4.1. Soil properties under different land uses types
In this study, the physical, chemical and biological indicators of soils varied significantly amongst land-use types, indicating that land use change plays an important role in soil properties (Bravo, Marín, et al., 2017; Viana et al., 2014). However, in Amazonian conditions, it is important to consider the effect of soil-forming factors and processes and the historical background of land use (Nieto & Caicedo, 2012; Quesada et al., 2011; Viana et al., 2014). In this sense, the pedogenesis of Amazonian soils shows a very particular character, marked by different factors, including a climate characterised by high and intense rainfall with annual averages of around 4000mm, high temperatures that can range from 24 to 30°C, and habitats ranging from tropical rainforest to very humid rainforest (Nieto & Caicedo, 2012; Torres et al., 2019). In addition to this, traditional forest use generates a stratified profile, with a surface layer of average thickness between 10 and 12cm, with a higher organic matter content than the other horizons (Bravo, Torres, et al., 2017). This in turn generates better conditions in terms of physical and biological properties and nutrient recycling dynamics (Bravo et al., 2015). All of these assumptions should be analysed when assessing the impact of land use change on soil quality. On this basis, physical indicator values are strongly influenced by the normally high content of organic matter in the soil and therefore influence the physical variables associated with its structural quality, such as bulk density (BD), saturated hydraulic conductivity (Ksat) and porosity distribution (Blanco-Canqui & Ruis, 2018; Rabot et al., 2018). The soil’s BD is one of the variables most sensitive to changes in land use and has a great influence on other attributes, such as porosity distribution, especially macroporosity (AP) and Ksat, which affects aeration capacity, water penetration speed and, with it, the biogeochemical behaviour of the soil (Blanco-Canqui & Ruis, 2018; Pla, 2010). All this means that as the BD increases, the porosity distribution values decrease, especially total porosity and macroporosity. Due to the low density of the soil, these maintain an adequate volume of pores in the soil, improving aeration and drainage in the soil (Torres et al., 2019; Viana et al., 2014). In this study, regardless of land use, the values obtained for physical properties (BD, Ksat, TP and RP), especially the forest and some types of chakras, confirm the strong influence of the content and variation of organic matter in the soil. Soil structure is recognized to control many processes in soils. Although a change in the structural indicators is generated with land use change, the values show an adequate physical functioning that favours an adequate range of aeration, infiltration, root penetration and moisture retention without degradation problems such as compaction, which contributes to soil quality independently of the land-use type (Rabot et al., 2018).
In the case of the chemical indicators selected, they reflect the nature of the soils in the area (Espinosa et al., 2018). In this study, the results for TOC confirm the history of management and potential use of the Amazon Region (forest) and the use of analogous forestry systems such as agroforestry cocoa, silvopastoral systems (Bravo, Torres, et al., 2017; Torres et al., 2019). The components of soil acidity and pH associated with quality reflect the acidic and low fertility nature of these soils, which is confirmed by the values of the primary forest as a reference use. The amounts of available P in Amazon soils are almost always very low and generally phosphorus, becoming the main limiting factor for Amazon region (Müller et al., 2004; Quesada et al., 2011). Some studies report that for the surface of the humid tropical regions, like those in the Ecuadorian Amazon Region, the climate exerts a primordial influence on the pedogenesis that favours the ferralitisation of the soil, generating a dystrophic environment (Custode & Sourdat, 1986). This ferralitisation tends to a total hydrolysis of the modifiable primary materials and the complex clays of the rocks through the leaching of the bases (K+, Ca2+, Mg2+) and of the silica (Gardi et al., 2014). This causes a predominance of non-modifiable minerals and simple clays, such as quartz, kaolinite, haloisite, gibsite and iron oxides, which confer certain morphological characteristics and decrease the parameters, mainly pH and nutrient availability (Custode & Sourdat, 1986; Gardi et al., 2014). On the other hand, the movement of the cations to lower layers is related to the presence of anions, resulting from the mineralisation of organic matter that, forming ionic pairs, drag the cations in the soil profile with the movement of water (Espinosa et al., 2018). In addition, organic matter in the soil is decomposed with the help of microorganisms producing a constant supply of CO2 that is easily transformed into bicarbonate (HCO3), whose reaction releases H+ions into the soil solution, thereby reducing pH (McGrath et al., 2014). This study also shows that soil nutrients were higher in the surface layer than the second horizon. This is probably associated with the dead wood and leaf litter that accumulates on the surface and subsequently transformed into humus through microbial activity (Zhang et al., 2019). If we only consider the depth factor, AP, TOC, P, K+, Ca2+, Mg2+ and Zn decreased with soil depth, while pH and BD increased. This was probably due to the higher biological activity and root penetration in the soil surface compared to the subsurface layer (Zhang et al., 2019).
Effect of land use types on the soil quality
Land use change can improve soil quality, but the improvement varies according to land-use type. Ngo-Mbogba et al. (2015) indicate that SQI was significantly different amidst different vegetation types, with the forest exhibiting the highest SQI. Mishra et al. (2019) included four physicochemical indicators to the MDS for the calculation of the SQI in deciduous tropical forests in India, being these: the electrical conductivity, the apparent density, the exchangeable Mg and the available P. Other studies have shown that the differentiation between agroecosystems was more evident and significant when only three variables (minimum data set) were used in the soil quality index (Quintero, 2020). Variables that were maintained in the three minimum data sets were: bulk density, stability index, pH, dehydrogenase activity, density of heterotrophs and phosphate-solubilizing bacteria. In this study, land use has changed from primary forest to analogous systems such as agroforestry, including chakra and silvopastoral systems with little soil alteration. Within their arrangements, these systems combine crops and grasses with trees, a condition that improves soil quality. In general, the higher soil quality in the chakra system (A and B) and their respective arrangements together and forest systems was mainly due to the greater availability of Zinc, the greater aeration porosity and the higher content of soil organic matter in both layers. It is important to note that the use of agroforestry systems with cover not only protects the soil from erosion (Bravo et al., 2016; Vallejo-Quintero, 2013), but it also improves soil properties, mainly in organic matter and consequently in soil structure, due to the high amount of leaf litter generated on the surface of the soil (Blanco-Canqui & Ruis, 2018; Bravo, Torres, et al., 2017; Zhang et al., 2019; Zhao et al., 2017). Soil quality can be influenced by many factors, which include its inherent capabilities and environmental elements such as lithology and geography, land-use type, vegetation and human activity (Karlen et al., 2006). In our case, soil-forming factors, vegetation and management with agroforestry systems are essential to protect the soil, reduce erosion and improve the soil attributes associated with its quality (Bravo, Torres, et al., 2017).
The mean values at the two depths indicated that the Chackra C soil quality index and the three livestock systems (Cattle_A, B and C) were lower than the chakra A, Chakra B and forest systems (figure 4), with SQI categorized from moderate to low (Cantú et al., 2007; Quintero, 2020). These results highlights natural fragility of the soils in the amazon ecosystems , in which is common that soils present an acid edaphic environment, high presence of aluminum and low availability of nutrients that do not contribute to soil quality. These characteristics is in agreement with other soils of amazon region which environments are acidic, have low fertility and low cation exchange capacity (CEC), and contain silicate minerals with low activity as kaolinitic and oxidic (Quesada et al., 2011). However, it is normally common to find soils with a high content of organic matter that improves physical indicators (Da, Ksat, Porosity) and that positively influence the final value of the SQI. On the other hand, the results suggest that a change in use towards agricultural or livestock systems, although it may slightly improve soil quality, may also deteriorate it depending on management and the integration of good practices in agroecosystems, which was consistent with what he found in previous studies. (Bravo, Torres, et al., 2017). Soil Organic matter is considered as one of the most important factors among soil quality indexes and have a positive effect on soil properties and beside is the central indicator of soil quality and health, which is strongly affected by agricultural management (Kiakojori & Gorgi, 2014). Bulk density is a structural index that is strongly related to total porosity, aeration porosity and consequently influences hydraulic properties and defines air:water relationships in the soil (Blanco-Canqui & Ruis, 2018; Pla, 2010; Reynolds et al., 2009). The soil under the influence of decomposing wood differed significantly from the control sample in terms of capillary water capacity, which indicates a significant increase in the number of micropores capable of retaining water (Piaszczyk et al., 2020). It is important to discuss, however, that the bulk density values (BD) under the different use land showed ranges from 0.30 to 0.51 Mg m-3 in the surface layer and from 0.45 to 57 Mg m-3 in the subsurface layer (Table 2), which were below the that threshold values of bulk density deemed to be detrimental to seed germination, root development, and plant growth (Blanco-Canqui & Ruis, 2018). In this context, the interpretation of BD with respect to soil functions depends on soil type, especially soil texture and soil organic matter (SOM) content. The threshold values among soil textural classes can vary due to differences in size and shape of soil particles, the threshold values can be >1.40 Mg m-3 for clayey soils, >1.60 Mg m−3 for medium-textured soils, and > 1.80 Mg m−3 for coarse textured soils (Blanco-Canqui & Ruis, 2018; Pla, 2010).
The examined physical properties correlated with soil carbon content showed (Table 4). In the case of bulk density, it was a negative correlation with total organic carbon (r=-0.80), total porosity, macroporosity (r=-0.63). and saturated hydraulic conductivity(r=-0.35). These results are in agreement with previous studies in which a negative linear relationship was found between BD and macroporosity, and between BD and the saturated hydraulic conductivity (Reichert et al., 2009). According to Chen et al. 2017, organic matter content has a dominant effect on soil bulk density and organic matter concentration is used to predict soil bulk density (Perie & Ouimet, 2008; Prévost, 2004). A positive correlation (Table 4) was observed between TOC and aeration porosity (r=0.71) and between aeration porosity and saturated hydraulic conductivity (r=0.85). Organic remains released from deadwood in the forest and leaf litter in the agroecosystems are delivered to soil and determine soil structure and aggregation (Piaszczyk et al., 2020). Therefore, SOC, AP and BD could play an important role for monitoring soil quality.
Analysing the minimum data set (MDS) is an effective method for assessing soil quality, because it reduces duplication of data, provides good accuracy and is rapid (Cantú et al., 2007; de Paul Obade & Lal, 2014; Yigini & Panagos, 2016; Zhang et al., 2019). In our research, five indicators were selected (Zn, AP, TOC, BD and LL) with a high weighting factor in the evaluation of MDS. All five factors related to one or more soil functions (e.g., water and nutrient retention and transport, soil structure, aeration, etc.) to influence soil pore structure and the capacity of soil to accept, store and release water and nutrients. Previous studies have shown that TOC, BD and TN were potential indicators of soil quality (Karlen et al., 2006; Viana et al., 2014; Zhang et al., 2007; Zhang et al., 2019). We found TOC, BD, LL and AP to be indicators of the physical quality of the soil (structure), reflecting their importance due to their greater contribution to the integrated quality index, despite the lower contribution of available P content and the changeable bases (K+, Ca2+ and Mg2+). This reflects the chemical nature of soils in the Ecuadorian Amazon Region with its dystrophic environments, as pointed out in previous studies (Bravo, Marín, et al., 2017; Custode & Sourdat, 1986; Espinosa et al., 2018; Martín & Pérez, 2009; Müller et al., 2004).
Land use changes due to resource exploitation remain a serious threat in Ecuador, a country that is trying to transition towards modern wealthy society. It is clear that such pressures must be considered when discussing and implementing development and conservation plans (Torres et al., 2019). One approach to the prevention of deforestation and soil degradation is to use management and conservation techniques that are appropriate in this region and these methods depend on the knowledge of soil attributes The diversity of the Amazon ecosystem and studies relating to soils in this region should be considered during the application of techniques that can prevent the exploitation of unsustainable natural resources. Information relating to soil attributes can serve as a basis for public policies that target agricultural planning and technologies that increase land use efficiency and conserve biodiversity (de Souza et al., 2018).
CONCLUSIONS
The effect of land use types on the soil quality of a dystrophic landscape was assessed using the soil quality index. Significant differences in physical, chemical and biological soil properties were found between the land uses and the primary forest as a reference system, indicating that the land use change had a significant influence on soil properties. The soil quality index (SQI) varied with depth and was higher in the surface horizon. Regarding land-use type, the chakras and some livestock systems slightly improved soil quality. In general, the SQI values in the chakra systems and the forest in the surface horizon were slightly higher than the rest of the uses, which implies that the agroforestry chakra model was able to improve the quality of the soil. In summary, our study confirms that the SQI method is a useful and practical tool for evaluating and monitoring soil quality because of its flexibility and quantitative precision. However, in order to assess soil quality more completely and accurately, it is necessary for future SQI studies to also consider other chemical and biological properties of soils.