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Magnetic Susceptibility of Soil to Differentiate Soil Environments in Southern Brazil

ABSTRACT

The interest in new techniques to support digital soil mapping (DSM) is increasing. Numerous studies pointed out that the measure of magnetic susceptibility (MS) can be extremely useful in the identification of properties related with factors and processes of soil formation, applied to soil mapping. This study addressed the effectiveness of magnetic soil susceptibility to identify and facilitate the distinction of different pedogenic environments of a representative hillslope in the highland Planalto Médio in the state of Rio Grande do Sul (RS), Brazil. In a 350-ha area in the municipality of Santo Augusto, RS, a representative transect was selected, trenches opened for soil characterization and 29 grid points marked at regular distances of 50 m, where soil samples were collected (layers 0.00-0.05, 0.05-0.15, 0.15-0.30, and 0.30-0.60 m) to analyze soil properties. Data from the transect samples were subjected to descriptive statistics. Limits of the pedogenetic environments along the slope were identified by the Split Moving Window (SMW) Boundary Analysis. The combined use of soil magnetic susceptibility and the SMW technique was effective in identifying different pedogenetic environments in the study area.

pedometrics; geomorphology; detailed mapping

INTRODUCTION

Magnetic susceptibility (MS) is a measure that indicates to which degree a material is magnetizable, and is directly related to the compounds of this material (Dearing, 1999Dearing JA. Environmental magnetic susceptibility: using the Bartington MS2 system. 2nd ed. Kenilworth: Chi Publishing; 1999.). Magnetic susceptibility measurements are used in studies of archaeology (Muniz, 2014Muniz MS. Forte de Rathnadrinna: arqueologia e implicações ambientais. Rev Arqueol Púb. 2014;9:111-22. doi:10.20396/rap.v8i1(9).8635682
https://doi.org/10.20396/rap.v8i1(9).863...
), paleomagnetism (Jiang et al., 2015Jiang Z, Liu Q, Dekkers MJ, Tauxe L, Qin H, Barrón V, Torrent J. Acquisition of chemical remanent magnetization during experimental ferrihydrite-hematite conversion in Earth-like magnetic field-implications for paleomagnetic studies of red beds. Earth Planet Sci Lett. 2015;428:1-10. doi:10.1016/j.epsl.2015.07.024
https://doi.org/10.1016/j.epsl.2015.07.0...
), as well as in the identification and description of pedogenetic environments of different locations, for example in China (Liu et al., 2013Liu Z, Liu Q, Torrent J, Barrón V, Hu P. Testing the magnetic proxy χFD/HIRMfor quantifying paleoprecipitation in modern soil profiles from Shaanxi Province, China. Global Planet Change. 2013;110:368-78. doi:10.1016/j.gloplacha.2013.04.013
https://doi.org/10.1016/j.gloplacha.2013...
), Europe (Torrent et al., 2010Torrent J, Liu QS, Barron V. Magnetic susceptibility changes in relation to pedogenesis in a Xeralf chronosequence in northwestern Spain. Eur J Soil Sci. 2010;61:161-73. doi:10.1111/j.1365-2389.2009.01216.x
https://doi.org/10.1111/j.1365-2389.2009...
; Jordanova et al., 2013Jordanova N, Jordanova D, Liu Q, Hu P, Petrov P, Petrovský E. Soil formation and mineralogy of a Rhodic Luvisol - insights from magnetic and geochemical studies. Global Planet Change. 2013;110:397-413. doi:10.1016/j.gloplacha.2013.08.020
https://doi.org/10.1016/j.gloplacha.2013...
), and in the different regions of Brazil: Northeast (Santos et al., 2011Santos HL, Marques Jr J, Matias SSR, Siqueira DS, Pereira GT. Suscetibilidade magnética na identificação de compartimentos da paisagem em uma vertente. Rev Bras Cienc Agrár. 2011;6:710-6. doi:10.5039/agraria.v6i4a1347
https://doi.org/10.5039/agraria.v6i4a134...
), North (Oliveira et al., 2015Oliveira IA, Marques Jr J, Campos MCC, Aquino RE, Freitas L, Siqueira DS, Cunha JM. Variabilidade espacial e densidade amostral da suscetibilidade magnética e dos atributos de Argissolos da Região de Manicoré, AM. Rev Bras Cienc Solo. 2015;39:668-81. doi:10.1590/01000683rbcs20140496
https://doi.org/10.1590/01000683rbcs2014...
), Southeast (Camargo et al., 2014Camargo LA, Marques Jr J, Pereira GT, Bahia ASRS. Clay mineralogy and magnetic susceptibility of Oxisols in geomorphic surfaces. Sci Agric. 2014;71:244-56. doi:10.1590/S0103-90162014000300010
https://doi.org/10.1590/S0103-9016201400...
; Siqueira et al., 2015Siqueira DS, Marques Jr J, Pereira GT, Teixeira DB, Vasconcelos V, Carvalho Júnior AO, Martins ES. Detailed mapping unit design based on soil-landscape relation and spatial variability of magnetic susceptibility and soil color. Catena. 2015;135:149-62. doi:10.1016/j.catena.2015.07.010
https://doi.org/10.1016/j.catena.2015.07...
), and South (Inda Junior et al., 2014Inda Junior AV, Tomasi CA, Oliveira JS, Fink JR. Óxidos de ferro e área superficial de Latossolo subtropical sob campo e floresta nativa. Cienc Rural. 2014;44:289-92. doi:10.1590/S0103-84782013005000153
https://doi.org/10.1590/S0103-8478201300...
).

In tropical soils, MS has been used to indicate the relationship between landscape and iron oxides, considered indicators of the factors and processes of soil formation (Camargo et al., 2014Camargo LA, Marques Jr J, Pereira GT, Bahia ASRS. Clay mineralogy and magnetic susceptibility of Oxisols in geomorphic surfaces. Sci Agric. 2014;71:244-56. doi:10.1590/S0103-90162014000300010
https://doi.org/10.1590/S0103-9016201400...
; Matias et al., 2014Matias SSR, Marques Jr J, Siqueira DS, Pereira GT. Outlining precision boundaries among areas with different variability standards using magnetic susceptibility and geomorphic surfaces. Eng Agríc. 2014;34:695-706. doi:10.1590/S0100-69162014000400009
https://doi.org/10.1590/S0100-6916201400...
, 2015Matias SSR, Marques Jr J, Pereira GT, Siqueira DS. Ferramentas matemáticas, suscetibilidade magnética e modelos de paisagem aplicados na delimitação de áreas de manejo específico. Rev Bras Cienc Solo. 2015;39:968-80. doi:10.1590/01000683rbcs20140638
https://doi.org/10.1590/01000683rbcs2014...
). For being directly related with iron oxides, MS is a covariate of soil mineralogical properties and consequently of different pedogenetic environments, here called landscape compartments (relief). According to Ab’Saber (1969)Ab’Saber AN. Um conceito de Geomorfologia a serviço das pesquisas sobre o Quaternário. São Paulo: Instituto de Geografia, USP; Departamento de Geomorfologia; 1969., geomorphological studies validate the compartmentalization of landscapes as a method to understand events that explain relief evolution and global and local transformations of the landscape itself.

Studies on the magnetic variability of Oxisols at locations with a total iron content between 4 and 13 % associated the variation in MS to magnetite derived from the source material and to ferromagnetic maghemite and ferrihydrite formed in different pedogenetic environments (Camargo et al., 2014Camargo LA, Marques Jr J, Pereira GT, Bahia ASRS. Clay mineralogy and magnetic susceptibility of Oxisols in geomorphic surfaces. Sci Agric. 2014;71:244-56. doi:10.1590/S0103-90162014000300010
https://doi.org/10.1590/S0103-9016201400...
). Along this same line, Marques Jr. et al. (2014)Marques Jr J, Siqueira DS, Camargo LA, Teixeira DDB, Barrón V, Torrent J. Magnetic susceptibility and diffuse reflectance spectroscopy to characterize the spatial variability of soil properties in a Brazilian Haplustalf. Geoderma. 2014;219:63-71. doi:10.1016/j.geoderma.2013.12.007
https://doi.org/10.1016/j.geoderma.2013....
analyzed a sandy Haplustalf with low total iron content (<4 %) and could accurately reproduce the spatial distribution of soil physical and chemical properties using MS. In both studies, MS was used to construct detailed maps (scale <1:10,000) in the southeastern region of Brazil.

The properties of taxonomically similar soil landscapes located in different landscape compartments can vary greatly. According to Matias et al. (2013)Matias SSR, Marques Jr J, Siqueira DS, Pereira GT. Modelos de paisagem e susceptibilidade magnética na identificação e caracterização do solo. Pesq Agropec Trop. 2013;43:93-103. and Camargo et al. (2014)Camargo LA, Marques Jr J, Pereira GT, Bahia ASRS. Clay mineralogy and magnetic susceptibility of Oxisols in geomorphic surfaces. Sci Agric. 2014;71:244-56. doi:10.1590/S0103-90162014000300010
https://doi.org/10.1590/S0103-9016201400...
, MS was efficient to define different pedogenetic environments in apparently homogeneous areas in southeastern Brazil. Magnetic susceptibility was also effective in landscape compartmentalization and identification of different pedogenic environments of Entisols and Ultisols in the Northeast of Brazil (Santos et al., 2011Santos HL, Marques Jr J, Matias SSR, Siqueira DS, Pereira GT. Suscetibilidade magnética na identificação de compartimentos da paisagem em uma vertente. Rev Bras Cienc Agrár. 2011;6:710-6. doi:10.5039/agraria.v6i4a1347
https://doi.org/10.5039/agraria.v6i4a134...
). Different mathematical approaches with magnetic data were used to characterize the variability of soils and their properties. Some authors applied geostatistics to determine the perimeter of pedogenetically different environments (Camargo et al., 2014Camargo LA, Marques Jr J, Pereira GT, Bahia ASRS. Clay mineralogy and magnetic susceptibility of Oxisols in geomorphic surfaces. Sci Agric. 2014;71:244-56. doi:10.1590/S0103-90162014000300010
https://doi.org/10.1590/S0103-9016201400...
; Marques Jr et al., 2014Marques Jr J, Siqueira DS, Camargo LA, Teixeira DDB, Barrón V, Torrent J. Magnetic susceptibility and diffuse reflectance spectroscopy to characterize the spatial variability of soil properties in a Brazilian Haplustalf. Geoderma. 2014;219:63-71. doi:10.1016/j.geoderma.2013.12.007
https://doi.org/10.1016/j.geoderma.2013....
). The “Split Moving Window” (SMW) boundary analysis has been used in cases of landscape compartmentalization and to identify pedogenetic limits in a single direction (Matias et al., 2014Matias SSR, Marques Jr J, Siqueira DS, Pereira GT. Outlining precision boundaries among areas with different variability standards using magnetic susceptibility and geomorphic surfaces. Eng Agríc. 2014;34:695-706. doi:10.1590/S0100-69162014000400009
https://doi.org/10.1590/S0100-6916201400...
; Siqueira et al., 2015Siqueira DS, Marques Jr J, Pereira GT, Teixeira DB, Vasconcelos V, Carvalho Júnior AO, Martins ES. Detailed mapping unit design based on soil-landscape relation and spatial variability of magnetic susceptibility and soil color. Catena. 2015;135:149-62. doi:10.1016/j.catena.2015.07.010
https://doi.org/10.1016/j.catena.2015.07...
).

Numerous studies showed potential applications of the MS technique in soil science studies to stratify the landscape in more homogeneous compartments, serving as an important variable for digital soil mapping (DSM) on a detailed scale. According to Dalmolin and Ten Caten (2015)Dalmolin RSD, Ten Caten A. Mapeamento digital: nova abordagem em levantamento de solos. Invest Agrár. 2015;17:77-86. doi:10.18004/investig.agrar.2015.diciembre.77-86
https://doi.org/10.18004/investig.agrar....
, the potential for the application of new technologies to improve the quality of DSM-generated information is huge. Furthermore, along with morphometry, proximal soil sensing and others, this technique renewed the interest in soil science (Hartemink, 2015Hartemink AE. The use of soil classification in jornal papers between 1975 and 2014. Geoderma Reg. 2015;5:127-39. doi:10.1016/j.geodrs.2015.05.002
https://doi.org/10.1016/j.geodrs.2015.05...
). In the region of the Planalto Médio, Rio Grande do Sul (RS), no detailed information on soils and little information from semidetailed surveys is available, covering less than 1 % of the area of this region.

Thus, in an attempt to generate data that can be used in pedotransfer functions in future DSM studies, and considering the potential of magnetic soil susceptibility, we established the following hypothesis: the landscape compartments, representing different pedogenetic environments, can be identified by the SMW technique based on MS data. This study tested the effectiveness of magnetic susceptibility of the soil to identify and facilitate the distinction of different pedogenetic environments in a representative hillslope of the Planalto Médio of Rio Grande do Sul.

MATERIALS AND METHODS

Location and description of the study area and sample design

The study was conducted in an experimental area of 350 ha located in the region of Planalto Médio of Rio Grande do Sul (Figure 1), which is part of the Serra Geral Formation, where basalt, andesite basalt, rhyodacite, and rhyolite spills are predominant. According to the Köppen classification system, the regional climate is Cfa - subtropical. The soils in the area were classified (Santos et al., 2013Santos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Lumbreras JF, Coelho MR, Almeida JA, Cunha TJF, Oliveira JB. Sistema brasileiro de classificação de solos. 3a ed. Brasília, DF: Embrapa Solos; 2013.) as Latossolo Vermelho-Amarelo Distrófico típico; Nitossolo Vermelho Eutrófico latossólico; Latossolo Vermelho Distrófico típico; and Latossolo Vermelho Eutrófico típico and as Haplic Ferralsol (Dystric, Clayic); Rhodic Eutric Nitisol (Ochric); Rhodic Ferralsol (Dystric, Clayic); Rhodic Ferralsol (Eutric, Clayic) respectively, by the WRB system (IUSS Working Group WRB, 2014IUSS Working Group WRB. World Reference Base for Soil Resources .2014. International soil classification system for naming soils and creating legends for soil maps. Rome: World Soil Resources; 2014. (Report FAO, 106).).

Figure 1
Location of the municipality of Santo Augusto (a); Location of the experimental area in Santo Augusto (b); experimental area with altitude contours and transect with studied profiles (c); elevation profile based on the transect with profile location (d). P1: Haplic Ferralsol (Dystric, Clayic); P2: Haplic Ferralsol (Dystric, Clayic); P3: Rhodic Eutric Nitisol (Ochric); P4: Rhodic Ferralsol (Dystric, Clayic); P5: Rhodic Ferralsol (Eutric, Clayic).

A transect was outlined with sampling points repeated along the flanks from the top downhill along the direction of the slightest slope (landscape spike). Twenty-nine grid points were marked in the transect at regular 50-m distances, resulting in a total of 87 sampling points and their replications. All points were georeferenced and trenches were dug at each of them for soil sampling in the layers 0.00-0.05, 0.05-0.15, 0.15-0.30, and 0.30-0.60 m, as proposed by the consortium GlobalSoilMap.net (2011)GlobalSoilMap.net. Specifications Version 1 GlobalSoilMap.net products. 2011. [accessed: June, 2013]. Available at: http://www.globalsoilmap.net/system/files/GlobalSoilMap_net_specifications_v2_0_edited_ draft _Sept_2011_RAM_V12.pdf.
http://www.globalsoilmap.net/system/file...
. For a better description of the area, five soil profiles were described and sampled (Santos et al., 2013Santos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Lumbreras JF, Coelho MR, Almeida JA, Cunha TJF, Oliveira JB. Sistema brasileiro de classificação de solos. 3a ed. Brasília, DF: Embrapa Solos; 2013.) at locations along the hillslope.

Laboratory tests

The samples were dried (air-dried fine soil - ADFS) and particle- size distribution analyzed as proposed by Donagema et al. (2011)Donagema GK, Campos DVB, Calderano SB, Teixeira WG, Viana JHM. Manual de métodos de análises de solos. 2a ed. Rio de Janeiro: Embrapa Solos; 2011.. The soil organic carbon content (SOC) was determined by wet combustion with external heating, according to Yeomans and Bremner (1988)Yeomans JC, Bremner JM. A rapid and precise method routine determination of organic carbon in soil. Commun Soil Sci Plant Anal. 1988;19:1467-76. doi:10.1080/00103628809368027
https://doi.org/10.1080/0010362880936802...
. In the samples of the representative profiles of the area, particle-size distribution was determined and chemical analysis carried out as described by Donagema et al. (2011)Donagema GK, Campos DVB, Calderano SB, Teixeira WG, Viana JHM. Manual de métodos de análises de solos. 2a ed. Rio de Janeiro: Embrapa Solos; 2011..

The free iron-oxide contents were extracted with dithionite-citrate-bicarbonate (DCB) solution (Mehra and Jackson, 1960Mehra OP, Jackson ML. Iron oxide removal from soils by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 1960;7:317-27. doi:10.1346/CCMN.1958.0070122
https://doi.org/10.1346/CCMN.1958.007012...
). The levels of low-crystallinity oxides were determined by extraction with ammonium (acid) oxalate solution (pH 3.0) in the dark, as proposed by McKeague and Day (1966)Mckeague JA, Day JH. Dithionite and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. J Soil Sci. 1966;46:13-22. doi:10.4141/cjss66-003
https://doi.org/10.4141/cjss66-003...
. Total iron contents were determined by high-temperature digestion (at ±170 °C) with sulfuric acid at a concentration of 1:1 (Donagema et al., 2011Donagema GK, Campos DVB, Calderano SB, Teixeira WG, Viana JHM. Manual de métodos de análises de solos. 2a ed. Rio de Janeiro: Embrapa Solos; 2011.). After preparing the samples from the surface (Ap) and subsurface (Bw1) horizons of each profile, the clay and fine sand fractions were mineralogically analyzed with a Philips PW 3710 diffractometer, equipped with a Cu tube anode. In the clay fraction samples, the Fe oxides were concentrated (procedure of Norrish and Taylor (1961)Norrish K, Taylor RM. The isomorphous replacement of iron by aluminium in soil goethites. J Soil Sci. 1961;12:294-306. doi:10.1111/j.1365-2389.1961.tb00919.x
https://doi.org/10.1111/j.1365-2389.1961...
modified by Kämpf and Schwertmann (1983)Kämpf N, Schwertmann U. Goethite and hematite in a climosequence in Southern Brazil and their application in classification of kaolinitic soils. Geoderma. 1983;29:27-39. doi:10.1016/0016-7061(83)90028-9
https://doi.org/10.1016/0016-7061(83)900...
and the iron extracted (Mehra and Jackson, 1960Mehra OP, Jackson ML. Iron oxide removal from soils by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 1960;7:317-27. doi:10.1346/CCMN.1958.0070122
https://doi.org/10.1346/CCMN.1958.007012...
).

Magnetic susceptibility was determined in 10 g ADFS from the sampling points and in the fractions clay, fine sand and ADFS from the profiles. For this, a Bartington Instruments Ltd MS2 magnetic susceptibility meter was linked to a MS2B dual frequency sensor, and read at low frequency (0.47 kHz) (Dearing, 1999Dearing JA. Environmental magnetic susceptibility: using the Bartington MS2 system. 2nd ed. Kenilworth: Chi Publishing; 1999.).

Statistical analysis

The MS data from 87 samples collected in the transect and the lateral replications were analyzed by descriptive statistics, calculating the mean, minimum and maximum values, variance, standard deviation (SD), and coefficient of variation (CV). The means of MS in different layers and compartments were compared by the Tukey test at 5 % probability, calculated with the statistical package R (R Core Team, 2015R Core Team. R: A language and environment for statistical computing. R Foundation for Statistical Computing. Vienna: 2015. Available at: https://www.R-project.org/.
https://www.R-project.org/...
). With the data from the soil profiles, MS of the clay fraction and the goethite, hematite and maghemite contents were correlated, by building regression models.

All data from the 87 sampling points of the properties sand, silt, clay, and organic carbon contents and MS were subjected to Split Moving Window (SMW) autocorrelogram analysis (Siqueira et al., 2015Siqueira DS, Marques Jr J, Pereira GT, Teixeira DB, Vasconcelos V, Carvalho Júnior AO, Martins ES. Detailed mapping unit design based on soil-landscape relation and spatial variability of magnetic susceptibility and soil color. Catena. 2015;135:149-62. doi:10.1016/j.catena.2015.07.010
https://doi.org/10.1016/j.catena.2015.07...
), to confirm the landscape compartments (Matias et al., 2015Matias SSR, Marques Jr J, Pereira GT, Siqueira DS. Ferramentas matemáticas, suscetibilidade magnética e modelos de paisagem aplicados na delimitação de áreas de manejo específico. Rev Bras Cienc Solo. 2015;39:968-80. doi:10.1590/01000683rbcs20140638
https://doi.org/10.1590/01000683rbcs2014...
; Siqueira et al., 2015Siqueira DS, Marques Jr J, Pereira GT, Teixeira DB, Vasconcelos V, Carvalho Júnior AO, Martins ES. Detailed mapping unit design based on soil-landscape relation and spatial variability of magnetic susceptibility and soil color. Catena. 2015;135:149-62. doi:10.1016/j.catena.2015.07.010
https://doi.org/10.1016/j.catena.2015.07...
). This evaluation is based on statistical calculations of dissimilarity between the sequences of sample points collected along the transect. In this analysis, a series of “n” points is selected and called window. This window is shifted, point by point, from the beginning to the end of the transect. At each window position, the selected points are divided into two parts, and the means between them are calculated and compared. Comparisons are performed using the t statistic and the results expressed in a distance graph. The window size is determined by autocorrelation analysis of the properties. Aside from the window size, the application of the Mullion index (van den Berg, 1988van den Berg M. Transect. For, listagem na língua Fortran. Piracicaba: ESALQ/USP, Departamento de Ciências do Solo; 1988.) also indicates the pre-set distance between two windows.

In this study, windows from seven observations were used and the Mullion index from one; these parameters were chosen by testing and validation of field observations. The peaks expressed in the chart indicate the boundaries between the different compartments and pedogenetic environments.

RESULTS AND DISCUSSION

The sand, silt and clay contents (Table 1) are distributed in the profiles along the hillslope with a slight increase in clay contents in the deeper layers of all profiles. Variations in soil properties are mainly expressions of the relief that affects water flows in the soil surface and subsurface (Anjos et al., 1998Anjos LH, Fernandes MR, Pereira MG, Franzmeier DP. Landscape and pedogenesis of an Oxisol-Inceptisol-Ultisol sequence in Southeastern Brazil. Soil Sci Soc Am J. 1998;62:1651-8. doi:10.2136/sssaj1998.03615995006200060024x
https://doi.org/10.2136/sssaj1998.036159...
).

Table 1
Soil morphological, physical and chemical properties of the five studied profiles

The silt/clay ratio varied from 0.3 to 0.4 in the surface layer and from 0.1 to 0.2 in the Bw horizon of the five profiles. Soil organic carbon (SOC) was highest in the surface horizons, decreasing in the deeper layers, as expected. The SOC levels were lowest in P5, located below the altimetric level of the other profiles.

The Fe content extracted with sulfuric acid (Fes) from the soils ranged from 164.1 g kg-1 at the top to 74.5 g kg-1 at the slope bottom. In profile 1, the Feo/Fed ratio varied from 0.06 in profile 4 to 0.19 in profile 1. The variation in the Feo/Fed ratio along the landscape showed that the most suitable environment for crystallization formation is at the insertion point of profile 4, as confirmed by XRD (Table 2 and Figure 2). The better crystallized these pedogenetic iron oxides, the more intense was the influence of pedogenic processes in this environment. The high Fed/Fes ratio indicates the occurrence of highly weathered soils, with values similar to those found by Inda Junior and Kämpf (2003)Inda Junior AV, Kämpf N. Avaliação de procedimentos de extrações dos óxidos de ferro pedogênicos com ditionito-citrito-bicarbonato de sódio (DCB). Rev Bras Cienc Solo. 2003;27:1139-47. doi:10.1590/S0100-06832003000600018
https://doi.org/10.1590/S0100-0683200300...
in B horizons of soils in different regions of Brazil, and by Dalmolin et al. (2006)Dalmolin RSD, Gonçalves CN, Dick DP, Knicker H, Klamt E, Kögel-Knabner I. Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil. Eur J Soil Sci. 2006;57:644-54. doi:10.1111/j.1365-2389.2005.00755.x
https://doi.org/10.1111/j.1365-2389.2005...
in Oxisols of the Planalto of Rio Grande do Sul.

Table 2
Structural characteristics of iron oxides and hydroxides (goethite, hematite and maghemite) in the clay fraction of the Ap and Bw1 horizons of the five studied profiles

Figure 2
X ray diffractograms of powder samples of the iron oxide concentration, showing the variation in the position of the reflections of goethite (Gt), hematite (Hm), quartz (Qt), and maghemite (Mgh) in the Ap and Bw1 horizons of the five studied profiles. P1: Haplic Ferralsol (Dystric, Clayic); P2: Haplic Ferralsol (Dystric, Clayic); P3: Rhodic Eutric Nitisol (Ochric); P4: Rhodic Ferralsol (Dystric, Clayic); P5: Rhodic Ferralsol (Eutric, Clayic).

The diffractograms (with concentration of iron oxides) of the horizons Ap and Bw1 of all soil profiles are in the figure 2.

The soils of the studied area developed from volcanic rock with a high weathering degree and are found in topographic positions that favor the predominance of hematite (Kämpf and Schwertmann, 1983Kämpf N, Schwertmann U. Goethite and hematite in a climosequence in Southern Brazil and their application in classification of kaolinitic soils. Geoderma. 1983;29:27-39. doi:10.1016/0016-7061(83)90028-9
https://doi.org/10.1016/0016-7061(83)900...
; Fernandes et al., 2004Fernandes RBA, Barrón V, Torrent J, Fontes MPF. Quantificação de óxidos de ferro de Latossolos brasileiros por espectroscopia de refletância difusa. Rev Bras Cienc Solo. 2004;28:245-57. doi:10.1590/S0100-06832004000200003
https://doi.org/10.1590/S0100-0683200400...
).

The characteristics of iron oxide and hydroxide structures (goethite, hematite and maghemite) (Table 2) indicate that the iron oxyhydroxides were better represented at lower altitudes; goethite was predominant in profile 3, hematite in profile 4 and maghemite in profile 5. The results showed that in the environment of profile 3 the conditions favor pedogenesis of goethite, while in the environment of profiles 4 and 5, conditions are conducive to pedogenesis of hematite and maghemite. Highest goethite and hematite levels were observed in the surface, and higher maghemite levels in the subsurface horizons. This behavior may be associated with different SOC levels, as stated by Dalmolin et al. (2006)Dalmolin RSD, Gonçalves CN, Dick DP, Knicker H, Klamt E, Kögel-Knabner I. Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil. Eur J Soil Sci. 2006;57:644-54. doi:10.1111/j.1365-2389.2005.00755.x
https://doi.org/10.1111/j.1365-2389.2005...
.

The diffractograms of the iron-free clay fraction (Figure 3) identified very marked reflections at 0.715 nm, 0.448 nm and 0.357 nm, corresponding to the clay mineral kaolinite and weaker reflections of gibbsite and quartz at 0.485 nm and 0.334 nm, respectively. The kaolinite contents in the soil decreased from the hilltop to the valley bottom, with predominance of poorly crystallized kaolinite in the environment of profiles 4 and 5. This behavior was most likely influenced by the landscape position that induces different redox conditions, and a better environment for iron oxide crystallization (Siqueira et al., 2015Siqueira DS, Marques Jr J, Pereira GT, Teixeira DB, Vasconcelos V, Carvalho Júnior AO, Martins ES. Detailed mapping unit design based on soil-landscape relation and spatial variability of magnetic susceptibility and soil color. Catena. 2015;135:149-62. doi:10.1016/j.catena.2015.07.010
https://doi.org/10.1016/j.catena.2015.07...
). In this environment (profile 4) the iron content extracted with sulfuric acid reached 164.1 g kg-1, consequently hampering kaolinite crystallization.

Figure 3
X ray diffractograms patterns of powder of iron-free samples, showing the variation in the position of reflections of kaolinite (Ct), gibbsite (Gb), quartz (Qt) and 2: 1 HE in the Ap and Bw1 horizons of the five profiles. P1: Haplic Ferralsol (Dystric, Clayic); P2: Haplic Ferralsol (Dystric, Clayic); P3: Rhodic Eutric Nitisol (Ochric); P4: Rhodic Ferralsol (Dystric, Clayic); P5: Rhodic Ferralsol (Eutric, Clayic).

Correlations of MS measured in the clay fraction with goethite (0.52; p<0.01), hematite (0.36; p<0.01) and maghemite contents (0.38; p<0.01) were found (data not shown in the table). Although goethite and hematite are antiferromagnetic minerals with low magnetism, they are present in the formation of other ferromagnetic minerals contained in the clay fraction, e.g., ferromagnetic maghemite and ferrihydrite (Barrón et al., 2000Barrón V, Mello JWV, Torrent J. Caracterização de óxidos de ferro em solos por espectroscopia de reflectância difusa. Tópicos Ci Solo. 2000;1:139-62.; Michel et al., 2010)Michel FM, Barrón VJ, Torrent J, Morales MP, Serna CJ, Boilye JF, Liu QS, Ambrosinig A, Cismasu AC, Brown GE. Ordered ferrimagnetic form of ferrihydrite reveals links between structure, composition and magnetism. Proc Nat Acad Sci USA. 2010;107:2787-92. doi:10.1073/pnas.0910170107
https://doi.org/10.1073/pnas.0910170107...
. According to Bahia et al. (2015)Bahia ASRS, Marques Jr J, Siqueira DS. Procedures using diffuse reflectance spectroscopy for estimating hematite and goethite in Oxisols of São Paulo, Brazil. Geoderma. 2015;5:150-6. doi:10.1016/j.geodrs.2015.04.006
https://doi.org/10.1016/j.geodrs.2015.04...
, the estimation of soil minerals by MS in basaltic soil is ideally improved by the construction of calibration curves after selective dissolution, to reduce possible interferences of lithogenic minerals such as magnetite.

The MS values were highest in the clay fraction of profile 1 (Figure 4), where the maghemite levels were highest (Table 2). This is in line with Fontes et al. (2000)Fontes MPF, Oliveira TS, Costa LM, Campos AAG. Magnetic separation and evaluation of magnetization of Brazilian soils from different parent materials. Geoderma. 2000;96:81-99. doi:10.1016/S0016-7061(00)00005-7
https://doi.org/10.1016/S0016-7061(00)00...
, who emphasized that magnetism is most evident in clayey soils. Among the ferromagnetic minerals, magnetite is most easily found in coarser soil fractions, such as sand and silt, and maghemite in finer fractions, such as clay. The MS values of the clay fraction in the soil profiles indicated a decrease in deeper layers throughout the profiles (Figure 4). This behavior may indicate different water flows according to the landscape position, carrying minerals with magnetic expression contained mainly in the clay fraction, e.g., magnetic maghemite and ferrihydrite, which influence MS. Another explanation is based on the age of soils in the landscape. Older soils are associated with more intense pedogenesis, favoring the formation of pedogenic minerals with magnetic expression. The magnetic susceptibility of profile 1 confirms that this environment is the oldest, since the values are homogeneous in the deeper layers, unlike in profile 5.

Figure 4
Magnetic susceptibility (MS) of the clay, fine sand and air-dried fine soil (ADFS) measured at low frequency for each layer and profile (P). P1: Haplic Ferralsol (Dystric, Clayic); P2: Haplic Ferralsol (Dystric, Clayic); P3: Rhodic Eutric Nitisol (Ochric); P4: Rhodic Ferralsol (Dystric, Clayic); P5: Rhodic Ferralsol (Eutric, Clayic).

In the profiles 2 and 5, the MS values were lowest in the clay and ADFS fraction, due to the poor drainage of these profiles, impairing the formation of ferromagnetic minerals such as ferromagnetic maghemite and ferrihydrite in the soils (Souza Jr et al., 2010Souza Jr IGS, Costa ACS, Vilar CC, Hoepers A. Mineralogia e susceptibilidade magnética dos óxidos de ferro do horizonte B de solos do Estado do Paraná. Cienc Rural. 2010;40:513-9. doi:10.1590/S0103-84782010000300003
https://doi.org/10.1590/S0103-8478201000...
). The variation in MS among the profiles in the clay fraction was directly related with the Fes content (Table 1), in agreement with the findings of Matias et al. (2013)Matias SSR, Marques Jr J, Siqueira DS, Pereira GT. Modelos de paisagem e susceptibilidade magnética na identificação e caracterização do solo. Pesq Agropec Trop. 2013;43:93-103. and Bahia et al. (2015)Bahia ASRS, Marques Jr J, Siqueira DS. Procedures using diffuse reflectance spectroscopy for estimating hematite and goethite in Oxisols of São Paulo, Brazil. Geoderma. 2015;5:150-6. doi:10.1016/j.geodrs.2015.04.006
https://doi.org/10.1016/j.geodrs.2015.04...
.

The MS variations in fine sand were similar to those in the clay fraction, but with higher relative quantities, mainly in profile 3 (Figure 4), on the mid-slope, with medium declivity, which is conducive to the transport of minerals in the clay fraction and higher mineral deposition in the sand fraction. According to Fontes et al. (2000)Fontes MPF, Oliveira TS, Costa LM, Campos AAG. Magnetic separation and evaluation of magnetization of Brazilian soils from different parent materials. Geoderma. 2000;96:81-99. doi:10.1016/S0016-7061(00)00005-7
https://doi.org/10.1016/S0016-7061(00)00...
, higher MS values in the sand fraction indicate the presence of ferromagnetic lithogenic minerals, probably magnetite. In profile 2 and 4, MS was lowest in the sand fraction, which may be associated with the position of these profiles in the mid-slope of the transect, a position that favors the loss of fine fractions (clay and silt) and sand accumulation, which may be related to the presence of (diamagnetic) quartz (Dearing, 1999Dearing JA. Environmental magnetic susceptibility: using the Bartington MS2 system. 2nd ed. Kenilworth: Chi Publishing; 1999.).

The systematic analysis of the SMW autocorrelogram (Figure 5) was sensitive in relation to the limits of the different segments along the transect. The reflections were most marked in the 0.30-0.60 m layer, comprising the most stable pedogenetic soil horizons, but some peaks in the surface layer coincided with the peaks of the deepest layer. Findings of Matias et al. (2014)Matias SSR, Marques Jr J, Siqueira DS, Pereira GT. Outlining precision boundaries among areas with different variability standards using magnetic susceptibility and geomorphic surfaces. Eng Agríc. 2014;34:695-706. doi:10.1590/S0100-69162014000400009
https://doi.org/10.1590/S0100-6916201400...
, based on the same MS analysis procedure, indicated more pronounced peaks in the 0.30-0.80 m layer.

Figure 5
Elevation profile of the transect (a) and the results of the “Split Moving Window” (SMW) analysis and correlation between the t-statistic values for magnetic susceptibility in the layers 0.00-0.05 m (b), 0.05-0.15 m (c), 0.15-0.30 m (d), and 0.30-0.60 m (e) along the transect with points spaced 50 m apart. The peaks above the dotted line (...) indicate significance and the dashed line (---) indicates the peaks of magnetic susceptibility. C: Landscape compartment. P1: Haplic Ferralsol (Dystric, Clayic); P2: Haplic Ferralsol (Dystric, Clayic); P3: Rhodic Eutric Nitisol (Ochric); P4: Rhodic Ferralsol (Dystric, Clayic); P5: Rhodic Ferralsol (Eutric, Clayic).

The identification of the limits of landscape compartments was validated by the peaks of the SMW analysis. Three peaks were found in all layers (at 280 m, 430 m and 985 m below the top of the landscape), delimiting four compartments (Figure 5). These results agree with those of Siqueira et al. (2015)Siqueira DS, Marques Jr J, Pereira GT, Teixeira DB, Vasconcelos V, Carvalho Júnior AO, Martins ES. Detailed mapping unit design based on soil-landscape relation and spatial variability of magnetic susceptibility and soil color. Catena. 2015;135:149-62. doi:10.1016/j.catena.2015.07.010
https://doi.org/10.1016/j.catena.2015.07...
, who used SMW analysis to separate landscape compartments in places considered homogeneous for having the same soil type. In their studies, Campos et al. (2012)Campos MCC, Marques Jr J, Souza ZM, Siqueira DS, Pereira TP. Discrimination of geomorphic surfaces with multivariate analysis of soil attributes in sandstone - basalt lithosequence. Rev Cienc Agron. 2012;43:429-38. doi:10.1590/S1806-66902012000300003
https://doi.org/10.1590/S1806-6690201200...
found that soils in different taxonomic classes can have similar properties if they belong to the same landscape compartment, which may contribute to increase the errors in the mapping units.

The MS values ranged from 19.3 (C2) to 47.7 10-6 m3 kg-1 (C3) (Table 3), very close to the results of Bahia et al. (2015)Bahia ASRS, Marques Jr J, Siqueira DS. Procedures using diffuse reflectance spectroscopy for estimating hematite and goethite in Oxisols of São Paulo, Brazil. Geoderma. 2015;5:150-6. doi:10.1016/j.geodrs.2015.04.006
https://doi.org/10.1016/j.geodrs.2015.04...
for soils derived from basalt (10-45 10-6 m3 kg-1). The coefficients of variation (CV 20.38-23.22 %) were highest in C2, in agreement with results of Montanari et al. (2005)Montanari R, Marques Jr J, Pereira GT, Souza ZM. Forma da paisagem como critério para otimização amostral de Latossolos sob cultivo de cana-de-açúcar. Pesq Agropec Bras. 2005;40:69-77. doi:10.1590/S0100-204X2005000100010
https://doi.org/10.1590/S0100-204X200500...
, who reported greater variability of soil properties in concave topography.

Table 3
Descriptive statistics magnetic susceptibility in the four compartments

In the construction of detailed mapping protocols based on mathematical models such as SMW, geostatistics or cluster analysis, the “internal error” of the defined mapping units can be reduced, since the soil factors and formation processes are more homogeneous within than among the compartments.

The protocol of landscape compartmentalization using SMW of MS is considered an effective indicator of soil environments, representing a contribution to the mapping of specific management areas, detailed soil surveys and the understanding of the variability of soil properties (Campos et al., 2007Campos MCC, Marques Jr J, Pereira GT, Montanari R, Camargo LA. Relações solo-paisagem em uma litossequência arenito-basalto na região de Pereira Barreto-SP. Rev Bras Cienc Solo. 2007;31:519-29. doi:10.1590/S0100-06832007000300012
https://doi.org/10.1590/S0100-0683200700...
; Cortez et al., 2011Cortez LA, Marques Jr J, Peluco RG, Teixeira DB, Silveira DS. Susceptibilidade magnética para identificação de áreas de manejo específico em citricultura. Rev Energia Agric. 2011;26:60-79. doi:10.17224/EnergAgric.2011v26n3p60-79
https://doi.org/10.17224/EnergAgric.2011...
; Barrios et al., 2012Barrios MR, Marques Jr J, Panosso AR, Siqueira DS, La Scala Jr N. Magnetic susceptibility to identify landscape segments on a detailed scale in the region of Jaboticabal, São Paulo, Brazil. Rev Bras Cienc Solo. 2012;36:1073-82. doi:10.1590/S0100-06832012000400002
https://doi.org/10.1590/S0100-0683201200...
). The results of this study will be useful in future research using predictive models of soil properties and extrapolation to neighboring areas by DSM.

CONCLUSIONS

The mineralogical description of the soil profiles showed that the variations in iron oxide forms and contents are related to the profile position in the hillslope.

The limits indicated by SMW analysis based on MS values were effective to outline representative compartments of pedogenetic environments on a hillslope without geological transition, on the Planalto Médio of Rio Grande do Sul.

ACKNOWLEDGMENTS

The first and the second authors would like to thank the Brazilian Council for Scientific and Technological Development (CNPq) for the scholarship. The authors are grateful to the CNPq for the financial support of the research project.

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Publication Dates

  • Publication in this collection
    2017

History

  • Received
    7 Apr 2016
  • Accepted
    15 July 2016
Sociedade Brasileira de Ciência do Solo Secretaria Executiva , Caixa Postal 231, 36570-000 Viçosa MG Brasil, Tel.: (55 31) 3899 2471 - Viçosa - MG - Brazil
E-mail: sbcs@ufv.br