Andic properties in soils with histic horizon “O” in the highlands of Southern Brazil

Soils with andic properties are characterized by a low apparent density, variable charges, large amounts of allophanes, imogolite, ferrihydrite, and/or organo-metallic complexes with Al, and present high phosphate retention. Soils derived from non-pyroclastic materials rich in silicates, formed under a cold and humid climate, a large amount of organic carbon, acid weathering, andic properties can manifest when a large amount of Al is present in the form of organo-metallic complexes. This study aimed to evaluate the characteristics and geographical expression of soils with such properties in areas of altitude in the extreme south of Brazil, on the escarpment edges of the Serra Geral Formation, under a cold and humid climate. The sampling points were selected based on environmental characteristics such as geomorphology, geology, the coloration of the superficial horizon of the soil, and position in the landscape, covering a linear distance of approximately 185 km at the escarpment edge between the states of Santa Catarina and Rio Grande do Sul. Soil samples were described and collected from ten soils with histic horizon O, three soils with humic horizon, and one soil with histic horizon H. The undisturbed soil samples were collected using volumetric metallic cylinder to determine the soil bulk density. Organic matter, pH(H2O), P retention, and selective dissolutions of Al, Fe, and Si analyses were performed. The Cambissolos Hísticos and Organossolos Fólicos showed andic properties, while the Cambissolos Húmicos and Organossolo Háplico did not meet one or more criteria, as required by the Brazilian Soil Classification System and the World Reference Base for Soil Resources. The horizons with andic properties were classified as aluandic, based on the predominance of Al associated with organic complexes. The cold climate and high cloudiness of the highest altitude areas in the extreme south of Brazil, occurring in a narrow strip of the escarpment of the Serra Geral Formation in the states of Santa Catarina and Rio Grande do Sul, allow the formation of a constantly humid environment. This environment favors the acid weathering of the source material, accumulation of organic matter in the soil, and its stabilization by the formation of organo-metallic complexes, mainly Al-humus. The combination of these factors gives the soils with histic horizons O a low bulk density, high phosphate retention, and Alo + 1⁄2Feo ≥2 % values, meeting the criteria required for andic properties.


INTRODUCTION
Soils with andic properties are characterized by low bulk density, variable charges, large amounts of allophanes, imogolite, ferrihydrite, and/or organo-metallic complexes with Al, in addition to high phosphate retention (Soil Survey Staff, 2014;IUSS Working Group WRB, 2015). The presence of short-range-order minerals and/or organo-metallic complexes in soils with andic properties generally results from part of the moderate weathering sequence of pyroclastic materials (IUSS Working Group WRB, 2015). However, andic properties can also manifest in soils with large amounts of Al in the form of organo-metallic complexes and non-pyroclastic materials rich in silicates in cold and humid climates, with a large amount of organic carbon (IUSS Working Group WRB, 2015) formed under weathering in acidic conditions (Driessen et al., 2001).
There is growing evidence of soils' occurrence with andic properties formed on non-pyroclastic material, indicating that their characteristics are mostly due to organo-metallic complexes instead of short-range-order minerals (Caner et al., 2000). The rapid weathering of volcanic glass can result in the accumulation of stable organo-metallic complexes or the formation of short-range-order minerals, such as allophanes and imogolite, additionally forming ferrihydrite. However, the weathering of other materials rich in silicates in a humid climate also leads to the formation of stable organo-mineral complexes (Garcia-Rodeja et al., 1987;IUSS Working Group WRB, 2015).
The genesis of the andic properties depends essentially on the rapid weathering of the porous, permeable, and fine-grained mineral material in the presence of organic matter. The ions released in the hydrolysis of primary minerals, especially Fe 2+ and Al 3+ , can form stable complexes with organic substances. However, the iron quickly oxidizes, and not all ions form complexes, allowing their precipitation as ferrihydrite. The concentration of silica in the soil solution increases if most or all the aluminum is stabilized in the complexes with organic substances. Part of this silica is leached, while another part precipitates as opaline silica. If a considerable proportion of released aluminum is not complexed, it can co-precipitate with silicon to form allophanes of varying composition, often in association with imogolite (Driessen et al., 2001).
When most or all of the aluminum is complexed with organic compounds, the silica concentration in the soil solution increases (Driessen et al., 2001); and while part of the silica is lixiviated, another part precipitates as opaline silica. If not, all aluminum is bound in complexes, the rest can co-precipitate with silicon to form minerals of low structural order, such as allophanes and imogolites. The formation of Al-humus complexes and the formation of allophanic associations are competitive. While allophanes and imogolites are stable under moderately acidic to neutral conditions, Al-humus complexes prevail in more acidic environments.
With excess aluminum available under such acidic conditions, it can combine with silicon to form 2:1 and 2:1:1 phyllosilicate clay minerals, which are often found in association with Al humus complexes (Ndayiragije and Delvaux, 2003). The occurrence of 2:1 and 2:1 of hydroxy-Al interlayer silicates in Andosols has often been associated with the presence of Al-humus complexes in non-allophanic Andosols . The incorporation of Al in the organic complexes and / or in the intermediate Al layers of 2:1 clay minerals can induce an anti-antibiotic effect and inhibit the formation of allophane and imogolite , as observed by Dahlgren et al. (1993) and Ndayiragije and Delvaux (2003). The abundance of organo-metallic associations resulting from pedogenetic processes, presenting high physical and biological stability, low mobility, and high accumulation of organic matter, constitutes an original characteristic of soils with non-allophanic andic properties (Aran et al., 2001).
In Brazil, the possibility of the contribution of allophanes in the manifestation of properties that would resemble andic soils in an altitude environment in areas of effusive rocks in Rev Bras Cienc Solo 2021;45:e0200152 the southern region of the country is briefly discussed by Bennema and Camargo (1964). Fasolo et al. (1980) also mention soils with potential andic properties in the state of Santa Catarina. However, these soils had no such properties (Ker, 1988;Ker and Resende, 1990). Volkoff et al. (1984) studied humus and mineralogy of altitude field soils in the states of Minas Gerais, Paraná, and Santa Catarina, focusing on the mobility of fulvic and humic acids, and found that the humus in these environments is similar to the humus of andic soils, of which properties were attributed to organo-metallic complexes, mainly with Al.
Studies of the soils from Trindade Island, in Brazil, indicate the presence of volcanic glass, amorphous materials, and low bulk density and high pH values in NaF, suggesting the presence of andic properties (Clemente, 2006;Clemente et al., 2009;Sá, 2010;Machado, 2016;Machado et al., 2017). The presence of soils with andic properties in this location was later confirmed by Mateus et al. (2020). Furthermore, soils with andic properties in Brazil were described for the first time, by Dümig et al. (2008), in the municipality of São Francisco de Paula, Rio Grande do Sul. This was the first confirmation of andic soils in South America outside the areas of recent volcanism in the Andes. This occurrence is mentioned in the World Reference Base for Soil Resources (IUSS Working Group WRB, 2015).
There is an expressive area of soils that resemble those studied by Dümig et al. (2008) along the escarpment edge of the Serra Geral Formation in southern Brazil. The cold and humid climate of this environment favors the high accumulation of organic matter in the soil, which, associated with the acid weathering of the source material, rich in silicates, can generate andic properties on soils in a much more extensive area than that studied by Dümig et al. (2008). However, this phenomenon can be restricted to a stretch on the escarpment edges, where the climate conditions favor the formation of histic horizons.
The general objective of this study was to evaluate the presence, characteristics, and extent of the occurrence of andic properties in soils with significant carbon content, formed on the escarpment edges of the Serra Geral Formation, in the southern plateau of the state of Santa Catarina (SC) and northeast plateau of the state of Rio Grande do Sul (RS).

Study area
The study area includes the Serra Geral Formation's escarpment edge in the southern plateau of Santa Catarina, and the extreme northeast plateau of Rio Grande do Sul ( Figure 1). The studied environment is predominated by undulating relief areas, with smoothly wavy and flat parts, which contrast sharply with adjacent rugged reliefs that characterize the entire escarpment of the Serra Geral Formation in the extreme south of Brazil. Such areas are mainly in the municipalities of São Francisco de Paula, Cambará do Sul, and São José dos Ausentes, in RS, and Bom Jardim da Serra, Urubici, and Urupema, in SC.
The Serra Geral Formation of the Lower Cretaceous is the record of a fissure extensive volcanic event that covered approximately 75 % of the Paraná Sedimentary Basin (Stewart et al., 1996;Milani et al., 1998;Nardy et al., 2002). This event was caused by the rupture of Gondwana, which formed after the Pan-African/Brazilian orogenic cycle and remained stable in its southern portion for approximately 400 million years, giving rise to the South Atlantic Ocean (Peate, 1997;Roisenberg and Viero, 2000;Orlandi Filho et al., 2009).
The escarpment of the Serra Geral Formation extends diagonally across southern Brazil, with an abrupt east face, slowly declining to the west towards the Paraná and Uruguay rivers. Close to the border between the states of Santa Catarina and Rio Grande do Sul, the Serra Geral Formation rises and approaches the coast, bending in the north-northeast (NEE) direction for approximately 230 km following the coastline (Besser et al., 2015). It consists of a series of rocks originating from basaltic spills, interspersed with andesitic spills, and the most recent spills of more acidic characteristics, resulting in the formation of more siliceous rocks, such as rhyodacite, dacites, and rhyolites (Bellieni et al., 1986).
The region's climate, according to the Köppen classification system, is Cfb, wet temperate (C), with well-distributed rains throughout the year (f), and an average temperature of the hottest month below 22 °C (b) (Mota, 1951;Kuinchtner and Buriol, 2001;Potter et al., 2004). There is a high water condensation in the highest portions of the escarpment, with daily fog formation near the top of the slopes and at the edges of the escarpments. These fogs can last up to weeks (Falkenberg, 2003).

Soil characterization
The sampling points were selected based on environmental characteristics such as geomorphology, geology, coloring of the superficial horizon of the soil, and position in the landscape, covering a stretch at the escarpment edge of the Serra Geral Formation, to the east from the municipality of São Francisco de Paula (RS) to Urupema (SC), in a linear distance of approximately 185 km ( Figure 1 and Table 1).
Soil samples were described and collected from ten soils profiles with diagnostic histic horizon O (P2, P3, P4, P6, P8, P9, P10 P11, P12, and P13), three soils profiles with diagnostic humic horizon A (P1, P5, and P7), and one soil profile with histic horizon H (P14). The diagnostic histic horizon O is formed from deposited organic materials under free drainage conditions, without stagnant water, and occurring closer to the escarpment edge. The diagnostic humic horizon A occur a little further from the escarpment. The histic horizon H formed from organic materials deposited under excess water conditions in a small depression surrounded by soils with the histic horizon O. In 14 soil profiles were evaluated the possible presence of andic properties in this environment.
The basic characterization of all soil profiles, including the analysis of the main physical and chemical properties, are in tables 2 and 3. Details of these soils can be found in Santos Junior (2017).

Soil classification
The determination of superficial and subsurface diagnostic horizons was carried out according to the criteria defined in the 5th edition of the Brazilian Soil Classification System -SiBCS (Santos et al., 2018). The most acidic spills originated the rhyodacite and constituted the dominant source material of the soils in the studied area. The studied soils were classified according to the criteria established by the SiBCS (Santos et al., 2018) and the World Reference Base for Soil Resources (IUSS Working Group WRB, 2015).

Laboratory analyses and other determinations
The soil samples were oven-dried at 40 °C, ground, and sieved to separate the fractions with a diameter smaller than 2.0 mm. Undisturbed soil samples were collected from the studied horizons using volumetric metallic cylinders to determine the soil bulk density (BD).
Organic carbon (C org ), pH(H 2 O), selective dissolution of Al, Fe, and Si, and P retention were analyzed using the air-dried fine earth fraction (ADFE). Hydrogen potential was measured using a combined electrode immersed in soil:water suspension in the proportion of 1:2.5, while C org was quantified via muffle-drying by incineration, both according to methodologies described by Teixeira et al. (2017). The contents of Al, Fe, and Si in the soil were determined through selective dissolutions, being extracted by ammonium acid oxalate solution (Al o , Fe o , and Si o ), according to McKeague and Day (1966), described by Teixeira et al. (2017), and sodium pyrophosphate (Al p ), according to Bascomb (1968), described by Teixeira et al. (2017), using an optical emission spectrometer with ICP-OES plasma. Phosphorus retention was determined according to the methodology described by Van Reeuwijk (2002), adapted from Blakemore et al. (1987).

RESULTS
The lithologies, location of the soils in relation to the hillside, slope, altitude, and distance to the edge of the escarpment are shown in table 1. Except P7 and P8, which were developed on basalt and andesite / basalt, respectively, the other studied soils are on rhyodacites. Three pedons are below 1000 m altitude (P1, P2, and P3), two pedons (P11 and P13) are above 1500 m, and the others are within this range of altitudes. The pedons P1, P5, P7, and P14 are in places with a slope less than or equal to 10 % and have the greatest distances to the Serra Geral Formation's escarpment edge.
The physical and morphological properties of the 14 soil profiles surveyed are shown in table 2. According to the soil depth classes established by Santos et al. (2018), these soils can be classified as shallow (P13 and P14), fairly deep (P4, P10, and P12), and deep (P1, P2, P3, P5, P6, P7, P8, P9, and P11). The superficial horizons have a dark color and low BD ranging from 0.44 to 0.83 Mg m -3 (Table 4). The subsurface horizons are predominantly clayey with 100 % flocculation degree and at least incipient structure development, and with considerable variations in consistency: from slightly hard to extremely hard, very friable to firm, non-plastic to plastic, and non-sticky to sticky. Table 3 shows the chemical properties of the soils. They are chemically poor soils, with low nutrient reserves, from strong to extremely acidic, according to the definitions of the soil reaction classes (Santos et al., 2018), with very low base saturation, containing high levels of extractable aluminum and consequently with high aluminum saturation. The pH(H 2 O) of the soil samples varied between 4.1 and 5.3 and the pH(KCl) varied between 3.4 and 4.7, however, in most horizons the pH(H 2 O) was below 5.0. and the pH(KCl) varied between 3.4 and 4.7. Extremely low P assimilable and C org of surface horizons relatively high.  part of the criteria for andic properties according to IUSS Working Group WRB (2015).
There was a great variation in BD with very low values as in O1 and O2 of P4 with 0.44 and 0.54 kg dm -3 , respectively, up to higher values as in horizons A1 and A2 of P1 with 0.75 and 0.83 kg dm -3 , respectively. However, all surface horizons met the requirement of BD ≤0.9 Mg m -3 , required for andic properties. The other horizons were not analyzed to verify all the criteria because they no longer met this BD requirement, making it impossible to frame these horizons as soils with andic properties.
The requirements for Alo + 0.5Feo ≥2 % did not meet the horizons of P1, P5, P7, and P14. For the retention of P ≥85 %, the horizons A1 and A2 of P1, A1 of P5, O and A of P7, and H1 and H2 of P14 did not achieve the requirement. All surface horizons of the studied soils showed Si o values <0.6 % and Alp / Alo ratio ≥0.5, so the soils with andic properties were classified as aluandic, with a predominance of Al forming complexes with organic acids (

DISCUSSION
The studied soils presented structures with moderate to strong degree of development with a yellowish color in the subsurface horizons, were clayey, acidic, with low natural fertility, and had superficial horizons (O, H, and A) of dark coloration with relatively high C org values, low BD, and high P retention (Tables 2, 3, and 4).
According to the criteria of IUSS Working Group WRB (2015) and Santos et al. (2018), the horizons O histic of the soils these study have andic properties. The occurrence of andic properties are associated with the highland environment with high humidity and low temperatures, favored by a relatively fast weathering, in an acid environment, rich in organic material, associated with the formation of organometallic complexes, with a predominance of Al in these forms (as suggested by the low Si o values and the high Al p / Al o ratio in (1) According to the Manual de Descrição e coleta de solo no campo (Santos et al., 2013); (2) P assimilable; pH(H 2 O): pH in water-saturated soil paste (1:2.5); K + (Mehlich-1); Ca 2+ , Mg 2+ , and Al 3+ (KCl 1 mol L -1 ); H+Al (Calcium acetate 0.5 mol L -1 at pH 7.0); EB: exchangeable bases sum; T: potential cation exchange capacity (at pH 7.0); V: bases saturation; m: aluminum saturation; C org : organic carbon (Teixeira, 2017).
Rev Bras Cienc Solo 2021;45:e0200152 The pH(H 2 0) of the horizons with andic properties of the surveyed soils ranged from 4.3 to 5.2, with most samples with values ≤5.0. Considering that the formation and maintenance of allophanes occurs at pH(H 2 0) >4.9 (Shoji and Fujiwara, 1984), the environment of this study does not favor the occurrence of these minerals, but the formation of organometallic complexes with a predominance of Al complexed by organic acids, such as was evidenced by the high Al p / Al o ratio.
In non-allophanic andic soils, a large part of Al complexed with humus can be preferentially dissolved by sodium pyrophosphate; therefore, the Al p / Al o ratio is often used to identify these soils (Dümig et al., 2008). According to the criteria of Nanzyo et al. (1993), this ratio should be between 0.1-0.4 for allophanics and between 0.8-1.0 for non-allophanics. Aran et al. (2001) found non-allophanic andic soils in northeastern France, on ancient volcanic rocks, with a large accumulation of organic matter and low pH, and considered the Al p / Al o ratios >0.8 relatively high, also suggesting that Al was complexed mainly with organic compounds.
In soils with andic properties, aluminum protects the organic part of Al-humus complexes against biodegradation. These complexes have limited mobility and moderate solubility; (1) Brazilian Soil Classification System (Santos et al. 2018).
(2) World Reference Base for Soil Resources (IUSS Working Group WRB, 2015). and this combination promotes the accumulation of organic matter in the topsoil, culminating in the formation of a surface horizon with intense dark color and high content of organic matter (Driessen et al., 2001), giving to these soils high porosity of strongly developed aggregate structures, which is mainly responsible for the low BD of non-allophanic andic soils, and a high water retention capacity (Nanzyo, 2002;McDaniel et al., 2012).
The high phosphate retention in soils with andic properties occurs due to the large specific surface area and strong affinity of allophanes, imogolite, ferrihydrite and/or organometallic complexes with phosphorus (Parfitt, 1990;Nanzyo, 2002). Phosporous retention occurs through the formation of internal sphere complexes at sites of high and low affinity and precipitation of Al-phosphate minerals (McDaniel et al., 2012).
In summary, according to the criteria recently adopted in SiBCS (Santos et al., 2018), as suggested by Santos Junior (2017)     P12, and P13), all with a superficial diagnostic horizon O hystic, with andic properties, aluandic, while Cambissolos Húmicos (P1, P5, and P7) and Organossolo Háplicos (P14) did not meet one or more of the required criteria.

CONCLUSIONS
The Cambissolos Hísticos and Organossolos Fólicos that occur in a narrow band on the escarpment edges of the Serra Geral Formation in the southern Santa Catarina plateau and northeastern plateau of the state of Rio Grande do Sul, above 900 m altitude, have andic properties.
The cold climate and high cloudiness of these high-altitude areas favor the formation of a constantly humid environment and the acid weathering of the source material, accumulation of organic matter in the soil, and its stabilization by the formation of organo-metallic complexes, especially Al-humus. The combination of these factors gives the diagnostic histic horizons low density, high phosphate retention, and Al o + ½Fe o ≥2 % values, meeting the criteria required for andic properties.

ACKNOWLEDGMENTS
Our thanks to CAPES/FAPESC for granting the scholarship to the first author and to Embrapa Solo, for the financial contribution and for carrying out the soil characterization analyses of this study, through the SiBCS Project (PA 02.14.01.008.00.03).