Depth distribution of exchangeable aluminum in acid soils: A study from subtropical Brazil

Due to potential crop toxicity, high aluminum (Al) availability requires increased attention when preparing agricultural soils. However, research examining the relationship between depth distribution of Al and soil weathering has received little priority in Brazil, particularly regarding the number of soil profiles investigated. This study analyzed 38 acid soils selected from Soil Surveys in southern Brazil to identify and isolate the effects of organic and mineral components on depth distribution of exchangeable Al extracted with KCl (AlKCl). These soil profiles were divided into the following three groups based on AlKCl depth distribution: Group I – decrease with depth; Group II – little variation with depth; and Group III – increase with depth. High AlKCl found near the surface of well-developed soils (Group I) was influenced by organic matter content, while mineral fraction quality defined the occurrence of high AlKCl in subsurface horizons of Group III. The depth distribution of AlKCl was defined by the degree of weathering in these subtropical soils. Possessing a knowledge of these soil groupings may aid in soil acidity management to optimize crop productivity in southern Brazil.


Introduction
Acidic soils have a pH lower than 7; however, much of the pedosphere has higher acidity (pH < 5.5) that favors increased toxic forms of aluminum (particularly Al 3+ ) in soil solution.Most agricultural plant species do not attain maximum production potential when grown in high acidity soils due to Al toxicity and nutritional deficiency (Kochian, Piñeros, Liu, & Magalhães, 2015;Goulding, 2016;Sade et al., 2016;Barbosa, Motta, Consalter, & Pauletti, 2017a).Acidic soils directly affect the health and nutrition of people living in rural and urban areas by constraining production of cultivated species.
The effects of lime, management system/land use practices, and organic residue addition on Al KCl levels have been widely evaluated (Brunetto et al., 2012;Barcellos, Motta, Pauletti, Silva, & Barbosa, 2015;Costa, Crusciol, Ferrari Neto, & Castro, 2016;Baquy, Li, Xu, Mehmood, & Xu, 2017;Barbosa et al., 2017b;Machado, Camara, Sampaio, Pereira, & Ferraz, 2017;Rocha et al., 2017).These data were obtained from the most superficial soil layers, excluding less weathered deep horizons with organic matter contents close to zero.In contrast, there are limited studies on Al KCl as a function of soil weathering, organic matter, and clay mineralogy.Quesada et al. (2010) studying surface layers (0 -30 cm) of 71 tropical forest soils (i.e., Brazil, Venezuela, Bolivia, Colombia, Peru, and Ecuador) noted higher Al KCl values in Chernozems, Cambisols, and Plinthosols than those in Acrisols and Ferralsols.Cunha, Almeida, and Barboza (2014) reported high values of Al KCl due to 2:1 minerals in Brazilian soils.However, little attention has been given to explaining variations of Al KCl in both superficial and subsurface layers of such acid soils.Understanding factors that govern depth distribution of Al KCl in soil can contribute to the knowledge base concerning the nature and management of acidic soils.
The aim of this study was to identify and isolate the effects of organic and mineral components on the depth distribution of Al KCl in acid soils with different degrees of weathering.Our goal was not to correlate lithology or climate with Al KCl levels because consistent variations in Al KCl within soil profiles occurred independent of these factors.

Study areas
This study was developed based on the following three soil surveys of the southern region of Brazil: 1) Soil Recognition Survey of the Paraná (Embrapa, 1984); 2) Soil Recognition Survey of the Santa Catarina (Embrapa, 1998); and 3) Soil Recognition Survey of the Rio Grande do Sul (Brasil, 1973).These surveys were conducted between the 1960s and 1990s and covered the territory of each southern Brazilian state.Soils were carefully chosen to adequately represent the taxonomic unit in terms of morphological, chemical, and physical attributes along the profile for classification.Regarding soil characterization, it is important to note that standard methodologies were used to evaluate soils in all surveys.

Data collection
Mineral soil profiles were selected to evaluate the potential for creating acidity.Distribution profiles of Al KCl were the only selection criterion.In selecting acid soil profiles, one of the horizons typically had Al KCl higher than 4 cmol c kg -1 , but all profiles had at least one horizon in which Al KCl was higher than 2 cmol c kg -1 .The profiles were divided into the following three Groups: (I) decrease of Al KCl with depth (12 profiles), (II) insignificant variation of Al KCl with depth (9 profiles), and (III) increase of Al KCl with depth (17 profiles) (Figure 1; Tables 1, 2, and 3).The following soil profile variables were also considered: depth, clay content, organic carbon (C), Ki index, H (potential non-exchangeable acidity), cation exchange capacity (CEC) at pH 7.0, pH in KCl (pH KCl ), and Al KCl saturation (m).
The Ki index was obtained by the following equation: where: SiO 2 and Al 2 O 3 were obtained by sulfuric attack (g kg -1 ).
The relationship between potential exchangeable acidity (Al KCl ) and non-exchangeable potential acidity (H) was determined as Al KCl /H.
The CEC at pH 7.0 was obtained by the following equation: where: Ca 2+ , Mg 2+ , K + , Na + , and Al 3+ represent the contents (cmol c dm -3 ) of these elements extracted using KCl solution; H represents the content (cmol c dm -3 ) of this element extracted using calcium acetate.Saturation by Al KCl (m) was obtained by the following equation: where: m in percentage; Al 3+ , Ca 2+ , Mg 2+ , K + , and Na + represent the contents (cmol c dm -3 ) of these elements extracted using KCl solution., 1984), Santa Catarina (Embrapa, 1998), and Rio Grande do Sul (Brasil, 1973). 2 Page number in each soil survey. 3Number of soil profiles collected in each survey. 4Brazilian system of soil classification (Embrapa, 2013). 5Corresponding global classification system (IUSS Working Group WRB, 2015); classification adopted in the present study.(Brasil, 1973). 2 Page number in each soil survey.3Number of soil profiles collected in each survey. 4Brazilian system of soil classification (Embrapa, 2013).5 Corresponding global classification system (IUSS Working Group WRB, 2015); classification adopted in the present study.

Data processing and analysis
Selected data were entered into Microsoft® Excel spreadsheets and organized according to each soil group.Because of wide depth variability between soil profiles and soil horizons, the following average ranges (cm) were used: 0 -10; 10 -20; 20 -30; 30 -40; 40 -50; 50 -70; 70 -100; 100 -150; and > 150 (average depth of 225 cm).The soil profile was evaluated based on horizon analysis, and the reported depth was considered to be half the depth of each horizon.Mean values and standard deviations for each attribute per depth range were calculated.All data were subjected to Pearson's simple correlation analysis using Sisvar statistical software (Ferreira, 2014).

Result and discussion
Climate (Alvares, Stape, Sentelhas, Gonçalves, & Sparovek, 2013) and lithology (Figure 1) were variable within each soil group and were similar across groups.For this reason, climate and lithology were not considered in the discussion data.In Group I, the average Al KCl content was close to 5 cmol c dm -3 in the 0 -10 cm layer and was reduced to ~1.5 cmol c dm -3 at the greatest depth.For Group II, a lower Al KCl variation (4.6 to 3.2 cmol c dm -3 ) was observed from the most superficial soil layer to the deepest layer.In Group III, a clear increase occurred in average Al KCl with depth, which varied from 3 to 11 cmol c dm -3 (Figure 2).
Comparing the two groups of highest contrast, Group I was predominantly Ferralsols (Table 1), while Group III (Table 3) included soils with moderate weathering and diagnosed as having incipient developed B horizons or B horizons with clay accumulation (Brasil, 1973;Embrapa, 1984;Embrapa, 1998).Additionally, when comparing contrasting Ki index values (Group I: from 1.2 to 2; Group III: from 2 to 3.3), well-developed soils have Ki indices < 2.0 (IBGE, 2015).Dalović et al. (2012) also found a clear increase in average Al KCl content in lowdeveloped soils (102 profiles) from a basin in Serbia.Similar results are reported for tropical and subtropical Brazilian soils (Marques et al., 2002;Motta & Melo, 2009;Cunha et al., 2014).The increased level and buffering of Al KCl with depth are even more significant for lowdeveloped soils that have high levels of amorphous minerals of allophane and imogolite types, high Al/Si molar ratios, and low environmental stability.This is observed under conditions of extremely low soil weathering, such on the Peninsula Keller (Antarctica) where soil profiles were developed from sulfide-bearing andesites (rich in amorphous minerals) having high Al KCl contents of 18.2 cmol c kg -1 in the A horizon and 27.8 cmol c kg -1 in the B horizon (Poggere, Melo, Francelino, Schaefer, & Simas, 2016).Motta and Melo (2009) established the following relationships for the evolution of Al KCl in subsurface horizons with weathering of tropical and subtropical soils: i) Low weathering soils with 2:1 dioctahedral (smectite) minerals: incipient weathering is not sufficient to release Al from octahedral sheets, and all negative charges of minerals are occupied by bases (V = 100%), ii) Moderate to intense weathering: partial or total dissolution of 2:1 minerals in the B horizon promotes Al release and acidifies soil, iii) Intense weathering: neoformation of kaolinite from Al and Si released by 2:1 minerals reduces acidity of the B horizon, iv) Very intense weathering: transition to an oxidic system that stabilizes Al in gibbsite structure.Considering chemical equilibrium reactions and equilibrium constants for 2:1 (Mg-montmorillonite) and 1:1 minerals (kaolinite) presented by Lindsay (2001), it is possible to exemplify more solubility and Al release from 2:1 minerals.As an example (using ionic forms of elements), given a pH of 6.0, H 4 SiO 4 in soil solution is in equilibrium with quartz (10 -4 mol L -1 ) for Mg-montmorillonite and kaolinite, and Fe 3+ is in equilibrium with goethite with Mg 2+ equal to 10 -3 mol L -1 (Lindsay, 2001) for montmorillonite, the following concentrations of Al 3+ are present in soil solution under equilibrium conditions: Mgmontmorillonite, Al 3+ = 10 -10.0 mol L -1 ; kaolinite, Al 3+ = 10 -11.3 mol L -1 .At lower pH values, the instability of minerals increases, and the difference in Al 3+ content in equilibrium solution favoring montmorillonite is even more significant.
According to these premises, the degree of soil weathering was classified intense to very intense for Group I and moderate to intense for Group III, which corroborated Ki index results (Figure 2).The expressed variation in average Ki levels among groups allowed for the establishment of positive correlations with Al KCl content considering all samples (0.53, p < 0.01) or excluding those in the 0-50 cm depth range (0.70, p < 0.01; Table 4).Analysis of only subsurface diagnostic horizons increased the correlation coefficient between Ki index and Al KCl content, since the effect of surface horizon organic matter on Al KCl dynamics in soil was isolated.When soil groups were considered separately, correlation coefficients were less than 0.4, since Ki values were similar along the soil profiles within each group (Figure 2).
The presence of 2:1 soil minerals promotes low CEC variation in subsurface soil, such as that observed in Group III (Figure 3).In Group III, the correlation coefficient for Al KCl and CEC at pH 7.0 was 0.70 (p < 0.01).However, unlike other groups, the low correlation between CEC and C (0.33, p < 0.01) indicated that maintenance of high Al KCl levels in Group III soils was primarily controlled by negative charges of minerals in the clay fraction.As a consequence, more intense adsorption kept Al in the soil and prevented leaching.High Al KCl levels in soils with 2:1 minerals are also associated with the capacity of the extractor (KCl) to solubilize Al amorphous compounds and Al-hydroxy islands between layers of secondary 2:1 minerals (Marques et al., 2002;Cunha et al., 2014).
The correlation coefficient between C and Al KCl was 0.61 (p < 0.01) for Group I (Table 4).For Group III, the correlation coefficient between these same parameters was low (-0.16,p < 0.01).The correlation coefficient between Al KCl and CEC was also high in Group I (0.74, p < 0.01), and soil negative charges were due to humic compounds (correlation coefficient between CEC and C = 0.97, p < 0.01).The average C levels decreased markedly along the profiles within the three groups (Figure 3).However, the CEC at pH 7.0 was reduced with the same intensity only in Groups I and II; ranging from averages close to 20 cmol c dm -3 near the surface to 6 cmol c dm -3 in the deepest soil layers.By contrast, Group III initial mean values were approximately 20 cmol c dm -3 but remained close to 15 cmol c dm -3 as depth increased.These results indicated that the variation of Al in Group I was most associated with the organic fraction, whereas in Group III, this variation occurred with the mineral fraction of the soil.
In Group I, organic matter contributed more to higher Al KCl content near the surface, and the decrease in exchangeable potential acidity with depth could be attributed to Al stabilization primarily in the structure of gibbsite (Vendrame et al., 2013).Ghidin, Melo, Lima, and Lima (2006) worked with a toposequence of Ferralsols (similar to profiles 30 and 32 in Group I; Table 1) from Guarapuava (Paraná State) that originated from basalt.They observed a predominance of oxides in the clay fraction of the B horizon (i.e., 322 g kg -1 gibbsite, 309 g kg -1 hematite, and 294 g kg -1 kaolinite).Thus, with much of Al stabilized in the structure of gibbsite and kaolinite, organic matter becomes the source of this element.Despite the strong interaction between Al and organic matter, a KCl solution can extract the most labile fraction (Campos, Silva, Silva, & Vidal-Torrado, 2014;Cunha et al., 2014).However, some authors found Al-hydroxy islands between layers of secondary 2:1 minerals in Ferralsol clay fractions of several Brazilian regions, although these minerals are only residual in welldeveloped soils (Silva, Motta, Melo, & Lima, 2008;Schaefer, Fabris, & Ker, 2008).
The lowest values for Al KCl /H were observed in deeper layers of Group I profiles (Figure 3).Since C contents in the subsurface were similar between Groups I and III, the low Al KCl /H ratio in Group I was indicative of greater hydroxylated surface groups (pH dependent charge) in soil colloids, higher levels of 1:1 silicate minerals, and Fe and Al oxides in the clay fraction.The aluminol (-AlOH) and ferrol (-FeOH) groups common in these minerals cause low acidity (predominance of CEA over CEC at pH below 7 to 9; Schwertmann & Taylor, 1989), and elevation of natural soil pH to 7.0 during extraction with 0.5 mol L -1 Ca acetate results in a high release of H.The reduction of this ratio in Group I was favored by lower Al KCl content.
For the most active clay system (Group III), less H release most likely occurs due to reduced occurrence of aluminol and ferrol; approximately 95% of 2:1 secondary mineral charges are structural or independent of pH changes (Brady & Weil, 1996).In 2:1 clays, a proportion of silanol groups (-SiOH) occur that act as strong acid radicals deprotonating at pH 2 (Tarì, Bobos, Gomes, & Ferreira, 1999).Since each analyzed soil had a pH above 2, H had been previously released.Thus, these H groups were not computed in the determination of potential acidity since they were not exchangeable using Ca acetate (0.5 mol L -1 , pH 7.0).Therefore, in subsurface horizons of Group III soils, the primary component of acidity was the exchangeable potential (high Al KCl /H ratio).The effect of organic matter favoring non-exchangeable potential acidity was evident in the reduction of the Al KCl /H ratio for surface horizons of Group III soils.
The pH KCl also exhibited variation among groups (Figure 2), with a negative correlation coefficient for pH (KCl) and C significant at p < 0.01 only for Group I (Table 4).Significant increases in this parameter with depth for Group I reflected reduction of the positive effect of organic matter in forming negative charges near the surface, and the more oxidic mineralogy of Ferralsols favors positive charge formation in subsurface layers (Silva et al., 2008;Serafim, Lima, Lima, Zeviani, & Pessoni, 2012).Increase in the proportion of positive charges with depth of Group I profiles favors the adsorption of OH -and an increased pH KCl after the exchange of these anions by the Cl -in solutions of KCl (1 mol L -1 ).Regardless of group, saturation by Al KCl (m) increased as a function of soil depth (Figure 3).However, the highest m values occurred through different routes, particularly for more contrasting groups (I and III).For Group I, reduced variation in Al KCl (below 60 cm) indicated increased mean m values up to this layer and was a reflection of reduced CEC with depth.For Group III, the increase in m values with depth followed significant increases in Al KCl along soil profiles.
In practical terms, the differences in Al KCl content and depth distribution patterns in the studied soils require necessary variations in acidity management.When cultivating plants in acid soils with Ki index > 2.2 (usually soils with cambic B or argic B horizons), managing soil acidity in depth will be more intense due to higher Al KCl .

Conclusion
The distribution of Al KCl with depth was defined by the degree of soil weathering in subtropical Brazil.For soils with intense to very intense weathering, the organic fraction increased CEC and Al KCl in superficial horizons, with Al KCl reduction in subsurface layers due to reduced organic matter and probable predominance of minor minerals with lower Ki (1:1 + Al oxides) that reflected higher Al stability in structural forms.By contrast, soils with moderate weathering had higher Al KCl and increased average Al KCl content with depth that indicated greater influence of soil mineral fractions.These soil groupings may aid in soil acidity management.

Figure 1 .
Figure 1.Distribution of acid soil profiles used to form Groups I, II, and III in southern Brazil.PR -Paraná State; SC -Santa Catarina State; RS -Rio Grande do Sul State.

Figure 2 .
Figure 2. Mean values of exchangeable Al (Al KCl ), pH KCl , Ki index, and clay in acid soils from southern Brazil.Bars represent standard deviation.

Figure 3 .
Figure 3. Mean values of carbon (C), cation exchange capacity (CEC) at pH 7, m (Al KCl saturation), and Al KCl /H ratio in acid soils from southern Brazil.Bars represent standard deviation.

Table 1 .
Acid soils used to form Group I (decrease of Al KCl with depth) in southern Brazil.
1 PR -Paraná State; SC -Santa Catarina State; RS -Rio Grande do Sul State.Soil Recognition Survey of the Paraná (Embrapa

Table 2 .
Acid soils used to form Group II (insignificant variation of Al KCl with depth) in southern Brazil.

Table 3 .
Acid soils used to form Group III (increase of Al KCl with depth) in southern Brazil.