1. Introduction
The Cerrado-Amazonian ecotone occupies 4.85% of Brazil. This ecotone and the Midwest region of Brazil are among Brazil’s major grain-, fiber-, and beef-producing regions [
1]. Here, the conversion of native vegetation into pastures and agricultural areas has been intense in recent decades [
2]. Changes in land use and the intensification of agricultural activity can impair natural resources such as soil, water, and biodiversity and are associated with deforestation and the seasonal scarcity of water resources in the Amazon region [
3]. Removal of native vegetation can cause high rates of carbon dioxide emissions, loss of biodiversity, erosion, and interruption of hydrological cycles [
4,
5,
6].
In addition to environmental impacts, significant changes in land use/land cover, resulting from the conversion of native vegetation to pasture and agricultural areas, can cause negative impacts on the physical attributes of the soil itself [
7,
8]. Areas of poorly managed pastures and intensively managed conventional agricultural plantings of monoculture annual crops such as soybean, maize, and cotton may result in compaction and erosion, which can cause soil degradation [
9,
10,
11]. To overcome negative impacts on soil and water resources, it is essential to know the physical-hydric properties of the soil to enable efficient decision-making and agricultural management [
12]. In addition to environmental analyses, it is important to consider these soil properties and their possible variations on larger spatial scales, such as in river basins. These areas are sensitive to land use changes and may respond differently to soil and water losses depending on longer time scales and the drainage area [
13]. According to Ou et al. [
14], understanding variations in soil properties between land uses will help clarify the impacts of human activities on soil quality and health, soil and water conservation, ecological degradation, and environmental restoration, especially in watersheds.
Recent studies have shown that, in general, soil physical and chemical properties are highly affected by changes in land use/land cover (LULC) in watersheds [
15,
16,
17,
18,
19]. These changes, in turn, play a fundamental role in soil development due to their influence on nutrient cycling, hydrological processes, and soil erosion. Changes in LULC can also influence the ability of the soil to support plants and other organisms as well as the productivity of natural ecosystems or managed agricultural systems [
6,
15,
20].
Therefore, monitoring soil, water, and biodiversity losses in watersheds is essential. Initially, it is necessary to characterize the physical-hydric attributes of the soil, as these are responsible for short-term changes that can compromise the quality of the ecosystem [
21]. This allows for a better understanding of the existing relationships in the soil–plant–atmosphere system and to assess soil variability and quality, aeration, hydraulic conductivity, water redistribution, storage capacity, water availability for plants, and root growth [
22]. Among the various physical-hydric attributes that can be considered indicators of soil physical quality, due to the simplicity of field sampling and laboratory analyses, we have focused on soil density, particle density, porosity, saturation hydraulic conductivity, and water capacity of available soil water [
23,
24,
25].
There is a need for such measurements in agricultural frontiers such as the state of Mato Grosso, Brazil. Here, the Cerrado and the Amazon biomes are considered global hotspots where grain and beef production have increased through the expansion of agriculture and livestock farming [
26,
27]. There is a great need for physical diagnoses of soils under different agricultural uses compared to areas of native vegetation (e.g., forests, savannah).
The Renato and Caiabi Rivers are both tributaries of the Teles Pires River in the Southern Amazon region of Brazil. Given the importance of characterizing soil physical attributes to help in better water and soil management and conservation, our goal was to verify if there are variations in soil physical-hydric attributes in two watersheds with different land uses in the Cerrado-Amazonian ecotone region. The specific objectives of this study were to (1) evaluate the effect of land use on soil physical-hydric attributes in different regions of the Renato River and Caiabi River watersheds and to (2) identify the possible relationships between the soil physical-hydric attributes of these soils.
4. Discussion
Previous studies in both watersheds [
11,
23] generated a spatialized database of soil attributes at different sampling points and with different objectives than the present study. In general, the sand fraction increased from the headwater to the middle region, followed by the mouth in both watersheds, with a consequent reduction in the clay fraction. The highest concentrations of clay and silt were observed in the headwater region, in soil with sandy clay loam texture. According to Rizzardi et al. (2014) [
55], soil texture influences soil physical-hydric behavior. Therefore, its evaluation is of great importance for using and managing agricultural soils. Sandy soils have higher macroporosity and lower total porosity, facilitating water movement. Meanwhile, clay soils have higher microporosity and total porosity, allowing for better water retention in the soil [
56].
Characterization of the particle size composition of soils in different regions of the watersheds is still important, especially in biomes and transitions with high potential for converting native forests to crops/pastures. In both watersheds’ middle and mouth regions, soils are typically more fragile from an environmental point of view for agricultural use. Therefore, it is necessary to use conservation practices [
57].
In pasture areas at the mouth of the basin, animal trampling caused changes in soil physical quality, with increased bulk density and consequent reduction in macroporosity. This compromises water infiltration into the soil, which can favor surface runoff, change the natural characteristics of soil drainage, and cause erosion [
58,
59]. When evaluating the soil attributes of a Latossolo (Oxisol) under different uses in the Amazon rainforest, Valladares et al. (2011) [
60] observed that in pasture areas, animal trampling caused an increase in bulk density and a reduction in macropore volume. Polanía-Hincapié et al. (2021) [
61] obtained similar results, with a decrease in macroporosity in pasture areas compared to the native vegetation of the Bolivian Amazon. Therefore, reducing macroporosity can lead to poor drainage, low root aeration, and soil degradation [
62].
The macroporosity in the middle region of watersheds increased due to increased sand content. In general, soils with higher sand content are more porous with greater macroporosity when compared to soils with higher clay content. In both watersheds’ middle and mouth regions, microporosity values ranged from 0.20 to 0.32 m
3 m
−3, below 0.33 m
3 m
−3, which Lima et al. (2007) considered to be an ideal minimum value [
63]. However, as the soils of the river basin regions have different textures (
Table 2 and
Table 7), being loamy sand and sandy loam for the middle and mouth regions, respectively, low microporosity values are explained by the high concentrations of sand. Microporosity is highly influenced by texture since soils with higher clay contents favor greater microporosity due to the micro aggregates of clay particles, while sandy soils, having larger particles, show a porous space consisting of pores of larger diameters (macropores), and thus also have lower total porosity [
64].
The effect of land use on total porosity was observed only in the surface layer of the middle and mouth regions of the Renato River, with higher values in the areas with native forest than for either crops or pasture. The total porosity of 0.50 m
3 m
−3 is ideal for well-structured soil with satisfactory physical conditions for plant development [
63]. However, lower values were observed for crops and pasture for the middle and mouth watershed regions. Production systems associated with intense soil management reduce total porosity (TP) compared to native vegetation areas [
9]. Lower TP can reduce the capacity of soils to provide ecosystem services, compromising water resources and limiting agricultural production [
6]. On the other hand, the highest values of TP for crop and pasture areas were observed in the headwater regions, which is consistent with the highest clay contents (
Table 2 and
Table 7). Soil TP is mainly related to soil structure and texture, and sandier soils tend to have higher macroporosity, while clay soils tend to have higher microporosity and TP [
65].
Reichert et al. (2003) [
65] established critical limits for bulk density (BD) of 1.55 and 1.65 Mg m
−3 for soils with medium texture (20% to 55% clay) and sandy texture (<20% clay), respectively. Values higher than these can restrict root development. Although BD did not show values above the critical limit, BD in the surface layer increased by up to 20% in crop areas in the middle region and 27% and 37% for pasture and crops, respectively, in the mouth region compared to native forests. This effect may be associated with reduced macroporosity caused by animal grazing and intensive use of agricultural machinery, which can cause compaction [
66,
67]. Poorly managed pasture and crop areas can induce soil compaction and surface sealing (especially in clay soils), resulting in low water infiltration and increased surface runoff. Accelerated erosion can increase soil losses, organic matter, and nutrients. This can also cause silting of river and stream beds, compromising local biodiversity [
61,
67].
Although land use and occupation affect bulk density, this attribute is also related to texture and organic matter content [
68]. Soils under native vegetation, as they were not subjected to machine traffic and animal trampling, generally had higher macroporosity values and lower bulk density values [
68]. The best soil physical conditions in native forest areas are promoted by the increase in organic matter from the decomposition of leaves, branches, and roots, which, in turn, causes a reduction in bulk density due to better structuring of the soil and formation of biopores by edaphic macrofauna [
69,
70].
Permanent wiling point (PWP) values were homogeneous across land use, regardless of watershed region, except for the surface layer of the headwater, where the PWP was higher for crops. According to Jin et al. (2018) [
71], the PWP is an important soil hydraulic attribute for agricultural production and has been widely used in determining water availability. However, the amount of water at the PWP is seldom influenced by management and is fundamentally determined by clay content [
72].
Field capacity values ranged from 0.22 to 0.47 m
3 m
−3, with lower values in both watersheds’ middle and mouth regions. These were influenced by variations in sand concentration. An inverse relationship was observed in the behavior of field capacity in crops and pasture when compared with native forests in the headwater and mouth regions. According to Reynolds et al. (2002) [
72], the water content at field capacity is determined by the complex relationship between clay, bulk density, and organic matter, attributes that soil management alters.
There was a significant increase in available water capacity for crops (109.33 mm), followed by pasture (91.86 mm), in the headwater region of the Renato River watershed. These values may be related to the higher organic matter content under these land uses in this region, being consistent with lower particle density, bulk density similar to that found in native forest, and the occurrence of higher soil microporosity in this region, which may have contributed to improved water storage in the soil promoted by crops and pasture in this region. Cruz et al. (2014) [
73] observed that the conversion of native forests to fertilized pastures led to an increase in soil organic matter, which favored better available water capacity (AWC). When evaluating soil water retention in two types of
Latossolo (Oxisol) under different uses, Beutler et al. (2022) [
74] observed that microporosity and greater aggregation are the main factors that influence AWC because they allow greater infiltration and retention of water in the soil.
In the Renato River watershed, in the top soil layer, AWC was positively correlated with microporosity and negatively correlated with macroporosity. Andrade et al. (2020) [
75] observed a better correlation of microporosity with AWC. Thus, soils with a predominance of micropores tend to store more water [
76]. On the other hand, in the two deeper soil layers between 0.1 and 0.4 m, in addition to the correlations observed in the surface layer, AWC was also directly correlated with total porosity, PWP, and clay and inversely correlated with bulk density (BD) and sand content. According to Costa et al. (2016) [
77], water storage in the soil is influenced by its texture, structure, pore distribution, and soil management. Thus, inadequate management that increases BD from soil compaction causes porous space to decrease, reducing the potential for water storage in the soil.
PWP was positively correlated with field capacity, microporosity, total porosity, clay, and silt and negatively correlated with sand content and BD. The same correlations were observed for field capacity (FC), negatively correlated with macroporosity and positively correlated with AWC. Similar results were observed by Ghanbarian-Alavijeh and Millán (2009) [
78], who reported a positive correlation between clay and PWP. Andrade et al. (2020) [
75] observed a correlation between clay and BD in the variability of FC. Kirkham (2014) [
79] found that variations in water retention at FC and PWP are explained by soil texture and compaction.
Higher values of BD influence the quantity and size of pores, given the very strong negative correlation between microporosity and total porosity (TP). These correlations are justifiable since TP is inversely related to BD, and soil compaction reduces pore volume and increases BD [
80]. In addition, BD was negatively correlated with clay and positively correlated with sand content, thus justifying the highest values of BD in the middle and mouth regions of watersheds, which had sandy texture (<20% clay). Similar results were also observed by Tanveera et al. (2016) [
81], who found that sand content was positively correlated with BD, while clay content and TP were negatively correlated with BD.
Regarding soil texture, silt and clay contents are strongly associated with PWP. Anaba et al. (2020) [
76] observed that the relationship between the physical-hydric attributes could explain this behavior since the finer fractions (silt and clay) of the soil have greater participation in water retention at high potentials due to larger specific surfaces. Other soil attributes, such as BD and particle density (PD), were also related to particle size fractions, mainly associated with sand content. BD and PD are directly related to soil texture and organic matter, which, in turn, depend on minerals related to the origin of the soil. In this case, minerals such as quartz and feldspars in the sand are denser than clay minerals [
82]. However, in addition to mineralogy, management influences BD, unlike PD.
Studies conducted by Rocha Junior et al. (2020) [
83] corroborate the results obtained in this study. When evaluating the physical and chemical attributes of a
Latossolo Vermelho-Amarelo (Oxisol) under different uses and landscapes, these researchers observed that the conversion of forest to coffee cultivation and pasture areas reduced soil quality, decreasing the capacity of these areas to provide ecosystem services. In this context, better soil physical conditions in areas of forest or native vegetation may be associated with the absence of agricultural activities, such as tillage and exposure of the soil, which can accelerate erosion [
84].
In a common scientific sense, the distribution of soil attributes in different watershed regions is influenced by several factors, such as climate, geomorphology, and vegetation cover. Generally, the source region, usually located in mountainous or hilly areas (steep terrain) that generate rapid water flows and sedimentation, is expected to be relatively weak, resulting in the predominance of sandy soils. The soil in the upstream region can be rich in minerals such as quartz and feldspar, which are eroded and transported downstream by long-term hydraulic action. In the intermediate regions of watersheds, reliefs are normally gently undulating and the slower flow of water leads to sedimentation, resulting in greater amounts of silt and clay in the soil. In turn, the river mouth regions are found in plains or estuarine areas, with relatively flat terrain, and the slower flow of water in this region increases sedimentation, leading to the prevalence of clayey soils. The soil in the downstream area contains a higher concentration of clay particles due to the slower flow of water, causing suspended particles to settle more easily. However, we emphasize that in the two hydrographic basins studied, there is an inversion of this common knowledge, with a predominance of more clayey and more sandy areas in the regions of headwaters and the mouth, respectively. In this case, there is a mineralogical dependence on the formation of regional soils [
30], which, associated with low variations in relief, indicate proportionality of water flows with the increase in drainage area, not entailing significant differences between the concentrations of sediments in suspension transported between source and mouth [
85]. We also highlight that in the case of the Renato River, the regions with higher altitudes (
Figure 1) in the north of the watershed currently do not contribute directly to the river since the Colider hydroelectric power plant (on the river Teles Pires) has caused the region at the mouth of the Renato River to remain permanently flooded since 2015.
The main causes of erosive processes are intrinsically related to changes in land use, especially following the conversion of native forests to poorly managed pastures and resulting from poorly conducted tillage of agricultural soils, which causes compaction and hinders the natural dynamics of water in the soil, promoting surface runoff and increasing soil susceptibility to erosion [
86]. Therefore, evaluating the physical-hydric quality of the soil is important to assess the potential of its use to ensure both food productivity as well as the sustainability of agroecosystems. Physical-hydric surveys are necessary to provide information on soil management, ensure decision-making for better use of this resource [
86,
87], and maintain water quality and sustainability in river basins [
88]. Changes in land use can alter ecosystem services provided by the soil. Land use change should be carried out cautiously, respecting current legislation, especially in cases such as that of the Renato River watershed, which still has areas mostly occupied by native Amazonian forests.
This study contributes valuable information about the impact of changes in land use on soil properties in the southern Amazon region. However, it is emphasized that changes in soil properties depend on complex processes, which require a longer observation period to capture broader trends and variations. In this context, recognizing the limitations resulting from the short-term nature of the study, we emphasize that this study presents a characterization of the physical and water properties in the same hydrological year, aiming to minimize the effects of seasonal variations in temperature and precipitation. Furthermore, we recommend the development of long-term investigations based on the attributes that showed significant differences and that are considered more representative of the effects of changes in land use (BD, TP, AWC, and Ksat), for evaluation of the influences of seasonal variations on the physical-hydric behavior of soils in the region.