4.1. Litter stock
The higher litter stock observed in the FLO ecosystem compared to AFS 1 is likely linked to the maturity and diversity of this ecosystem, considering not only the lower litter deposition but also the reduced interaction of this litter with soil microfauna in younger ecosystems (Saj et al., 2021). Analogously, the litter stock of FLO compared to AFSs 2, 3, and 4 reveals a trend of increasing nutrient cycling efficiency along the temporal gradient due to the gradual increase in favorable environmental conditions such as humidity, temperature, oxygen, and pH (Froufe et al., 2020). In this case, the quality and composition of the litter are variables that directly influence nutrient availability for the soil-plant system.
Our results also highlight the importance of species selection to compose AFSs. The high quantity of reproductive material quantified in AFS 2, for example, suggests a possible association with the predominance of A. magium, a species that blooms and fruits multiple times a year (Matos et al., 2023). Although this predominance benefits soil protection due to the considerable increase in stored litter layer, ecosystems dominated by this species generally also exhibit lower decomposition rates, leading to slow release of nutrients for the soil-plant system (Jambul et al., 2020).
Species composition also plays a crucial role in litter quality and, consequently, decomposition rates, as it directly interacts with soil biota (Sena et al., 2020). For example, leaves of Theobroma cacao, present in AFSs 2 and 3, despite high lignin content, decompose rapidly due to notable concentrations of cellulose (Firmino et al., 2021). On the other hand, leaves considered hard (hardness > 372 g), such as those of Mangifera indica and Musa x paradisiaca found in AFSs 1 and 3, respectively, are characterized by lower palatability and longer decomposition time, although they may constitute a stable substrate (Matos et al., 2022).
4.2. Soil chemical attributes
We found similarities between the FLO and AFS 4 regarding most chemical attributes at both depths. This resemblance can be attributed to the presence of shading tree species in both environments, which contribute to the formation of a sub-canopy environment with high biomass production (Fujii et al. 2018). This microclimate favors the proliferation and activity of microorganisms, which decompose biomass and mineralize nutrients from this organic matter, highlighting the importance of this environment for nutrient biogeochemical cycling (Sadeghi et al. 2023; Bastos et al. 2023).
Overall, soils in the Amazon region are characterized by high acidity and low natural chemical fertility, so we can infer that despite AFS 4 and the FLO being older, the low levels of Ca, Mg, and V % in these ecosystems may be associated with the absence of liming, since Ca and Mg are the main inputs for this soil preparation stage (Siepel et al. 2019), consequently influencing the V % (Macedo et al. 2023).
The higher CECpH7 in the AFSs and FLO areas in the 0–10 cm layer may be justified by the lower OM content present in AFS 1 and 2 in the superficial layer, as expected due to the youth of the plants in these ecosystems and their vegetative structure in full development. OM presents OH groups in its aromatic groups, with the potential to generate negative charges at pH values above 3.5, justifying the lower soil pH value in the FLO (Chien et al. 2018). The lower pH in the reference ecosystem may reflect the higher values of potential acidity in this system, as these two factors have a directly proportional relationship.
Our results are aligned with previous research on the impacts of prolonged fire use in forest ecosystems (Cecilio Rebola et al. 2021; Mukul et al. 2022; Primo et al. 2023), highlighting the potential of AFSs to restore functional aspects in the Amazon. The practice of burning plant residues and the use of mineral fertilizers may have contributed to the increase in soil acidity over time, as these practices reduce soil pH and favor the release of hydrogen ions (H+) and aluminum (Al + 3) (Chaves et al. 2020). This continuous acidification process may be intensified due to base losses by leaching, dissolution reactions of CO2 in the soil solution, dissociation of H + ions from organic and inorganic radicals by microbial activity (Ontman et al. 2023).
Considering that soil acidity negatively affects nutrient availability to plants (Malavolta, 2006; Matschullat et al. 2019, 2020), our results emphasize the importance of litter stock for maintaining forest ecosystems. Despite the more acidic pH, the FLO showed higher OM levels in the superficial layer compared to the other evaluated ecosystems, which may be justified by the high litter stock. However, soil acidity and Al+ 3 toxicity have little effect on native tree species, such as those cultivated in AFSs, because the high concentration of Al and m % in Amazonian soils, mainly in Oxisols, is a natural condition resulting from the high degree of weathering and abundant presence of aluminum and iron oxides, associated with the region's climatic conditions, with high rainfall and temperatures (Quesada et al. 2011).
4.3. Soil physical attributes
The higher Sd recorded in AFS 1 compared to AFSs 3 and 4, as well as FLO, can be explained by the lower accumulation of soil organic matter, greater interference of anthropogenic activities, and consequently, greater soil compaction. On the other hand, the lower SD observed in the 0–10 cm layer in FLO and AFSs 3 and 4 are related to the age and increased soil organic matter, as over time, the soil tends to accumulate organic matter from plant residues, animal debris, and microorganisms (Siqueira et al., 2020), facilitating gradual infiltration and percolation of water from the surface to subsurface (Cherubin et al., 2019).
The positive correlation of Gm and TP with K and Mg highlights crucial aspects in soil dynamics, as under adequate moisture conditions, these nutrients mobilize in the soil and are incorporated into the profile through pores, becoming accessible for root absorption by plants. In this regard, attention is drawn to soil decompaction before liming, as it assists in the process of water infiltration and percolation, reducing surface runoff and consequently, the transport of particles and nutrients in erosive processes (Borisov et al., 2022), which can contaminate downstream water bodies (Zhu et al., 2020).
In mature ecosystems with proper management, soils have higher organic matter content, better structure (Saputra et al., 2020), and higher moisture (Mutuku et al., 2021), with high water retention at lower matric potentials, as evidenced for FLO and AFS 4, especially at the 10–20 cm depth. Such characteristics are related to the age and degree of soil cover (Cherubin et al., 2019), where older ecosystems have a more developed root system, tending to have higher microporosity and reduced water loss through evaporation (Huang et al., 2022; Lichner et al., 2020). AFS 3, due to wider spacing between plants, may experience a more pronounced evaporation rate than other systems (Hernández-Ochoa et al., 2022). The discrepancies observed for AFSs along the matric potential indicate that areas with lower vegetation cover have reduced water availability, with a narrow range in water concentration.
The decrease from field capacity to permanent wilting point was extremely drastic for the younger AFSs (1 and 2), resulting in a much narrower optimal water range, especially in the 0–10 cm soil layer. Regarding field capacity (FC) and permanent wilting point (PWP), θ was higher in both soil depths in AFSs 1 and 2, indicating greater vulnerability of these systems to drought events due to higher exposure to high temperatures during these periods (Carvalho et al., 2021). In contrast, AFS 4 may be less impacted by water stress, as suggested by the decrease in CC among systems at the 10–20 cm depth. The similarity of these attributes in the 0–10 cm layer among these systems may be related to rapid water absorption in the soil and consequent evaporation due to high temperatures during drier periods of the year (Flammini et al., 2018). PWP is influenced by soil clay content concentrations, while FC is influenced by a complex interaction between clay content, structure, SD, and organic carbon in the soil (Wiecheteck et al., 2020). Thus, in FC, changes in soil moisture can be partially compensated for by the sandy soil texture (Reynolds et al., 2002).