1. Introduction
The Amazon biome, globally recognised as a biodiversity hotspot, is facing an urgent threat from deforestation, particularly evident in Maranhão State [
1]. The rapid expansion of beef cattle production since the mid-1960s has led to the conversion of vast forested areas into pastures, significantly reducing the original extent of the Amazon forest in this region [
2,
3]. This extensive deforestation and subsequent land-use changes have triggered soil degradation issues. These issues are primarily attributed to inadequate management practices and the prevalent use of fire to control natural regeneration and promote pasture growth [
4,
5]. Consequently, only a fraction of the original forest area remains, with estimates suggesting a decline to 24–25% of its initial coverage [
6,
7,
8].
Understanding the implications of these land-use transformations on soil quality is crucial, especially considering the vital role soil organic matter (SOM) plays in sustaining ecosystem health. Within the complex web of tropical ecosystems, SOM plays an important role, influencing essential soil attributes and fertility [
9,
10].
However, while studies on SOM mostly focus on carbon levels and stocks, there is a need to delve deeper into the dynamics and stability of SOM fractions to comprehensively gauge the impact of land use changes on soil quality [
9,
11]. Fractionation techniques, including chemical methods [
12] and oxidisable approaches [
13], can be used to evaluate the dynamics and stability of SOM by employing solubility in various agents. These techniques are essential and sensitive tools for detecting changes associated with different land use and management practices, as well as the plant species used [
10].
Chemical fractionation provides humic substances (fulvic acids—FA, humic acids—HA, and humin—HUM), which constitute a highly reactive compartment of SOM, accounting for 70 to 90% of the soil’s carbon content. These substances participate in various chemical reactions in the soil and strongly influence soil fertility [
14,
15]. Oxidisable fractionation, on the other hand, yields four fractions (F1 to F4) with different degrees of oxidation, representing varying levels of nutrient availability and chemical stability [
13]. Measurements of these fractions offer insights into the labile and resistant forms of carbon in the soil [
9,
16].
By understanding the changes in carbon within different fractions of SOM, it is possible to assess soil quality and detect potential impacts caused by specific management systems. Therefore, the present study hypothesises that pasture recovery modifies the humic substances and oxidisable fractions of SOM. The objective of this research is to evaluate the carbon contents of chemical and oxidisable soil fractions in pastures with different recovery periods and compare them to those of secondary forests.
4. Discussion
The similarity in TOC content between the P8 and SF areas in the surface layers, as well as the prevalence of PP and P8 areas in the deeper layers, might be connected to the gradual carbon deposition in the soil of secondary forests facing water stress conditions [
26]. Secondary forests are susceptible to water stress, resulting in altered carbon balance and reduced growth rates during dry periods [
26], which are frequent in the state of Maranhão due to its low rainfall periods [
17].
Bastos et al. [
27] found no differences in TOC content in a Plinthosol in the State of Rondônia when compared to pasture and forest areas. Similar patterns were also noted by Reis et al. [
5] and Santos et al. [
28] for Plinthosol in the Maranhão state.
Pastures cultivated in the Amazon biome can deposit an average of 18.90 Mg ha
−1 of dry biomass on the soil surface [
29], resulting in an input of over 7 Mg ha
−1 of C on the soil surface [
29,
30]. Below the surface, tropical grasses can produce about 10 Mg ha
−1 of dry root biomass, with approximately 21% and 12% of root and shoot biomass, respectively, converting into TOC [
31,
32].
Sarto et al. [
33] observed that after eight years of pasture establishment, C levels similar to those found in forest areas were present in the surface layer of the soil. This pattern can be attributed to several factors, such as higher residual deposition of aerial and root plant parts, contributions from animal residues enhancing biological activity, and the higher C/N ratio of grasses. In the deeper layers, the improved results in the P8 and PP areas can be attributed to the capacity of
Brachiaria brizantha cv. Marandu to form a voluminous, fast-growing, and well-developed root system. Due to constant renewal through grazing, dead roots are decomposed by soil microorganisms, releasing nutrients, and promoting the increase of organic compounds [
34,
35].
Pasture recovery eight years ago contributed beneficially to the increase of the HUM fraction compared to the PP area. This pattern is evident through the analysis of ΔCS-HUM and the principal component analysis (
Figure 2 and
Figure 3). This increase may have been facilitated by the preservation of HUM inherited from the forest, which was protected from biological activity during the initial development of the pasture. The higher biomass input in the form of easier-to-degrade material for microorganisms could explain this pattern [
34,
36]. The higher levels of C in the HUM fraction, across all areas and layers, as confirmed by the low values of the EA/HUM ratio, are common in regions with tropical climates. In these regions, the dominance of the most stable and insoluble fraction is a result of intense residue mineralisation and edaphic limitations on biological activity, which reduce the humification process, indicating the strong stability of SOM [
14,
37,
38].
The predominance of the HUM fraction in all areas and layers can be associated with its recalcitrance, low solubility, and association with iron oxides [
39], which are abundant in Plinthosols [
20]. Other factors that contribute to the predominance of the HUM fraction include the aliphatic chemical nature of the molecule, derived from highly resistant macromolecules from plants and/or microorganisms [
40], as well as the high molecular weight and greater stability compared to other fractions, resulting in greater protection of fixed carbon. The HUM fraction is considered the most significant reserve of organic carbon in the soil [
36].
Regarding the Alkali-soluble fractions (FA and HA), which showed better results in the recovered pasture areas (5 and 8 years) compared to the perennial pasture, this difference may be due to the lower deposition of residues in the PP area. This decrease in crop productivity after 20 years of establishing perennial pasture, along with inadequate soil management practices, could be responsible for this result [
41]. The recovery of pasture areas (5 and 8 years) led to an increase in HA fraction levels. Although the ΔCS-HA values were negative, they were still better compared to the PP area (
Figure 2). The principal component analysis also highlighted their greater contribution to the PC2 axis (
Figure 3), suggesting greater stabilisation of SOM with higher carbon content in a humified form [
36].
The high C contents of the FA fraction in the P8 area, as well as the positive values in the ΔCS-FA (
Figure 2), can be attributed to the greater deposition of residues from aerial and root biomass, as well as animal residues, which contribute to the accumulation of decomposable material and the formation of this fraction. In the deeper layers, these results can be attributed to the greater mobility and less condensation of the FA fraction compared to HA, allowing it to be present in higher proportions. The predominance of the FA fraction compared to HA, particularly in the deeper layers, was observed across all areas, as indicated by the low values in the HA/FA ratio. This pattern suggests limited SOM evolution due to edaphic reasons, possibly due to soil wetting and drying cycles and/or water table movement, as well as recent inputs of SOM [
14,
24].
Araújo et al. [
42], in their study on the impact of converting the Amazon forest into pasture in the State of Acre, Brazil, found similar behaviour of humic substance fractions as observed in this study. The authors observed higher levels of TOC in pasture areas with a predominance of the HUM fraction and greater formation of FA compared to HA at depth.
The analysis of oxidisable organic carbon fractionation confirms the predominance of the most recalcitrant forms of C (F3 and F4) and NLC, which aligns with the predominance of the HUM fraction observed. Additionally, low values of the F1/F4 and LC/TOC ratios indicate that even the low levels of C in the soil are largely associated with compounds of greater stability and interaction with the mineral fraction of the soil. This limits microbial attack, resulting in a higher molecular weight and a longer residence time of these products in the soil. [
9,
35,
43].
The recovery of pasture areas eight years ago led to increases in C levels in the labile (F1), moderately recalcitrant (F3), and NCL fractions compared to the perennial pasture area. These increases can be attributed to greater initial contributions and plant vigour, resulting in higher biomass inputs on the surface through shoot deposition and in the soil through the root system [
41,
43]. These increases reflect the soil’s capacity for C recovery, and in some cases, the quantified values were higher than those found in the forest area. The accumulation of C in the P8 area for the F1, F3, and NCL fractions, as well as the F1/F4 ratio, indicates that pasture recovery contributes to increased carbon storage in the soil, reducing emissions to the atmosphere. The results observed for labile fractions resulting from pasture recovery demonstrate the benefits of the implemented soil changes, indicating changes in soil attributes caused by management shifts [
44,
45]. Similar findings have been observed in other studies that reported changes in soil attributes due to management changes, as well as for moderately recalcitrant and non-labile fractions, demonstrating their contribution to soil organic carbon stabilisation [
43,
46].
Xu et al. [
44] also observed increases in the labile fraction (F1) of soil C levels, which were associated with improvements in soil structure and nutrient content, as well as increases in carbon reservoirs and biological activity. Chemical transformations undergone by SOM, combined with the physical protection of C, contribute to the accumulation of recalcitrant fractions, leading to greater C stability and enhancing soil quality [
46].
The improved quality of the soil in the pasture areas that underwent recovery can be attributed to several factors. Firstly, the establishment of perennial pasture led to increased biomass input, both above and below ground, through plant residues and root systems. This increased organic material contributed to the accumulation of labile fractions (F1 and FA) and promoted nutrient cycling and biological activity in the soil [
44,
45]. Secondly, the recovery process likely involved changes in soil management practices, such as reduced soil disturbance and improved soil cover, which enhanced the protection and stabilisation of SOM [
9]. These changes favour the formation and preservation of recalcitrant fractions (F3 and HUM), which are more resistant to decomposition and have longer residence times in the soil [
36].
Overall, the findings support the positive impact of pasture recovery on SOM fractions and highlight the potential of this management strategy for soil conservation and improvement in the Amazon region of Maranhão. The results also emphasise the importance of natural regeneration processes, as exemplified by the secondary forest, in promoting soil quality and SOM accumulation.
In contrast, the perennial pasture area showed the lowest soil quality among the studied areas. Continuous pasture management practices, such as grazing and soil compaction, can lead to reduced SOM inputs and increased decomposition rates, resulting in lower levels of labile and recalcitrant fractions. The lower soil quality in the perennial pasture area highlights the need for sustainable land management practices, such as pasture recovery, to improve soil health and preserve SOM stocks.