Contreras-Santos et al., (2020) reported that silvopastoral systems accumulate between 60.6 and 65.1 Mg ha− 1 of organic carbon in the soil, compared to typical treeless grazing systems, which accumulate just 28.3 Mg ha− 1 of C. Similarly, Rojas et al., (2009) obtained values of 121.7 and 121.2 Mg ha− 1 of C for silvopastoral systems with Pithecellobium saman and Diphysa robinioides associated with B. brizantha, respectively, in comparison to 87.7 Mg ha− 1 of C for B. brizantha monoculture pasture.
Rojas et al., (2009) demonstrated that the depth considerably influenced the organic carbon content of the soil, with the first 20 centimeters in both silvopastoral and conventional systems having the highest organic carbon content. On average, 55% of the soil's total carbon is in the top 20 cm, compared to 25% and 20% at depths of 20 to 40 cm and 40 to 60 cm, respectively. Vasquez et al., (2020) discovered that at a depth of 0 to 15 cm, it was substantially greater than at a depth of 15 to 30 cm; in the first depth (0–15 cm), averages ranged from 108.85 to 76.73 Mg ha− 1 of C, while in the second depth (15–30 cm), averages ranged from 40.20 to 24.57 Mg ha− 1 of C. According to Ibrahim et al., (2007), soil carbon varies with depth and depends on the type of soil, its organic matter content, and the decomposition rate. The results obtained in this essay concur with these findings.
The increased soil moisture in SPS reported in this study is likely attributable to the function of tree shade in preventing soil moisture loss (da Silva et al., 2021). The low moisture obtained in both systems may be attributable to the scant precipitation reported during the season of data collection (July, Fig. 1). Due to the high likelihood of senescent material on the soil's surface, the moisture percentage was greater at the top 15 centimeters than at 30 centimeters. In addition, Bucheli et al., (2013) note that the composition of the tree species influences the soil moisture content differentially, and the SPS had a different tree composition than the CPS.
The SPS appeared to have more moisture than the CPS, so it was anticipated that they would have larger pores. However, the SPS exhibited smaller pores. This is likely due to the fact that pore spaces may be filled with water, making them inaccessible for ventilation and soil moisture retention. In addition, certain soils have a denser, more compact structure that restricts the quantity of available pore space (Hao et al., 2019). This information correlates with the higher apparent density and reduced total porosity of the SPS studied.
In addition, to soil physical characteristic, according to Sanchez (2021), the low bulk density values of the soil are influenced by high moisture contents and senescent material, such as tree leaf litter and pasture roots that accumulate in the superficial layer. Furthermore, Donoso (1994), indicates that pores with excellent drainage, aeration, and water infiltration have low BD values. Similarly, our results concur with Nachtergaele et al., (2023), which recommends that soils have a BD between 1.1 and 1.2 g/cm3. Regardless of system type, the lowest BD was discovered at depths between 0 and 15 cm, possibly due to the porous space, well-aerated soils, and the accumulation of plant remains on the soil surface (Alegre et al., 2017).
According, to Junet et al. (2013), the soil organic C is present between 20 and 100 centimeters deep. The soil organic matter is a heterogeneous mixture of organic compounds of various origins. Humins is the most stable fraction of soil organic matter and is considered the most resistant to microbial attack, as it is less active than humic acids and has lost a portion of the reactive groups (carboxyls and phenols) responsible for ionic exchange and the formation of stable aggregates (Conti and Giuffre, 2011 as cited by Rios et al., 2016). The low microbial biomass in CPS may not be sufficient for humins degradation and carbon utilization.
The fraction of organic matter that is the most stable contains humic acids. It is the fraction that has the highest ionic exchange capacity and plays a significant role in the formation of stable aggregates by acting as a binder for the particles, and it is typically found in the top 0 to 15 cm of the soil, as in this study. In contrast, fulvic acids have the lowest molecular weight and the highest solubility; in general, they contain a greater proportion of aliphatic compounds than humic acids and humins (Rios et al. 2016). These characteristics make them more susceptible to washing and loss of diffusion in the profile due to the action of the rains- Fulvic acids are carried from the most superficial layers to deeper layers, with the deepest values found at a depth of 15 to 30 centimeters on this study. While this investigation did not identify any disparities in fulvic acid between the two systems, another investigation conducted by Loss et al. (2014) observed that SPS had a positive impact on the proportions of both fulvic acid and humic acid. The effect can be linked to the accumulation of organic matter from fallen leaves and the release of organic compounds by grassroots into the soil, known as rhizodeposition. The deposition of organic material with a high carbon-to-nitrogen ratio promotes the process of humidification of soil organic matter (SOM) and subsequently leads to the accumulation of organic matter in protected compartments, as noted by da Silva et al. (2020) and Lima et al. (2008).
The first 10 to 15 cm of the SPS in this study exhibited less mechanical resistance, most likely due to the contribution of leaf litter, moisture, and porous spaces. Between 20 and 30 cm, the mechanical resistance of both systems is comparable. This is likely due to soil moisture. Although the BD of the CPS was lower than that of the SPS, compaction typically results from an increase in BD due to the pressure exerted by cattle, which exerts mechanical forces and lead to degradation of soil structure by decreasing aggregate stability (Sanchez, 2021).
The classification criterion utilized for the SPS and CPS was based on the proportion of tree covering inside the livestock area, with a threshold of greater than 15% for SPS and less than 15% for CPS. The observed disparities are likely attributable to the existence of trees inside the CPS, which can influence the levels of organic matter, bulk density (BD), organic carbon, and mechanical resistance through the shading impact exerted by the scattered trees. This outcome aligns with expectations for a SPS. This finding demonstrates that even a limited presence of trees has the potential to modify the physical properties of soil, so facilitating the storage of carbon.