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.