Soil organic carbon in a toposequence in the semiarid of Paraíba , Brazil

Organic carbon is a sensible indicator to evaluate the environmental quality of the soil. The objective of this study was to evaluate the organic carbon content of the soil in a toposequence in Serra do Teixeira, municipality of Teixeira, PB. Soil samples were collected in the upper third (UT), upper middle third (UMT), lower middle third (LMT) and lower third (LT) on three depths (0-5, 5-10 and 10-20 cm), with five replicates for each depth, resulting in a total of 60 samples. The organic carbon was evaluated using the methodology of Walkey-Black. Physical and chemical soil analysis were also carried out. The highest mean of carbon content was found in the first 5 cm (19.83 g dm), significantly differing from the other depths. It was also observed that the mean content of soil organic carbon on LMT was significantly higher than the other thirds, with 19.39 g dm. It is concluded that the highest contents of organic carbon are found on the most superficial layer of the soil. The organic carbon content variations found along the toposequence indicates influence of the relief and the anthropic action.


Introduction
The soil organic matter is considerate one of the main sources of energy and nutrients that exists in the system, being able to maintain soils productivity in general. In this context, understanding the dynamics of the carbon existing in the organic matter and knowing the stock of this element in the ecosystems became a challenge to life maintenance on the conditions in which it is currently (Xavier et al., 2006;Congo et al., 2011).
Unlike fossil carbon reserves, the flux of soil carbon is a dynamic process and can, in short or long term, be transferred to the atmosphere, being unable, in this way, to fully compensate the emissions from fossil fuel combustion. Thus, although the soil is the largest carbon compartment of terrestrial ecosystems stocking carbon, depending on how used, it can slow or mitigate the negative impacts of global climate change, or increase the problem with its inadequate use, releasing CO 2 (Machado, 2005).
According to Souza (2012), forests plays an important role in the equilibrium of the carbon global cycle balance, because of their long term storage capacity of great quantities of carbon in their biomass and on other forest compartments, such as the underbrush, the litter and in the soil.
In the case of soil compartment, carbon addition occurs through the deposition and decomposition of organic matter, being the added amount dependent on edaphoclimatic conditions and species present in the area. The losses occurs basically by the release of CO 2 in the respiration of organisms, microbial decomposition of residuals and the soil organic matter and losses of its organic form by leaching and erosion (Giongo et al., 2011).
The losses of organic carbon in the soil through erosive processes are considered important, mainly in areas with steep declivity, where these processes are potentialized and constant. Souza et al. (2003) and Silva et al. (2007) report the influence of declivity on the superficial runoff, and consequent organic matter carriage, and the accentuation of erosive Research, Society andDevelopment, v. 9, n. 5, e164953365, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3365 4 processes, being that in well-drained areas and with the other soil formation factors kept constant, the lower the declivity the higher will be the accumulation rate and organic matter.
The semi-arid region of the Brazilian northeast also faces problems due to the continuous use of the soil, by agricultural activity, and the phytomass removal for energy production, not adequately planned, reducing the soil carbon stock in general, as well as increasing the emission of CO 2 to the atmosphere. Added to these factors is the susceptibility of soils to erosion, which also have fundamental importance in the installation of the desertification process and the degradation of the semi-arid and the Caatinga, its main biome (Giongo et al., 2011). As consequence of these factors, the climate change can affect the magnitude of the processes, modifying the rates of addition and decomposition of carbon and, consequently, changing the amount of carbon stored in the soil.
According to Souza (2012), it is imperative that researches directed to determination of carbon content in the vegetation compartments and in the soil of the Caatinga biome are conducted, since there is a difficulty in estimating the total average of biomass produced by the vegetation, which has as a predominant characteristic a floristic composition formed by different vegetation typologies, due to the great variability of soils in the same area.
Therefore, studies that quantify carbon, considering the topography of the terrain also as a factor that determines its variation in the soil, are indispensable to fill the gaps on the behavior of this element in soils of the Caatinga biome. In this context, we hypothesized that the content of soil carbon content will be higher in areas with lower altitudes.
Thus, this work aims to evaluate the organic carbon content of the soil in a toposequence under a Caatinga vegetation in the municipality of Teixeira, PB.

Methodology
For this research, field collections and laboratory analyzes were necessary, whose nature can be characterized as quali-quantitative (Pereira et al., 2018).
The study was carried out in a toposequence, located in the Serra do Teixeira, municipality of Teixeira, PB, in the central-west region of the State of Paraíba, with an area of 182 km² (Figure 1). The region's climate, according to the classification of Köppen, is BSh type, semiarid, marked by a dry season and a rainy one (Alvares et al., 2013).
The average annual rainfall stays around 800 mm. The relief varies from wavy to heavily wavy, with heights that vary from 630 m in the southeast portion, to higher than 750 m, reaching 960 m to the north, in the Serra of Teixeira (Mascarenhas et al., 2005). Research, Society and Development, v. 9, n. 5, e164953365, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3365 The soils of the municipality have characteristic classes of the semi-arid region, as the Cambisols and the Lithic Neosols, with little depth and average texture (Cavalcante, 1989).
For the data collection, four areas were selected and georeferenced, following the ascendance of the terrain, whose information is showed in Table 1. The vegetation of the studied area is typical of the Caatinga, presenting different phytophysiognomies and preservation stages, in a way that the steeper it is, the more preserved the vegetation also is, because in the lower and lower middle thirds occurs agriculture and cattle raising practices with some predominant species of each studied third, On Table 2 is presented the data of the toposequence's soil granulometric analysis and it is verified that on the upper third (UT) the clay content was higher than the others, falling under the sandy-clay loam textural class; while on the upper middle third (UMT), lower middle (LMT) and lower (LT) the most predominant were sand particles, being included then in the sandy loam textural class. Samples used to determine the soil organic carbon were collected following the completely randomized design, in a 4 x 3 factorial scheme (four areas: UT, UMT, LMT and LT and three depths: 0-5, 5-10 and 10-20 cm) with five replicates, totaling 60 samples.
The organic carbon was quantified by the Walkley-Black method, adapted from Cantarella et al. (2001), which adaptation was the non-use of Whatman 540 type filter paper, in which the soil was allowed to decant at the bottom of the erlenmeyer and the supernatant was collected without it.
The organic carbon contents had their significance evaluated by analysis of variance and the difference between the means obtained were compared by the Tukey test at 5% Research, Society and Development, v. 9, n. 5, e164953365, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3365 probability, with the aid of the statistical program ASSISTAT Version 7.6 beta (Silva & Azevedo, 2013).

Results
The toposequence soil chemical analysis showed that the pH values tended from slightly acid in the upper thirds (UT and UMT) to acid in the lower middle and lower, fact in which higher values of P, Ca, Mg and K were identified on the upper thirds (Table 3).  It can be observed in Table 4 with the analysis of variance that there were significantly differences at 5% of probability in the carbon content of the toposequence for the two studied variables (thirds and depths), as well as in the carbon evaluation through the interaction between the thirds and the depths at 5% of probability by the Tukey test, in a way that if there is an interaction, it is necessary to evaluate each factor individually, presenting the third and depths analysis separately. The highest mean of organic carbon content was found on the first 5.0 cm of the soil with 19.83 g dm -3 , significantly differing from the other depths, where it was registered lower

Discussion
It was found that the content of nutrients P, Ca, Mg and K were lesser on the lower third (LT), with the reduction of P much more accentuated on the lower thirds, fact that can be explained by the increase of acidity and the significant presence of agricultural and livestock activities in these areas of lower declivity in the toposequence, suggesting that these practices reflect negatively on soil fertility, corroborating with this statement, Fraga & Salcedo (2004) and Sampaio et al. (1995) report that there is a reduction of nutrients, mainly P, as there is the substitution of native vegetation for agricultural activities.
Regarding the bases saturation (V%), it was verified that with the reduction of the toposequence declivity, the anthropic activity pressure on the vegetation of the Caatinga was increased, the Ca 2+ , Mg 2+ and K + values were lowered, with consequent reduction of V%. On the other hand, the CEC had its values more elevated in the upper third (UT) and lower middle third (LMT), in the first case, probably because it had higher values of pH, P, Ca, Mg and K, as well as more preserved native vegetation.
Higher values of pH and P are found on the upper thirds (UT and UMT), corroborating with the results reported by Zos et al. (2009), who stated that the increase of the soil pH can reduce the P adsorption in the soil, increasing its availability for plant absorption. In this case, the reduction of P presence may be due the agricultural activities practiced on the lower thirds, as well as its availability reduction due to pH reduction. Research, Society andDevelopment, v. 9, n. 5, e164953365, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3365 According to Souza (2012), the acidity of the soil, quantified through pH, is one of the factors that directly affects the agricultural areas productivity, since acidic soils substantially compromise the cation-exchange capacity (CEC). Martinazzo (2006) affirms that, according the technical indications of Soil Chemical and Fertility Commission (CQFS, 2004), the soil pH should be higher than 5.5 and base saturation (V%) exceed 65% of the CEC, since this condition provides an adequate ambient suitable for the developing of crops. To Oliveira et al. (2005), the range from 5.8 to 6.2 in the soil pH is the one that presents greater essentials nutrients availability for vegetables.
The superficial soil layers receive the addition of organic residues coming from the established vegetation of the area, being degraded over the time by action of the edaphic fauna, which concentrates in greater number on the soil superficial layers, favoring the decomposition. Thus, the soil carbon content is regulated according to the amount of residue to be decomposed, their chemical composition, besides the edaphoclimatic conditions that could alter this process, resulting in an increase or decrease of the soil organic carbon stock. Fracetto et al. (2012), while quantifying the carbon in a Caatinga area with native vegetation, verified a similar situation to the present work, where higher contents of C were found on the superficial layer (0-5 cm) and decreased as the depth increased. According with the same authors, this behavior is common in areas of native forest, due to the vegetable residues accumulation on the soil surface, promoting slow and gradual decomposition, ensuring one continuous incorporation of organic material into the soil, in a way that the concentrations are in the superficial layers. Other authors have also corroborate this idea, such as Passos et al. (2007), Potes et al. (2010) & Calonego et al. (2012. Souza (2012), studying the soil organic carbon contents on the rainy and dry seasons in a vegetation of the Caatinga, observed that on the two studied seasons the of 0-5 cm layer had higher soil organic carbon content. The author salient that one possible justification for the presence of higher carbon contents on the superficial layer is the vegetable material decomposition that comes from herbaceous species, which have a short life cycle and die mostly in the dry period.
There was interaction between the thirds and soil depths in the soil organic carbon content, thus, as long there is absence of anthropic actions and maintenance of the vegetation cover due to the difficult use access of this area (LMT), there is greater accumulation of organic matter and consequently increase of organic carbon content on the superficial layer depth (0-5 cm). This result may be related to the fact that in areas with accentuated declivity, like is the case of the UT and UMT, the organic carbon is more easily lost, resulting from the carriage of the particles from the most superficial layers of the soil to the lower parts of the relief, tending to an accumulation of this element in the lower areas of the landscape, such as occurred in the LMT. This result is similar to that observed by Silva et al. (2007) in a toposequence on the south of Minas Gerais, where they concluded that the soil organic carbon content decreased as the declivity increased.
In the LT, the lower values of carbon can be attributed to the more intense anthropic action, with inadequate management practices such as the cutting and burning the native vegetation, following the soil inversion to cultivate monoculture crops like maize or beans and livestock, resulting in the decrease of soil organic carbon content. Fraga & Salcedo (2004) identified a reduction in the carbon content when altering the vegetation cover and posterior soil use without conservationist precepts.
According to Barreto et al. (2006), when the natural vegetation is removed to install an agricultural system, there is an imbalance on the soil organic carbon content occurs, because the mineralization of the organic matter intensifies, initially causing the releasing of some nutrients, favoring the vegetal nutrition. When the organic matter addition process is inferior to that of decomposition, this system does not reach a new equilibrium, becoming exhausted and causing soil degradation. The authors also emphasizes that in tropical and subtropical soils, the organic matter has great contribution to fertility, increasing the cation-exchange capacity, improving their chemical, physical and biological characteristics, being then of fundamental importance in the maintenance of sustainability. Calonego et al. (2012) compared different management systems with areas of native vegetation from Cerrado, and found that human intervention through farming practices reduced the carbon stock in the soil to levels much lower than what was found in native forest conditions. Fracetto et al. (2012), evaluating the alterations of C and N contents due to the change of soil use on a native vegetation in the Caatinga biome for castor bean cultivation, observed low organic carbon contents on the cultivated soils with this species, regardless of the implantation time of the crop. Just like Tiessen et al. (1998);Fraga & Salcedo (2004) observed that the removal of vegetation from Caatinga with subsequent agricultural use resulted in an expressive decrease of 40 to 50% in the soil C contents.

Conclusions
The highest organic carbon contents were found on the soil superficial layer of 0-5 cm.
The organic carbon contents variations found along the toposequence indicates influence of the relief and anthropic action, such as deforestation, burnings, livestock and agriculture.
There was interaction effect between studied factors, thus, the LMT presented higher organic carbon content on the superficial layer of 0-5 cm depth.
As the content of organic carbon in the soil was influenced according to the level of anthropization of the place, it is suggested that this parameter can be used as an indicator of degraded areas, and that they be taken into account in soil conservation studies.