Chemical properties of an Oxisol affected by different land use and soil management systems

Agricultural crop management practices that guarantee soil quality are necessary for the sustainability of agrosystems. The use of agroforestry systems to make food production viable with less loss of soil fertility is a possible alternative for sustainable agriculture. This work evaluated the chemical characteristics of an Oxisol in three systems of use and management: 1) peach for palm heart production; 2) peach palm for fruit and seed production; and 3) Urochloa decumbens pasture. In these areas, located at Fazenda Piloto of the Agricultural Sciences Department at the University of Taubaté UNITAU, Taubaté-SP, Brazil, samples were collected at the depths 0-10cm, 10-20cm and 20-40cm. In these soil samples, levels of P, K, Ca, Mg, B, Cu, Fe, Mn, Zn, pHCaCl2, H+Al, total organic carbon and organic carbon stock were determined. The sums of bases (SB), cation exchange capacity (CEC) and base saturation (BS) were calculated. The results were subjected to analysis of variance and the means were compared using the Tukey test at 5%. The results indicate that soil cultivated with peach palm for palm heart production showed better fertility compared to peach palm for fruit and seed production and pasture. The upper layers 0-10cm and 10-20cm are the most fertile, concentrating P, Ca e Mg, high CEC, SB and BS.


INTRODUCTION
Agricultural, livestock and forestry activities, when handled improperly, cause changes in the respective agrosystems, either by depleting of mineral elements in the soil or by reducing organic matter.
It is a great challenge to develop and maintain agricultural production systems with high productivity and environmental sustainability, especially in tropical regions, where soils present a high degree of weathering (Netto et al., 2009).
In highly weathered soils, organic matter becomes essential for maintaining or improving soil chemical quality, promoting cation retention, providing nutrients to plants, as well as complexing toxic elements (Portugal et al., 2010;Schiavo et al., 2011).
Soil quality has been continuously improving since the publication of the report "Soil and water quality -an agenda for agriculture" (NRCC, 1993). Among its different approaches, the report highlights the use of indicators related to soil functionality is an indirect way of measuring soil quality. Here the functionality characteristics would be related to the supply of nutrients and water to the plants, as well as nutrient cycling . Freitas et al. (2017) highlight that the study of soils' chemical properties, in addition to allowing the understanding of present fertility, makes it possible to observe possible changes suffered due to the management adopted in the area, and are therefore good soil quality indicators. These include acidity, salinity, organic matter, calcium, magnesium, potassium levels, available phosphorus, ion exchange capacity (Maia et al., 2013;Morais et al., 2015), carbon total and stock values (Almeida et al., 2016).
In several Brazil regions, peach palm cultivation (Bactris gasipaes) has become an important income source (Silva et al., 2007;Fernandes et al., 2013), given its potential for producing palm hearts and fruits. Proper crop residue management can improve physical and chemical soil characteristics, provide protection against erosion, and increase soil fertility (Souza and Piña-Rodrigues, 2013). Litterfall is the main pathway for nutrient transfer to the soil. Ribeiro et al. (2020) observed that this plantation produced 9.2 Mg ha -1 of litter, which once decomposed provides the return of significant amounts of N, P, K, Ca, and Mg to the soil.
Another way to promote the supply of biomass to the soil is the implantation of pastures, which in most cases replace native vegetation with pasture planted under continuous grazing, affecting soil chemical quality, since inadequate management of these is common in areas after their formation (Cardoso et al., 2011).
In the formation of Brazilian pastures, Urochloa grass is widely used, given the adaptability of this species to various edaphoclimatic conditions, growing well in acidic and infertile soils, as well as being a disease-resistant (Kluthcouski et al., 2013). It also has a high production capacity and continuously renews its root system, which can improve soil over time 3 Chemical properties of an Oxisol affected by … Rev. Ambient. Água vol. 7 (supplement) -Taubaté 2020 by increasing soil organic matter and, consequently, soil fertility.
In this context, as soil is a complex and dynamic natural resource, the objective of this study was to evaluate the chemical properties of a Oxisol in three use and management systems: (1) peach palm for palm heart production; (2) peach palm for fruit and seed production; and, (3) Urochloa decumbens pasture in fallow. The study also evaluated the contribution of each system to the gradient of chemical attributes at depths of 0-10, 10-20, and 20-40cm, where greater presence of roots of these plants is found.

MATERIAL AND METHODS
The study was carried out in an Oxisol area (Santos et al., 2018), located at Fazenda Piloto of the Agricultural Sciences Department at the University of Taubaté -UNITAU, Taubaté-SP, Brazil, coordinates 23º01' S and 45º30' W, 565 m altitude, with an average annual rainfall of 1,350 mm and an average temperature of 21.9°C (Folhes and Fisch, 2006).
In 1998, a Urochloa decumbens pasture was subdivided into sub-areas for installation of experiments to evaluate the chemical properties of an oxisol in three use and management systems ( Figure 1). The area for planting of peach palm was plowed and lime was added to the entire area. After 45 days, each plant was planted in pits, with spacing of 2x1 meters.
The rest of the Urochloa pasture remained without any type of mineral fertilization. The average size of each system under study was 0.5 ha. The systems were subjected to frequent clearings, with all biomass kept in place.
Each year fertilizer was used for production of peach palm for palm hearts with 100 kg ha -1 of ammonium sulfate and 40 kg ha -1 of potassium chloride; and every three years, fertilization was with 30 kg ha -1 of simple superphosphate and 2 kg ha -1 of borax. The fertilization was done in a total area, and all residues from the annual extraction of palm hearts were mixed into the soil. However, for the production of peach palm for fruits and seeds, fertilization, in total area, was made every three years, with 2 kg ha -1 of borax.
The experimental design used was completely randomized with the treatments (1) peach palm for palm heart production; (2) peach palm for fruit and seed production; and, (3) Urochloa decumbens pasture -the control soil management system because there was no soil correction and fertilization ( Figure 1).
In each experimental plot, after 15 years of peach palm plantation, 10 simple sub-samples with four repetitions per system were collected with a drill auger for carrying out chemical analysis. This procedure was performed for samples collected at depths of 0-10, 10-20 and 20-40cm. Each sample was dried in the shade, ground and sieved (2 mm), and subsequently analyzed for (pH) in CaCl2, potential acidity (H+Al), total organic carbon (TOC), phosphorus (P) in resin, K, Ca, Mg, Fe, Cu, Mn, Zn and B contents according to van Raij et al. (2001). These analyses were performed at the Laboratory of Soil Analysis and Plant Nutrition of the Department of Agricultural Sciences at the University of Taubaté -UNITAU, Taubaté-SP, Brazil.
From these results, the values of the sum of bases (SB), cation exchange capacity at pH 7.0 (CEC), base saturation (BS%) and the relationship between the elements that make up the CEC, K/CEC, Ca/CEC, and Mg/CEC were calculated.
The results were subjected to analysis of variance by F test (ρ ≤0.05) to detect differences in all factors and the comparison between the means by the Tukey test (ρ ≤0.05) using the SAS statistical software (SAS Institute, 1999).

RESULTS AND DISCUSSION
The analysis of variance showed that the soil use and management systems and the depths showed significance (ρ ≤0.05) for most of the analyzed chemical attributes. However, there was no significant interaction between land-use and management systems and depths (Table 1). Table 1. Summary of the results of the analysis of variance (p<0.05) for the chemical properties of an Oxisol as a function of the different management systems (peach palm for production of fruits and seeds, peach palm for production of palm hearts, and pasture of Urochloa decumbens) and depths (0-10cm, 10-20cm and 20-40cm). Soil chemical properties presented variability, with peach palm for production of palm hearts being the best system compared to the others ( Table 2). Regardless of the management system, the pH and H+Al values found characterize the soil as medium-to high-acidity (Raij et al., 1997).
We observed a reduction in the potential for acidity and consequently an increase in pH in the peach palm system for the production of hearts of palm. The lower pH and high potential acidity in peach palm for fruit and seed production cause the need to correct acidity in long plantings. Pasture, our control system, reflected the lack of soil correction and basic fertilization by other authors such as Benites et al.  Table 2. Chemical properties of an Oxisol as a function of management systems (peach palm for production of fruits and seeds, peach palm for production of palm heart, and pasture of Urochloa decumbens). The fertility results, considering the assessments made at depths of 0-10cm, 10-20cm and 20-40cm of the three management systems are shown in Table 3.
The upper layer (0-10cm) presents an intermediate value of H+Al, and does not differ from the other layers analyzed. The depths of 0-10cm, 10-20cm and 20-40cm showed lower (2.07 cmolc dm-³) and higher (2.40 cmolc dm-³) values, respectively. The pH in the superficial layers (0-10cm, 10-20cm) did not differ significantly. The deepest layer (20-40cm) presented a lower pH value (4.79). The increase in potential acidity and reduction of pH in depth were also verified in other studies carried out in Oxisol under different management systems (Guareschi et al., 2012;Montanari et al., 2016;Portugal et al., 2010;Schiavo et al., 2011).
The upper layers, 0-10cm and 10-20cm are the most fertile, concentrating P, Ca e Mg, high CEC, SB and BS ( Table 2).
The peach palm for the production of fruits and seeds and pasture of Urochloa decumbens systems showed low levels of P and Ca. In the fruit and seed production system, we observe that dynamics of mineral cycling and availability for the soil solution was not sufficient to maintain or improve the chemical conditions of the soil compared to pasture. We therefore recommended corrective practices in this management for soil enrichment, whether in chemical or organic form, as suggested by Butzke et al. (2020) and Silva et al. (2011). The peach palm heart palm management system showed high and medium P and Ca levels, respectively (Raij et al., 1997), that are justified by the use of maintenance fertilizers combined with greater nutrient cycling promoted by the cutting of the hearts of palm, since the biomass from the extraction was kept under the soil. Schiavo et al. (2011) andIwata et al. (2012) verified the increase in P and Ca soil levels management systems under which the biomass was kept under the soil without disturbance, which justified the tendency for the P and Ca gradients in depth.
The K and Mg soil levels were higher in the pasture system and lower in the peach palm system for the production of fruits and seeds (Table 2). This is understandable, since the pasture was not grazed for at least 15 years, that is a condition of less export of these nutrients, as pointed out by Benites et al. (2010).
The CEC was low in all systems under study similar compared to those observed by Tavares-Filho et al. (2011) in a study of soil fertility of pastures submitted to different management for more than a decade. In the Urochloa decumbens pasture, CEC was 5.36 cmolc dm -3 , which is similar (5.4 cmolc dm -3 ) to that verified by the same authors in a pasture system with grazed native Urochloa decumbens that were exposed to burns every 3 years.
BS was over 50% in all evaluated systems. Therefore, eutrophic soil with a slight gradient in depth, which is desirable for the good development of both peach palm and Urochloa decumbens (Raij et al., 1997). The results are similar to the studies that observed an increase in the nutrients availability over time with the enrichment of the superficial layers of the soil due to the constant supply of organic matter (Butzke et al., 2020;Iwata et al., 2012;Montanari et al., 2016). Except for Mg/CEC, the other relationships showed variation between the evaluated systems. But, despite these variations, Ca was the nutrient that contributed most to CEC, followed by Mg and K (Tables 2 and 3).
The results in Table 4 show that there was no significant interaction for the total organic carbon content between the land-use and management systems and the depths, despite a tendency to decrease in depth.  The total organic carbon was concentrated to a depth of 20 cm, reflecting in carbon stocks of 20.78, 23.20, and 24.56 Mg ha -¹ for the peach palm system for the production of fruits and seeds, peach palm for the production of palm hearts and pasture, respectively. The litter in the peach palm plantations proves this enrichment of the superficial layers of the soil, constantly adding new biomass (Ribeiro et al., 2020). Bernoux et al. (1999) found a significant increase in the carbon stock in a 15 year pasture (26.5 Mg ha -1 ), as in our study. Highest carbon stock was observed in the pasture, which corresponded with greater carbon storage (63.5 Mg ha -1 ) (Lopes et al., 2011). This greater carbon stock in pastures is because of the intense renewal of pasture root systems (Loss et al., 2014). Pereira et al. (2010) found decreasing levels of total organic carbon in depths up to 20cm in pasture area. However, these levels were lower than those verified in this study. According to the same authors, management systems in which the soil does not revolve contribute to the increase in organic carbon stock, particularly in the superficial layers, which improves both the physical and chemical characteristics of the soil, avoiding erosive processes over time.

CONCLUSION
The soil cultivated with peach palm for palm heart production showed better fertility compared to peach palm for fruit and seed production and pasture of Urochloa decumbens.
There was a higher concentration of nutrients and organic carbon in the depths 0-10cm and 10-20cm.