silva et al., 2021;Menezes et al., 2021).
Recently, some studies have sought to understand the relationships between biophysical variables and the fl ow of water and C in areas with diff erent stages of degradation in the Caatinga (Borges et al., 2020;Oliveira et al., 2021). These studies showed that the degradation of the Caatinga can modify the local microclimate by reducing atmospheric C sequestration and increasing greenhouse gas emissions.
Despite the importance of these studies, combined with the fact that soil properties and attributes can be safely used to assess soil quality (Maurya et al., 2020). Research that seeks to evaluate the physical, chemical and biological attributes of soils under Caatinga in diff erent stages of regeneration has been neglected (Silva et al., 2021).
Thus, in this research we seek to evaluate the physical, chemical and biological attributes of soil layers exposed on the surface by land degradation, where even after interventions and actions towards reforestation, it still has a slow capacity for vegetation regeneration. This scenario is similar to that observed for other regions in the SAB, where extensive heavily degraded areas have truncated soils, with subsurface horizons of high unavailability for agricultural use exposed on the surface (CGEE, 2016;Macedo et al., 2021).
In this context, this integrated study of soil attributes can contribute to the understanding of the eff ects of human actions on the dynamics and stock of nutrients in soils at diff erent stages of degradation, as well as elucidate the mechanisms involved in the recovery of fertility and ecosystem services of these soils with the restoration of vegetation. Certainly, this knowledge will also contribute to strategic actions aimed at reincorporating extensive areas of degraded soils in the SAB into agricultural activities, increasing the resilience and food security of family agroecosystems in the face of climatic adversities in the region.
The objective of this research was to evaluate physical, chemical and microbiological attributes of soils in two areas of Caatinga (dense shrubby Caatinga -CAD; sparse shrubby Caatinga -CAE) subjected to anthropic pressures in the SAB.

Study area
The research was carried out at the Experimental Station Prof. Ignácio Salcedo, belonging to the Instituto Nacional do Semiárido (INSA), municipality of Campina Grande, state of Paraíba, Brazil. An area under dense shrubby Caatinga (CAD) and another under sparse shrubby Caatinga (CAE) was studied ( Figure 1).
The CAE site (7º14'59.78"S and 35º56'49.70"W; 500 m) corresponds to a borrow area where the surface horizon of the soil was removed to supply material for the construction of a nearby road. This process caused the exposure of the saprolitic layer (horizon C) on the surface, which presents characteristics of the source material and ample occurrence of partially weathered minerals. As part of the recovery process of this degraded area, in 2018, reforestation was carried out with adapted and native species in the region, such as white jurema (Mimosa verrucosa), jurema preta (Mimosa tenuifl ora (Wild.) Poir), guandu (Cajanus cajan), cunhã (Clitoria ternatea), crotalaria (Crotalarea juncea), marmeleiro (Croton sonderianus Müll. Arg) and pereiro (Aspidosperma pyrifolium Mart). This confi guration results in a phytophysiognomy consisting of a low shrub stratum, with a maximum height of 3 m, being classifi ed as a sparse shrubby Caatinga (Rizzini, 1997).
The climate is semi-arid with low altitude and latitude (BSh) according to the Köppen classifi cation (Alvares et al., 2013). The average annual temperature is 23.3°C and the average annual rainfall is 503 mm. The native vegetation is the hyperxerophytic Caatinga, characterized as a dry xerophytic forest with sparsely distributed shrubs and small trees (less than 7 meters in height), and patches of grass that develop only during the rainy season (January to September).
The relief varies from gently undulating to undulating with elevations of fl at tops, composed of slopes of tens of meters with dry and open valleys (Brasil, 1972). Leptsols predominate in the area, formed from the weathering of cataclastic leucogneiss with biotite (Brazil, 1972).

Soil collection and analysis
Soils were collected in August 2020. Year in which annual precipitation was 551.2 mm, with a rainy season between March and July (494.2 mm), corresponding to 89% of annual precipitation. In this rainy period, rainfall was 270.6 mm in March and April and 223.6 mm between May and July.
The experimental design used was completely randomized. The sampling was performed in two homogeneous areas of 1000 m 2 , with fi ve collection points (experimental units), and at each point fi ve simple deformed soil samples were collected (0-20 cm), which constituted a composite sample that was dried in air and passed through a 2 mm sieve (TFSA). At each collection point fi ve undisturbed samples were also collected in 100 cm 3 volumetric rings to perform soil density (Ds) analysis. The deformed samples were morphologically described according to Santos et al. (2015). Additional features such as rock volume and percentage of aggregates were evaluated according to Schoeneberger et al. (2012).
Sample preparation and physical and chemical analyses of the soils were performed at the INSA Soils and Mineralogy Laboratory according to methodologies proposed by Embrapa (Teixeira et al. 2017). Particle size analysis and water dispersed clay were performed by the pipette method, using NaOH and H 2 O as dispersants, respectively. Ds was obtained by the volumetric cylinder method.
The pH was determined in water (1:2.5 -TFSA:H 2 O) and the electrical conductivity (CE) was obtained using a direct reading conductivity meter (1:5.0 -TFSA:H 2 O). The exchangeable contents of Ca 2+ , Mg 2+ and Al 3+ were extracted with KCl 1 mol L -1 , while P, K + and Na + were extracted with Mehlich 1 solution (HCl 0.05 mol L -1 + H 2 SO 4 0.0125 mol L -1 ). Potential acidity (H + Al) was extracted with 0.5 mol L -1 calcium acetate pH 7.0. Ca 2+ and Mg 2+ were determined by complexometry, Al 3+ by titration, K + and Na + by fl ame photometry and P by colorimetry. The total organic carbon (COT) was determined by the method proposed by Yeomans and Bremner (1988). COT contents were converted to soil organic matter (MOS) from multiplication by the factor 1.724 (Machado et al., 2003). Total carbon (CT) was obtained via dry combustion in a CHNS elemental analyzer. Organic carbon stock (EstC) was calculated according to the following equation (Du et al., 2017): Where: EstC (Mg ha -1 ) represents the concentration of organic carbon (g kg -1 ), Ds is the density of the soil (kg dm -3 ), E is the thickness of the evaluated layer (cm) and R is the content volumetric (%) of rock fragments > 2 mm in the soil.
With the results, the following indices were obtained: degree of fl occulation (GF), base sum (SB), eff ective and total cation exchange capacity (CTCef), base saturation (V%), aluminum saturation (m%) and percentage of sodium saturation (PST) (Teixeira et al. 2017). Soil analysis results were interpreted according to Sobral et al. (2015).
The analyzes of the microbiological attributes were carried out at the Laboratory of Environmental Microbiology of INSA, evaluating the microbial biomass from the quantifi cation of carbon (C-BMS), by the chloroform-fumigation-extraction (CEF) method (Vance et al., 1987) and the activity microbial growth by basal soil respiration (RBS) (Jenkinson and Powlson, 1976). The metabolic quotient (qCO 2 ) values, which express the ratio between basal respiration and soil microbial biomass per unit of time, were calculated by dividing CBM by RBS.

Data analysis
The data obtained were submitted to the nonparametric Mann-Whitney test (p < 0.05). In addition, principal component analyzes (PCA) were performed in order to reduce the large number of variables to a more signifi cant set. The R software (version 4.1.0) was used for such analyzes (CoreTeam, 2017).

Morphology and physical attributes
The morphological and physical attributes of the studied areas are shown in Table 1. The soil under CAD is dark yellowish-brown and under CAE it is dark gray. Mottles occur only on the ground under CAE. Very friable medium/large subangular blocks predominate in the area under CAD, while extremely hard/fi rm massive structure was identifi ed in CAE. Roots of varying sizes are common or abundant only in soil under CAD.
The soil under CAE is sandy loam, while the soil under CAD is sandy loam (Table 1). There is no signifi cant diff erence in sand, ADA and GF contents between the evaluated soils. Silt contents are signifi cantly higher in CAE, while clay contents are signifi cantly higher in CAD (p < 0.05). Silt contents are signifi cantly higher under CAE, while Ds is signifi cantly higher under CAD.

Chemical Attributes
The chemical attributes of the studied areas are presented in Table 2. Soil under CAE is practically neutral, while soil under CAD is moderately acidic.
The low content of Ca 2+ in the soil of CAE is signifi cantly lower than the content considered high in the CAD (CAE: 0.5; CAD: 3.3 cmol c kg -1 ; p < 0.05). The average Mg 2+ content in CAE is signifi cantly lower than in CAD (CAE: 1.0; CAD: 1.9 cmol c kg -1 ; p < 0.05). Both soils have high K + contents, although signifi cantly higher in CAD (CAE: 1.2; CAD: 7.9 cmol c kg -1 ; p < 0.05). Al 3+ and P contents are low in both evaluated soils, although P is signifi cantly higher in CAD.

Microbiological attributes
No signifi cant diff erence was observed in microbial biomass carbon content (C-BMS) for the evaluated soils (Table 3). On the other hand, RBS (1) Soil analyzes performed according to the methodology described by Vance et al., (1987) and Jenkinson & Powlson (1976); (2) Averages followed by diff erent letters in the column diff er by the Mann-Whitney test at 5% signifi cance (p<0.05); ns Not signifi cant. C-BMS: carbon from soil microbial biomass; RBS: basal soil respiration; qCO 2 : metabolic quotient.

Degradation and physical attributes of soils
Our results showed that the removal of the Caatinga followed by the removal of the surface layer of the soil signifi cantly altered soil properties. Such practices intensifi ed the erosive processes in the area currently under sparse shrubby Caatinga (the authors observations), which resulted in the loss of the soil surface horizon (horizon A) and the exposure of the saprolite surface (horizon Crn). The occurrence of ≥ 50% of rock structure, < 50% of soil aggregates and massive structure confi rm the occurrence of saprolite (Juilleret et al., 2016).
No variability was found in the sand contents, given the predominance of quartz in the soil source material and the low precipitation rates in the region. The higher levels of silt in CAE refl ect the incipient pedogenesis of the saprolytic layer. The high silt/ clay ratio confi rms the moderate weathering in the saprolite, delaying the transformation and/or dissolution of plagioclases and micas that remain in the soils at silt particle size ( Santos et al., 2018).
Studies have shown that seasonality and diff erent land use systems alter Ds in semi-arid regions (Mora and Lazaro, 2014;Silva et al., 2021). In this research, Ds refl ects the higher temperature on the surface of degraded soils, since it is more exposed (Oliveira et al., 2021), which contributes to the reduction of MOS from the oxidation of carbon with a consequent increase in Ds. In addition, in the soil under CAD there is a greater contribution of litter and roots, which contributes to an increase in MOS and a reduction in Ds.

Degradation and chemical attributes of soils
The more acidic reaction of soils in CAD is mainly due to the ionization of H + ions from carboxylic and phenolic acids, and notably from tertiary alcohols from organic matter (Silva and Mendonça, 2007). On the other hand, the practically neutral condition of the saprolite refl ects its origin from gneisses and amphibolites, which release considerable amounts of basic reaction cations ( Santos et al., 2017;Câmara et al., 2021).
Soils under CAE have a eutrophic character (V > 50%). However, in these soils Na + ions predominate in the exchange complex, while K + , Ca 2+ e Mg 2+ are dominant in soils under CAD. This fact confi rms that the removal of the surface layer of soils and the consequent exposure of the saprolite on the surface reduces soil fertility, which corresponds to 75% of the Ca 2+ and K + contents and 47% of the Mg 2+ contents. This lower availability of nutrients, together with the greater compactness of the saprolite, contributes to the slow regeneration of vegetation in the area.
Degradation promoted an increase in surface Na + contents. Surprisingly, these contents are much higher than the average values found in natric horizons in the SAB (Brasil, 1972;Oliveira Filho et al., 2020). These levels confi rmed the sodium character (PST ≥ 15%) of the saprolite, whose origin is mainly credited to the weathering of plagioclases (dos Santos et al., 2018). In semi-arid regions, where the potential for evapotranspiration exceeds precipitation, Na + tends to predominate in the exchange (sodifi cation) complex (Araújo Filho et al., 2017), which can lead to colloidal dispersion and reduced soil permeability, making regeneration diffi cult from degraded areas.
Signifi cant carbon losses in degraded soils in the Brazilian semi-arid region have been reported (Neves et al., 2021;Santos et al., 2022). Our results confi rm these observations, where the exposure of saprolite on the surface represents a reduction of 93.6% in CT and 87.8% in EstC, when compared to the surface layer of soil under CAD. Although the Caatinga vegetation in both areas acts as a carbon sink (Oliveira et al., 2021), our results indicate that only soils under CAD store carbon at levels reported for other areas of the Caatinga biome, even in amounts greater than found in Caatinga preserved under similar soils (Althoff et al., 2018).
Thus, considering that Leptsols sequester an average of 65 Mg ha -1 of total carbon, which corresponds to between 70 and 80% of the total carbon stock of ecosystems with Caatinga vegetation (Menezes et al., 2021), our results prove that degradation compromises atmospheric carbon sequestration, with direct impacts on the carbon cycle and the provision of ecosystem services, notably those related to regional climate regulation.

Degradation and microbiological attributes of soils
Soil microbiological attributes can be signifi cantly altered with changes in land use, management practices and soil degradation (Kaschuk et al., 2010;Araújo Filho et al., 2018). The C-BMS values found for CAD and CAE are similar to those found for soils under diff erent uses in the Caatinga (72 and 385 mg C kg -1 ) (Kachuck et al., 2010).
The highest RBS observed in the CAD area indicates high biological activity and decomposition of organic matter, with a consequent high level of productivity in the ecosystem . A study carried out in agroforestry systems found greater RBS in the surface layer of the soil, associated with a greater amount of organic residues in this layer (Pezarico et al., 2013), corroborating the results of our study, where the CAD presented a greater amount of MOS.
The highest values of qCO 2 in soil under CAD refl ect the release of CO 2 throughout the process of mineralization of organic matter. Higher values of qCO 2 were observed in forests and signifi cantly decreased in soil under cultivation (Dinesh et al., 2003). According to the authors, these high values suggest that soil microorganisms in forests need high energy compared to cultivated sites. Higher values of qCO 2 in an area of native Caatinga when compared to cultivated areas are indicative that the microbial biomass in areas without cultivation has greater metabolic activity.
On the other hand, the signifi cantly lower qCO 2 in CAE may indicate an economy in the use of energy by microorganisms in the saprolitic layer, which have become more effi cient in the use of ecosystem resources, releasing less CO 2 into the atmosphere and incorporating more carbon contents to microbial tissues. This fact indicates a stable system, but not necessarily with a high level of productivity.

Degradation and density of vegetation cover
PCA clearly individualized the topsoil under CAD and the saprolite of the CAE area. In this way, we show that the slow regeneration of the Caatinga vegetation is taking place in a saprolitic layer with: (i) imperfect drainage, inferred by the occurrence of gray mottling (7.5YR 4/1) indicative of oxidation-reduction processes (gleization); (ii) low clay content, which implies a lower amount of clay minerals responsible for the high cation exchange capacity in saprolites in the SAB (Santos et al., 2017); (iii) low levels of organic matter and carbon, with adverse implications for the phenomena of nutrient adsorption and water absorption; (iv) high sodicity; and (v) reduced microbiological activity, which can compromise the mineralization of organic compounds and, therefore, the release of nutrients to plants.
On the other hand, soils under CAD have higher levels of nutrients and organic matter, with carbon stocks at levels similar or superior to other areas with preserved Caatinga vegetation. This explains the higher stage of vegetation regeneration in these soils, presenting higher primary production and signifi cant contribution to atmospheric carbon sequestration (Oliveira et al., 2021). These conditions allow the characterization of this area as highly complex, given its capacity to maintain important ecosystem services for the semi-arid region, such as nutrient cycling, primary production (support services), erosion control and climate regulation (regulation services) .

CONCLUSIONS
The removal of native vegetation followed by degradation causes the removal of the surface horizon of the soils and promotes the exposure on the surface of a saprolitic layer with high compactness, low nutrient and organic matter contents and high sodium contents.
Degradation signifi cantly reduces the carbon stock and biological activity of the soil, resulting in an increase in carbon emissions into the atmosphere and savings in energy use by microorganisms, with adverse implications for the mineralization of organic matter and the release of nutrients.
Ecosystem services related to climate regulation and nutrient cycling are compromised with degradation. This adverse scenario increases the vulnerability of these areas to extreme weather events and the process of desertifi cation, the latter due to regressive changes in soils, vegetation and water regime, leading to local biological deterioration, with direct implications for both the regeneration of Caatinga, as for the establishment of its productive capacity.