Soil properties variation in a small-scale altitudinal gradient of an evergreen foothills forest, Ecuadorian Amazon region

The aim of this study was to evaluate variation in the soil physical and chemical properties in a small-scale elevation gradient at 601–1000 m above sea level in an Evergreen Andean-Amazonian Forest. The soil sampling was divided into four elevation zones, establishing five permanent monitoring plots for each one, where soil physical and chemical properties were determined at depths of 0–10 cm and 10–30 cm. For the altitudinal soil-gradient relationship analysis, multivariate statistical methods were used. The results suggest that some soil properties–such as bulk density, saturated hydraulic conductivity (Ksat), total porosity (TP), soil organic matter (SOM), total nitrogen (TN), available phosphorus (P), potassium (K+) and exchangeable calcium (Ca2+) were significantly affected by altitude. Soil pH, Al3+ and exchangeable acidity did not follow any defined pattern with respect to the altitudinal gradients. SOM, TN, Ca2+, Ksat, TP and retention porosity (RP) exhibited an increase according to altitudinal gradient, whereas soil bulk density, K+ and P showed an inverse behavior, with higher values at the lower elevation gradient. The principal components analysis and the redundancy component analysis revealed a clear separation of soil properties among altitudinal gradient and confirmed that these variables were necessary in order to explain variation in the soil physical and chemical properties as an elevation gradient function.


Introduction
Ecuador's Amazon region represents 45% of its territory and is considered one of the tropical areas with the highest concentration of biodiversity.It is characterized by very rainy weather conditions, a variety of soil orders with a predominance of Inceptisols that represent about 80% of its surface, while the other orders (including Andisols) make up a smaller proportion (Espinosa et al. 2018).Due to the humid and warm tropical climate, the soils are highly meteorized, which leads to a predominance of unalterable minerals, such as quartz and low-activity clays e.g., kaolinite, halloysite, gibbsite and iron oxides, all of which confer certain morphological characteristics and a deterioration of the soil chemical parameters (Herrera et al. 1978;Custode and Sourdat 1986;Gardi et al. 2014).Although the soils of the Ecuadorian Amazon region have a high organic matter content, their fertility is normally low due to the acid pH, the presence of exchangeable aluminum and low content of available phosphorus and exchangeable bases (K + , Ca 2+ , Mg 2+ and Na + ).These nutrients are easily washed away by the intense and constant rainfall and the high humidity characteristic of the region (Quesada et al. 2011;Nieto and Caicedo 2012;Bravo et al. 2017a).The region has large areas of natural forest, which in turn constitutes an ecosystem of wide local and global interest (Bravo et al. 2017b;Torres et al. 2019;Bravo-Medina et al. 2021).The forests in this region are considered part of the evergreen slow-growth forests in the northeastern Andean foothills (Sugawara and Nikaido 2014) and include about 50 tree families, with the richest being Moraceae, Fabaceae, Meliaceae, Cecropiaceae, Euphorbiaceae, Arecaceae, Annonaceae, Clusiaceae, Rubiaceae and Myristicaceae (Torres et al. 2019).
Cross-sectional studies in tropical forests have provided valuable information regarding the effect of changes in altitude over long distances on the availability of nutrients (Schawe et al. 2007;Wilcke et al. 2008;Unger et al. 2010;Magnani et al. 2018), which has allowed people to identify and relate the nutrients that limit plant growth as a function of elevation (Unger et al. 2010).However, studies in altitudinal gradients in small scale are very scarce, so this proposed research represents an initial step toward evaluating the effect of altitude on the variability of physical and chemical properties associated with fertility and subsequently at a second stage to relate these soil parameters with the distribution and growth of vegetation.Likewise, this study allows us to understand the support function of the soil associated with plant growth and its hydric behavior.This function is related to structural indicators, such as bulk density, saturated hydraulic conductivity and porosity distribution, and how they are affected by changes in altitude over at close range.
Several studies have indicated that soil properties are related to topographical positions in different forest ecosystems (Tsui et al. 2004;Begum et al. 2010;Unger et al. 2010;Boyden et al. 2012;Liu et al. 2020) and some researchers have related topography to soil types and their current state of soil fertility and trace elements (De Bauw et al. 2016;Tesfaye et al. 2016;Magnani et al. 2018).
Forest soil is an important source of nutrients for vegetation, including N, P, S, K, Ca, Mg, Na and some micronutrients (Tsui et al. 2004).Under natural conditions, soil nutrient availability and water availability often show high variability at small scales (Boyden et al. 2012) and topography is considered an important control factor (Behrens et al. 2014).In this sense, the terrain influences the spatial distribution of soil fertility through variables such as organic carbon, pH, cation exchange capacity (CIC) and nutrient availability (De Bauw et al. 2016).Important relationships between elevation and parameters associated with soil fertility have been indicated; for example, soil pH, available P, and exchangeable K + , Ca 2+ and Mg 2+ showed a significant decrease with respect to elevation (De Bauw et al. 2016).For Amazonian conditions, an increase in the availability of P and exchangeable bases has been reported with the increase in elevation associated with the thickness of the organic layer, which increases the nutrient reserve for the plants (Wilcke et al. 2008;Unger et al. 2010).Soil organic carbon and total nitrogen may be more influenced by soil depth than by elevation in native forests (Tesfaye et al. 2016).The interrelationship between fertility, topography and tree growth in a subtropical forest ecosystem in southeast China showed that topographic heterogeneity leads to ecological gradients across geomorphological positions.Therefore, small-scale soil-plant interactions in a young forest can serve as a driver for future vegetation development and biodiversity control over soil fertility (Scholten et al. 2017).Both the soil structure and floristic composition of the forest can undergo a dramatic change along the gradient, which in turn can lead to changes in the physical, chemical and biological properties of the soil (Chen et al. 1997;Torres et al. 2019).In some cases, soil properties, such as pH and nutrient availability, also affect the type of vegetation (Tsui et al. 2004).Moreover, the condition of development and distribution of vegetation types on different slope positions are controlled by the bioavailability of nutrients (De Bauw et al. 2016).
The soil physical properties and, in turn, plant growth are significantly controlled by variations in landscape attributes, including slope, appearance and elevation, which influence the distribution of energy, plant nutrients and vegetation (Rezaei and Gilkes 2005).The thickness of the surface layer of the soil, the bulk density, and the structure of the soil are all quite dynamic because they are sensitive to soil formation factors, and depend, to some extent, on land management factors (e.g., erosion, exploitation and conservation) (Blanco-Canqui and Ruis 2018).
In this context, the objective of this study was to determine the variation of soil properties in a small-scale altitudinal gradient in an evergreen foothills forest in the Ecuadorian Amazon region.In this paper, it was hypothesized that the soil physical and chemical properties associated with the support functions (bulk density, saturated hydraulic conductivity, soil porosity, SOM, TN, available P, and exchangeable K + , Ca 2+ and Mg 2+ ) are directly affected by the small-scale altitudinal gradient.An additional hypothesis is that the properties are indirectly influenced by landscape attributes with microclimates that support the growth of plant species with different characteristics in the absence of management factors.

Study area
This study was carried out in Andean-Amazonian forests in the province of Napo, Ecuador, in collaboration with the 'Centro Experimental de Investigación y Producción Amazónica' (CEIPA).The forests in this region are considered part of the evergreen slow-growth forests in the northeastern Andean foothills (Sugawara and Nikaido 2014) (Fig. 1) and include about 50 tree families, with the richest being Moraceae, Fabaceae, Meliaceae, Cecropiaceae, Euphorbiaceae, Arecaceae, Annonaceae, Clusiaceae, Rubiaceae and Myristicaceae (Torres et al. 2019).
The climate was similar across the altitude range, and was humid and hyper-humid (MAE 2012), with an average annual temperature of 23ºC and average annual rainfall of 4,119 mm.The lowest rainfall occurs from January to April and the highest from May to July, and the relative humidity varies from 65 to 70% throughout the year.The soils are also located in a pedogenetically homogenous zone since all the sampling plots along the different altitudinal gradients are on the Chambira formation (Christophoul et al. 2016).The soil in all the altitudinal gradients belongs to the Andisol order and Udands suborder, which is characteristic of volcanic soils typical of a humid climate (see Fig. 1) (Espinosa et al. 2018).
The site where the study plots were established was located in a mountainous area dominated by high and medium crests and rounded hills, composed of volcanic and sedimentary rocks of recent origin (MAE 2012).The plots were installed at four elevations in the Evergreen Andean-Amazonian Forest (EAAF) whose slope angle varied: it was between 3 and 12% in EZ1 (601-700 m.a.s.l) and EZ2 (701-800 m.a.s.l.), while in EZ (801-900 m.a.s.l) and EZ4 (901-1000 m.a.s.l) the slope angle ranged between 3 and 30% (Table 1).At each elevation, five permanent monitoring plots (PMP), each of 1000 m 2 (10 × 100 m), were established parallel to the slope to maintain consistency in terms of elevation (reducing micro-level variation).The selection criteria were accessibility, topographical homogeneity, elevation at four small-scale gradients (100 m in each gradient) and being apparently non-disturbed old-growth forests.

Soil sampling
In each PMP, 3 subplots (10 × 10 m) were identified in order to cover the entire altitudinal range.In each subplot, 5  soil sub-samples were collected at 2 soil depths (0-10 and 10-30 cm), which were subsequently homogenized to obtain a representative soil sample for each soil depth.

Soil physical analysis
Non-altered soil samples were taken using cylinders.We determined pore size distribution (TP: total porosity), aeration porosity (AP: pores of > 15 µm radius), and retention porosity (RP), using the saturation tension table method with − 10 kPa matric potential (Blake and Hartge 1986).The saturated hydraulic conductivity (K sat ) was evaluated using the variable loading method (Reynolds et al. 2002).

Soil chemistry analysis
Total Organic Carbon (TOC) was determined by the Walkley & Black wet digestion method (Nelson and Sommers 1982) and later the soil organic matter (SOM) was estimated from the TOC multiplied by uivalent factor 1.724 (Jackson 1964).The potentiometric method (soil-water ratio of 1: 2.5) was used to measure the soil pH.Acidity and exchangeable Al 3+ were measured by titration (McLean 1965).The analytical methods are described by Okalebo et al., (2002).
Total N was assessed by titration after Kjeldahl digestion with sulfuric acid and selenium as a catalyst.Available P and extractable cations (K + , Ca 2+ and Mg 2+ ) were removed using Olsen extraction solution.P was measured colorimetrically using the molybdenum blue method, while K + , Ca 2+ and Mg 2+ were determined using an atomic absorption spectrophotometer.

Data analysis
The data analysis was carried out using ORIGIN PRO 2020.A one-way ANOVA was used to detect significant differences (P < 0.05) between the altitudinal gradients.Prior to the ANOVA, an exploratory data set analysis was carried out using descriptive statistics, and normality was tested with the Shapiro-Wilk test.In order to display the soil-landscape relationships, redundancy (RDA) and principal component (PCA) analyses were carried out using Canoco 5.0 using the soil physical and chemical variables and elevation data set.All the data were transformed with log(y + 1) before implementing the PCA and RDA.The degree of correlation among soil properties was assessed by means of Pearson's correlation coefficient (r) (Hervé 2020).The nutrient stock in the soil (Ss) was calculated at the depths of 0-10 cm and 10-30 cm using the following equation (Bond 2010): and then a weighted average was performed to obtain the nutrient reserve in the 0-30 cm zone.

Soil physical and chemical properties as a function of the altitudinal gradient
The soil physical properties evaluated through structural indices registered significant differences (P < 0.05) for the different altitudinal gradients.A clear difference in soil physical properties along the altitudinal gradient was observed, showing a decrease in BD ranging from a minimum of 0.52 Mg m −3 dry soil to a maximum of 0.89 Mg m −3 in the surface layer (Table 2).BD was significantly lower at the highest altitudinal gradient (901-1000 m.a.s.l) compared to other altitudinal gradients at both soil depths (Table 2).
In the surface layer, the saturated hydraulic conductivity only showed significant differences (P < 0.05) in the higher altitudinal gradient (901-1000 m.a.s.l).In the first three gradients, the values ranged from 16 to 21 cm h −1 (Table 2), while the highest gradient showed values close to 60 cm h −1 .Within the 10-30 cm soil layer, the first altitudinal gradient (600-900 m.a.s.l) presented values between 6 and 7 cm h −1 and for the second altitudinal gradient, the values were around 37 cm h −1 .Regardless of the altitudinal gradient, K sat decreased with depth (Table 2).The highest proportion of pores was represented by retention pores (RP) as opposed to aeration porosity (AP) (Table 2).In general, for both depths, the RP increased gradually following a pattern according to the altitudinal gradient.Specifically, in the surface layer, the values ranged from 52 to 58% at 601 to 900 m.a.s.l, while at 901 to 1000 m.a.s.l, the value was around 65%.
For the subsurface layer, the values showed ranges from 53 to 58% (601 to 900 m.a.s.l) and 63% at the highest floor (901 to 100 m.a.s.l).AP did not follow a defined pattern based on the altitudinal floor and it decreased with depth regardless of the altitude.The surface layer showed values that ranged from 15 to 19%, while in the subsurface layer, the values ranged from 9 to 11%.
Soil bulk density and soil microporosity values increased with soil depth, whereas K sat , TP and AP showed an inverse behavior, decreasing with soil depth.The behavior of structural indices is closely related to the variation in SOM content (Table 2).
(1) The soil chemical properties associated with soil fertility showed significant differences (P < 0.05) for soil organic matter, total nitrogen, available phosphorus, potassium and exchangeable calcium at both soil depths of sampling points (Table 3).
At both soil depths, the pH values were very acidic and this did not change along the altitudinal gradient.The exchangeable acidity and Al 3+ showed high values without differences between the altitudes.The soil organic matter (SOM) and total nitrogen (TN) concentrations increased as elevation increased.In particular, at both soil depths, the SOM content obeyed the following trend: EZ1 = EZ2 < EZF3 = EZ4 and so on for TN was: EZ1 = EZ2 < EZ3 < EZ4.The available P content was slightly higher at the lower elevation areas for both soil depths with medium to low contents.Furthermore, at both soil depths, this nutrient followed the trend of EZ1 = EZ2 > EZ3 = EZ4.
The levels of exchangeable bases are affected by leaching processes throughout the altitudinal gradients; however, the K + concentration at both soil depths followed a downward trend according to the altitudinal gradients, with values that ranged from high to low.In contrast, the exchangeable Ca 2+ exhibited an increasing pattern, with higher values at the higher altitudinal gradients, which are categorized as medium to low.The exchangeable Mg 2+ did not show significant differences and did not follow any definite pattern with respect to the altitudinal gradient.

Soil parameter variation as a function of the altitudinal gradient
The analysis of main components, using the graphical representation method of the data matrix, showed the interaction of the soil physical and chemical variables associated with soil fertility in the two evaluated soil depths according to the altitudinal gradient (Fig. 2a, b).At a depth of 0.00-0.10m, both factors were responsible for explaining 82.03% of the variance of variables with eigenvalues greater than 1.PC1 explains 62.75%, being responsible for the variables altitude, BD, SOM, TN, Ca 2+ , TP, Ksat, RP, while PC2 explains 19.28% of the variance, being responsible for pH, Al 3+ + H + and Al 3+ .In PC1, BD, SOM, TN, Ca 2+ , TP, Ksat showed positive values, while P and K + presented negative values, this indicates that the attributes that showed the same signs have a direct correlation while those that have opposite signs have an inverse correlation.In PC2, Al 3+ + H + , Al 3+ and P showed positive values.
At a depth of 0.10-0.30m, both factors were responsible for explaining 82.14% of the explanatory variance.PC1 explained 65.99% being responsible for Altitude, BD, TP, TN, SOM, Ca 2+ , Ksat, RP, while PC2 explained 16.15% being responsible for Al 3+ + H + and Al 3+ .In PC1, most attributes showed a positive value while the BD, Al 3+ + H + and Al 3+ showed a negative value.
For the surface layer (Fig. 2a) along the first axis, it can be observed that the plots with higher altitudes (EZ3 and EZ4; see  Exchangeable aluminum; TN: Total Nitrogen; SOM: Soil organic matter; P: Available phosphorus; K + : Exchangeable potassium; Ca 2+ : Exchangeable calcium; Mg 2+ : Exchangeable magnesium TN, K sat , exchangeable Ca 2+ , TP and RP, while the available P, exchangeable K + and BD were higher in the plots located at medium altitudinal gradients (EZ1 and EZ2; see Table 2).The soil pH, acidity, exchangeable aluminum and aeration porosity were not related to the plot nor were they influenced by the altitudinal gradients.
The following variables presented relationships with altitudinal gradient: BD, K sat , SOM, TN and exchangeable Ca 2+ for depths of 0-10 cm and 10-30 cm.Soil bulk density (Fig. 3a, b) showed a linear and inverse behavior, decreasing with altitude.It had a determination coefficient of R 2 that ranged from 0.60 (0-10 cm) to 0.83 (10-30 cm), considered moderate to high, respectively.The saturated hydraulic conductivity (Fig. 3c, d) for the surface layer exhibited an exponential adjustment with altitude and a high determination coefficient (0.90), while for the subsurface layer, the correlations were very low.
The correlation of the SOM content and TN with altitudinal gradient showed a similar behavior with a linear adjustment.For SOM, the correlation value ranged from 0.76 to 0.63, which is categorized as high to moderate for the surface and subsurface soil layer, respectively (Fig. 4a,  b), while for TN, the value ranged from 0.84 to 0.90, which is considered a high correlation (Fig. 4c, d).
Exchangeable Ca 2+ (Fig. 4e, f) exhibited an exponential behavior with a moderate correlation that ranged from 0.47 to 0.34 in the surface and subsurface soil depths, respectively, indicating higher values in the higher elevation areas.
The results of the redundancy analysis (RDA) for soil properties at both soil depths (0-10 and 10-30 cm) showed a clear separation of some soil physical and chemical variables associated with soil fertility as a function of the altitudinal microgradient (Fig. 5).In both studied soil layers (Fig. 5a, b), the Monte Carlos test indicated the variation of some soil properties to be significantly related to altitude (P < 0.001).The variables K sat , AP, TP, SOM, TN and Ca showed a positive relationship with altitude, while BD, P and K presented an inverse behavior, reaching lower values in higher elevation areas.The soil nutrients stock as TN, P available, K, Ca and exchangeable Mg at 0-30 cm deep showed significant differences (P < 0.05) as a function of altitude gradient (Fig. 6).

Effect of altitudinal gradient on soil physical properties
In this study in a small-scale altitudinal gradient of a piedmont evergreen forest in the Ecuadorian Amazon region, some soil physical properties (BD, TP, RP, Ksat) showed a spatial distribution pattern that varied as a function of elevation zone.Bulk density (BD) showed a decreasing pattern as a function of the altitudinal gradient, while the values of saturated hydraulic conductivity (Ksat), total porosity (TP) and retention porosity (RP) exhibited an increase with respect to the altitudinal gradient with higher values in the zones of higher elevation (EZ3 and EZ4) (Table 2).This pattern indicates small-scale changes and may be reflecting an underlying gradient in soil organic matter content, whose value increased as a function of altitudinal gradient (Homeier 2010;Unger et al. 2010;Du et al. 2014;Saha et al. 2020).Different works have pointed out that some soil properties can change as a function of altitudinal zone among them soil organic matter and its decomposition rate showing higher values in higher elevation zone (Homeier et al. 2008;Unger et al. 2012;Lippok et al. 2014;López 2022).
In agreement with our results, some researchers report similar values, that means, a decrease in bulk density with elevation, which is associated with an increase in soil organic matter (Unger et al. 2010).In our research project, the small-scale altitudinal gradient also showed a pattern of the spatial distribution of soil organic matter (SOM), which values increases with altitude (Fig. 2).The present study indicated that SOM was significantly related (P < 0.001) to BD (r = − 0.78 to 0.92), K sat (r = 0.61 to 0.67), TP (r = 0.71 to 0.75), and RP (r = 0.61 to 0.63) at both depths.The negative correlation between soil bulk density, soil organic matter and altitude could be associated with an increase in tree density and basal area, which caused an increase in the entry of roots and leaf litter biomass (Viana et al. 2014).Bulk density values along the altitudinal gradient reflect an adequate distribution of pore size and a high speed of water penetration in the profile, which shows that the soil has adequate physical functionality (Table 2).
The results of the physical properties of the soil correspond with values observed in other works carried out in forest areas in Pastaza which reported similar values of structural indices associated with soil physical quality (Bravo et al. 2017c, a, b;Huera-Lucero et al. 2020;Bravo-Medina et al. 2021).In general, soils presented optimal values of the structural indices evaluated at both depths and for all altitudinal zone when they were compared to the critical values of each parameter (Pla 2010;Blanco-Canqui and Ruis 2018).
Soil bulk density (BD) is a property of great environmental and agricultural significance because it influences the soil's hydric and pore behavior, as well as the root penetration and development of the crop (Pla 2010;Blanco-Canqui and Ruis 2018).Its values can be interpreted according to the soil's texture; in the case of a fine texture, for instance clay, BD > 1.40 Mg m −3 should be the threshold value (USDA-NRCS 1996).Based on this, in all the altitudinal Fig. 5 Ordination diagram based on redundancy analysis (RDA) of soil data in the 0-10 cm a and 10-30 cm b depths.Arrows represent the directions of maximum variation of soil properties.BD: Bulk density; K sat : Saturated hydraulic conductivity; TP: Total porosity; AP: Aeration porosity; RP: Retention porosity; Al 3+ + H + : Exchangeable soil acidity; Al 3+ : Exchangeable aluminum; TN: Total Nitrogen; OM: Soil organic matter; P: Available phosphorus; K + : Exchangeable potassium; Ca 2+ : Exchangeable calcium; Mg 2+ : Exchangeable magnesium gradients, the soil texture classes in the plots studied varied from clayey loam (CL) to clayey (C), which are categorized as fine.Meanwhile, the BD values were below the indicated threshold, which confirms the adequate physical conditions of the soil regardless of the altitude (Table 2).
Despite that, significant differences were observed (P < 0.05) in the physical properties which are closely related to the changes in the organic matter content of the soil along the altitudinal gradient, which values increase with elevation zone (Table 3).The increase in soil organic matter has been pointed out as an important factor in improving the physical properties associated with the soil structure (BD, K sat , porosity), as well as the water infiltration and retention rates (Bravo et al. 2017a;Blanco-Canqui and Ruis 2018).The relationship between soil organic matter, physical properties and altitude were confirmed through the PCA (Fig. 2) and correlation values at both depths (Fig. 3).The two depths showed highly significant Pearson's correlation coefficients (P < 0.001) for BD (r = − 0.73 to − 0.80), K sat (r = 0.60 to 0.59) and TP (r = 0.64 to 0.56).Soil physical properties and plants growth are significantly controlled by the variation in landscape attributes, including appearance, slope, and altitude, which influence the distribution of energy, plant nutrients, and vegetation, affecting the availability of the organic component of the soil (Rezaei and Gilkes 2005).In this research, the dynamic nature of the soil physical properties assessed (BD, Ksat, TP, AP, RP) could be affected by the altitudinal gradient or other landscape attributes (e.g., shape and growth density), which indirect influences by providing different microclimates.Those support the growth of plant species with different characteristics in the absence of management factors.The soil and vegetation have a complex interrelation, because they develop together over a long period of time.The selective absorption of nutrient elements by different tree species and their capacities to return them to the soil bring about changes in soil properties (Sharma et al. 2010).
Extensive research has been conducted to assess the impact of topographic features or elevation gradients on physical, chemical and biological properties at large scales, both in tropical (Chen et al. 1997;Homeier 2010;Liu et al. 2020) and temperate climate regions (Begum et al. 2010;Saha et al. 2020;Gömöryová et al. 2022).Little is known about the soil properties variation in a small-scale altitudinal gradient in tropical mountain regions, and the controlling factors remain uncertain (Unger et al. 2010;Du et al. 2014), which limits the comparison of our results with other works under similar conditions, but in turn this situation makes our study an exploratory research and of great relevance in humid tropical forests.In this regard, it has been suggested by several authors that variation among topographic microhabitats are a crucial factor influencing abiotic conditions and vegetation characteristics at small spatial scales (Homeier et al. 2010;Lippok et al. 2014;Saha et al. 2020).

Soil organic matter and total nitrogen
In forest ecosystems, soil chemical properties are influenced by canopy composition and topographic factors (Tsui et al. 2004).In this study, we hypothesized that small-scale altitudinal gradient may affect soil chemical properties.In our study, several soil chemical properties (i.e., SOM, TN, available P, K + and exchangeable Ca 2+ ) showed statistically significant differences (P < 0.05) at both depths with respect to altitude (Table 3).SOM and TN exhibited an increase with respect to the altitudinal gradient with higher values in the higher elevation zones (EZ3 and EZ4) in both the organic and mineral horizons (Table 3), which corresponds with values reported in other tropical forests in soils of moderate fertility (Unger et al. 2010).In the present study, SOM and TN contents showed a similar pattern of spatial distribution with respect to altitude (Fig. 2), presenting a linear fit that varied from high to moderate for the soil surface and subsurface horizon, respectively (Fig. 4).These results were evidenced by Pearson's correlation coefficient with highly significant relationships (P < 0.001) between altitude, SOM (r = 0.76 to 0.63) and NT (r = 0.90 to 0.84), confirming the role of SOM as the main source of nitrogen (McGrath et al. 2014).
Several studies have shown the differences along an altitudinal gradient can affect changes in the balance of C inputs and losses, which in turn is potentially related to changes in abiotic and biotic factors (litter quality) (Egli et al. 2008;De Bauw et al. 2016;Magnani et al. 2018).Due to higher N concentrations, organic material in the topsoil generally has higher N mineralization rates than the belowground horizon (Unger et al. 2010).To assess nitrogen availability in this study, we used the C/N ratio as an indicator of organic matter decomposition (Unger et al. 2010).In both horizons the C/N ratio ranged between 8 and 9 along the altitudinal gradient, suggesting an adequate rate of mineralization that stimulates the proliferation of microorganisms that can mineralize organic matter, and consequently, nutrients are available to plants (McGrath et al. 2014).Also, another factor that could influence N availability to plants in central and northern Ecuador is that many summits are volcanics that develop andosols in higher altitude areas with relatively high nitrogen concentrations as the soil ages (Unger et al. 2012).

Elevational change in P availability
Phosphorus (P) availability to plants depends on a variety of geochemical and biological factors, several of which are difficult to measure under field conditions.Furthermore, a fully accepted analytical procedure for quantifying plantavailable P in forest soils does not exist yet (Unger et al. 2010).In this context, our analysis is based on available P thus approaching the plant-available fraction.Available P exhibited significant differences in both horizons without any clearly defined pattern in relation to the altitudinal gradient (Table 3).Due this, the available P exhibited a negative relationship with altitude with Pearson correlation coefficients ranging from r = − 0.41 to − 0.66, with higher average values of available P at lower altitudes (Table 3).Opposite results have been reported for tropical rainforest who found that available P increased as a function of altitudinal gradient, which was associated with a substantial increase in the thickness of the organic horizon (Unger et al. 2010).In this study, it was observed that available P was higher in the surface horizon (0-10 cm) compared to the second layer (10-30 cm), indicating a vertical gradient in soil P availability and emphasizing the role of organic matter as an important source of P in these soils.We could point out that, in our study, the low amounts of available phosphorus in both depths confirm that this nutrient is a limiting factor and that part of the P released by the decomposition of organic matter is used by plants, which does not allow its accumulation in the soil.Organic P turnover and rapid recycling of P from wastes are the main processes for providing P to plants in natural ecosystems (Johnson et al. 2003).The P values available from this study agree with previous work on different Amazonian landscapes (forests and other land uses), where it is highlighted that low P content in Amazonian conditions could be a limiting factor in ecosystem productivity (Cuevas and Medina 1986;Bravo et al. 2017a;Bravo-Medina et al. 2021).However, as has been pointed out by other researchers, this type of study needs to be expanded as additional P fractions, also it may be available to plants and that are not measured by this technique, in particular organic and inorganic P mobilized by external phosphatases and other rhizosphere processes (Unger et al. 2010).Phosphorus is the main macronutrient limiting ecosystem productivity in heavily weathered tropical soils (Elser et al. 2007;Hamer et al. 2013).
It is important to note that these soils have high amounts of iron (Fe) and aluminum (Al) sesquioxides that geochemically bind P and make it unavailable for plant uptake (Custode and Sourdat 1986;Soltangheisi et al. 2019).Understanding soil phosphorus transformation pathways and the determinants related to soil P nutritional status when land change uses occur is critical for developing better management practices, especially in the Amazon region (Soltangheisi et al. 2019).Given that phosphorus is stored in various forms in soil and that availability varies among plants (depending on their rooting strategies) and soils (depending on their properties), some researchers suggest using measurements of total P rather than availablePbecause many plants in natural ecosystems can access more forms of P in soil than crops (Bond 2010).Further studies are essential to discuss the relevant processes controlling soil available P stocks and its decline with altitude, as well as its potential impact on tree growth, tree species composition, and nutrient cycling in tropical forests at altitude.

Soil acidity and the availability of Ca, Mg and K
Soil pH is an important factor, as it directly affects other parameters associated with soil fertility and the ability of plants to absorb nutrients (McGrath et al. 2014).Based on this, the acidification process evaluated through soil pH, acidity and exchangeable aluminum were similar across the altitudinal gradient and without significant differences.Soil pH levels were categorized as very acidic (< 5), acidity and exchangeable Al 3+ at both soil depths did not follow any defined pattern with respect to the altitudinal gradient (Table 3).The high acidity and exchangeable aluminum values contributed to the low soil pH levels, reflecting a wide adaptation of plants to the oligotrophic environment characteristic of the Amazon region (Cuevas and Medina 1986;Bravo et al. 2017a).Different studies in tropical forests report a gradient related to soil pH showing the highest values in the lower zones, which is associated with the direction of the slope that favors the removal of Ca 2+ and Mg 2+ minerals and their accumulation in the lower zones (Chen et al. 1997).Likewise, in tropical montane forests, a significant decrease in soil pH and Mg 2+ and K + concentration with slope has been found (Homeier et al. 2010).Higher precipitation and lower temperatures at higher elevations promote nutrient leaching and a slowing of nutrient mineralization and reduction of organic matter turnover (Wilcke et al. 2008;Homeier et al. 2010).Also, other authors agree that the behavior of this parameter can be explained by more intense leaching and acidification in higher altitude areas, traditionally following the orographic rainfall gradient and resulting in a lower value of available Ca 2+ , Mg 2+ , K + and P (Vitousek and Chadwick 2013;De Bauw et al. 2016).Opposite results have been reported by other researchers who found an improvement in pH and nutrient availability (Ca 2+ , Mg 2+ , K + , P, N) with increasing altitudinal gradient due primarily to increased organic layer thickness in higher elevation zones (Unger et al. 2010).Such results highlight the fundamental role of the organic horizons, not only in the supply of N and P, but also in the proportion of basic cations (Wilcke et al. 2008).In this context, in our study, a higher amount of SOM in higher altitude zones was also associated with a higher exchangeable Ca 2+ concentration, which was confirmed by the degree of association between both variables with Pearson correlation coefficients (r = 0.79-0.74).The availability of exchangeable N and Ca 2+ is directly related to the accumulation of organic matter in the soil, which in turn is influenced by the amount of leaf litter falling on the forest floor (Tsui et al. 2004).In our study site, a higher presence of tree species of the legume family such as Fabaceae (Inga spp and Dusia spp) has been reported in the higher elevation floor (901-1000 m.a.s.l.), which could contribute to an increase in SOM, TN and Ca 2+ values (Torres et al. 2019).Redundancy analysis (RDA) (Fig. 5) showed that the altitudinal gradient can affect the behavior of some soil physical and chemical parameters associated with fertility (MOS, TN, available P, K + and Ca 2+ ); however, to complement these results, studies on vegetation composition and structure are needed to help explain the variation in soil parameters (Chen et al. 1997;Tsui et al. 2004).

Nutrient reserve according to altitudinal gradient
The available nutrient reserve was calculated from the nutrient concentrations in the soil and adjusted to the BD and weighted at the first 0.30 m depth within the tree root zone.The soil analysis generally extracts a part of the total amount of the element presents and the amount extracted is proportional to the amount available for plant absorption.The results in our study indicate that the nutrient reserves (TN, P, K, Ca and Mg) followed the same trend along the altitudinal gradient and according to the variation in the concentration of each element; therefore, the TN and Ca 2+ reserves increased significantly with altitude (Fig. 6, P < 0.001), while the P and K + reserves were higher in the lower altitude zones.For TN and Ca 2+ were clear that these reserves are associated with soil organic matter; however, their importance in the productivity of these ecosystems is evident, not only with respect to TN and Ca 2+ but also to available P and Mg 2+ (Wilcke et al. 2008;Unger et al. 2012).
These results suggest that even in dystrophic environments, such as in Ecuadorian Amazon region with medium to low levels of soil nutrient, there are nutrient reserve that plays an important role in the productivity of natural ecosystems through soil fertility and with potential input for plants, especially N and Ca 2+ .
Most of the weathered soils in humid regions act as weak acidic systems, where the total acidity is much higher than the active acidity and, therefore, the potential acidity is high (Lira-Martins et al. 2019).In our study, the N stock was higher than the exchangeable bases (K + , Ca 2+ and Mg 2+ ) and the available P. However, available P is the most limiting nutrient, which is common in the tropical forests of the Amazon, and represents a forest that grows on nutrientpoor soils (Johnson et al. 2003;Bravo et al. 2017b).Organic P mineralization and dissolution (or weathering) of stable P minerals are natural processes that provide labile P to crops.However, in soils that have low to medium available P values, this means that the supply for plants only covers 15 to 50% of the crop requirement (McGrath et al. 2014).It is important to note that laboratory assessment methods designed to evaluate nutrient availability for crops underestimate the nutrients available in natural ecosystems, because plants in forests often access more nutrients than agricultural crops, appealing to a highly diverse nutrient conservation mechanism (Cuevas and Medina 1988).Correlations of vegetation patterns with soil nutrients, particularly P, suggest that soil chemistry is an important factor influencing vegetation properties (Lloyd et al. 2008).Although soil nutrient concentration represents an important reserve for biomass production, it has been estimated that in some forests, 90% of the total reserve of the Ca 2+ , K + , P and N ecosystem may be trapped in aerial biomass reserves, whereby this proportion is lower if deeper soil layers are considered (Bond 2010).It should be noted that the nutrient reserves in our study were obtained from available P and exchangeable fractions (K + , Ca 2+ and Mg 2+ ).However, some studies recommend that the analysis should focus on the total nutrient stock rather than the fraction of the stock that is available to natural vegetation or the rate at which nutrients are available in decomposition processes (Johnson et al. 2003;Bond 2010;Paul et al. 2010).From this perspective, Bond, (2010) estimated that, as a general guideline, the nutrient stocks (kg ha −1 ) needed to build a forest would be > = P: 20-30, K: 200-350, Ca: 300-600 and Mg: 55-65.Therefore, when comparing the potential woody biomass requirements with the nutrient reserves obtained in our study, we could indicate that they do not limit the development of the forest as it is possible to satisfy these demands, especially in plots located at higher elevations.This is confirmed by the production reported in our study area, where aerial biomass varied between 246.8 and 320.9 Mg ha −1 (Torres et al. 2019).

Conclusions
In this study was found that some soil physical and chemical properties varied along a small-scale altitudinal gradient in the evergreen foothills forest of the Amazon region.Higher elevations generally exhibited more fertile soils, characterized by higher levels of soil organic matter, total nitrogen, exchangeable calcium, saturated hydraulic conductivity, total porosity, and retention porosity.Conversely, lower elevations showed higher values of soil bulk density, exchangeable potassium, and available phosphorus.Soil pH, exchangeable aluminum, and exchangeable acidity did not show a consistent pattern with respect to the altitudinal gradient, likely due to the influence of soil formative factors and high rainfall.The higher organic matter content at higher elevations improved the physical conditions and soil structure.The varying patterns of nutrient concentrations suggest that factors such as vegetation associated with altitude may influence nutrient stocks, particularly total nitrogen, available phosphorus, potassium, and exchangeable calcium.Further studies are necessary to examine the relationship between floristic composition, soil, microclimate, and altitudinal components.

Fig. 1
Fig. 1 Study area location and of the permanent forest plots at each elevation site

Fig. 6
Fig. 6 Soil stock nutrients in the 30 cm depth as a function of altitude.A TN; B Available P; C Exchangeable K + ; D Exchangeable Ca 2+ ; E Exchangeable Mg 2+

Table 2 )
presented higher values of SOM,