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Soil formation in loess-derived soils along a subhumid to humid climate gradient, Northeastern Iran
Geoderma 179–180 (2012) 113–122
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Soil formation in loess-derived soils along a subhumid to humid climate gradient, Northeastern Iran Farhad Khormali a,⁎, Shadi Ghergherechi a, Martin Kehl b, Shamsollah Ayoubi c a b c
Department of Soil Science, Faculty of Water and Soil Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran Institutes for Geography, University of Cologne, Albertus-Magnus-Platz, 50932 Cologne, Germany Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2011 Received in revised form 2 February 2012 Accepted 4 February 2012 Available online 22 March 2012 Keywords: Micromorphology Loess Clay illuviation Decalcification Climate gradient
a b s t r a c t In order to contribute to the understanding of carbonate enrichment and clay illuviation in loess-derived soils of subhumid to humid regions, the development of soils was studied along a climate gradient with xeric and udic soil moisture regimes (SMR) and thermic and mesic soil temperature regimes (STR), respectively, in the Golestan Province, Northeastern Iran. Six representative pedons along a climate gradient were investigated. Soils were classified mainly as Hapludalfs and Haploxeralfs. Stability of the geomorphic surface under forest vegetation associated with high leaching conditions has provided appropriate conditions for decalcification followed by clay migration through the profile and formation of argillic horizons in all the studied soils. Clay content of the Bt horizons, soil organic carbon concentration of the A horizons, and depth of the Bk horizons increased significantly with increasing precipitation and decreasing temperature. There was a considerable decrease in silt content with soil development. The main pedofeatures observed in the Bt horizons were clay coatings and decalcified zones. Nodules, coatings and hypocoatings were the main calcitic pedofeatures observed in the Bk horizons. Occurrence and preservation of clay coatings were more pronounced in the udic regions with illite and vermiculite as the dominant clay minerals. Type of clay minerals, shrink/swell properties, and precipitation rate are factors affecting the abundance and preservation of clay coatings. In the strongly developed horizons of the udic SMR, the occurrence of vermiculite clay minerals could reduce the shrink/swell potential and increase the amount of clay coatings. The presence of crystallitic b-fabrics and the high carbonate contents (CaCO3) in the lower horizons (Bk) were mainly related to decalcification processes under descending water flow in the overlying horizons. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Most of the physico-chemical properties of the modern soils are related to climate. Clay movement and illuviation are the major processes occurring in subhumid and humid loess derived soils. Clay increase and orientation, however, cannot always be attributed to a single genetic process. Illuviation, in-situ weathering of mica or feldspar, and shrink/swell activity of clay minerals affect the orientation and content of clay particles (Mermut and Pape, 1971; Ransom and Bidwell, 1990). The results of the study along a loess climosequence in Northern Iran showed that with increasing precipitation soil pH and calcium carbonate contents decreased, whereas soil organic carbon, clay content, and cation exchange capacity increased. From north to south representing a precipitation gradient from arid to humid, the relative proportion of smectite increases reaching almost dominance
⁎ Corresponding author. Tel.: + 98 171 4426523; fax: + 98 171 4420981. E-mail addresses: [email protected], [email protected] (F. Khormali). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2012.02.002
in regions with typic xeric soil moisture regime. In the more humid areas vermiculite is dominant (Khormali and Kehl, 2011). The degree of argillic horizon development strongly depends on the amount of calcium carbonate in parent material. Since Ca 2 + facilitates the flocculation of clay particles, leaching of calcium carbonate is often taken as a prerequisite for dispersion and vertical translocation of clay particles in soil (Fanning and Fanning, 1989). Argillic horizons in calcareous soils of arid to semiarid regions characterized by limited leaching may indicate periods of more humid climatic conditions in the past. Pietsch and Kuhn (2009) reported that calcium carbonate found at a depth of up to 30 cm in most of the layered soils is very typical of an increasing aridity in parts of the semiarid tropics, particularly in areas with high groundwater levels and interflow. In fact, clay pedofeatures in the presence of calcium carbonate (CaCO3) are common in soils of arid and semiarid climates (Abtahi, 1977; Pal et al., 1994, and Sehgal et al., 1975) and often related to a fluctuation in climate (Emadi et al., 2008; Gile, 1975; Reheis, 1987; Reynders, 1972). Khormali et al. (2003) illustrated that decalcification was the dominant process in the evolution of calcareous soils with argillic horizons in semiarid southern Iran and related both
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decalcification and clay illuviation to a former period of increased precipitation. On the other hand, Holliday (1985) indicated that abundant calcium carbonate may prevent argillic horizon development, but if pores are available and precipitation is high the clay particles can be translocated into the lower horizons of the soil, despite the tendency of clay to flocculate in the presence of Ca2+ ions. Available soil moisture, soil temperature, soil texture, vegetation and availability of calcium carbonate are among the main factors determining the type and morphology of calcitic features (Khormali et al., 2006). Size and frequency of microcrystalline calcite nodules and coatings increase from aridic to xeric, but decrease again towards the areas with an ustic moisture regime. Cytomorphic and acicular calcite are observed mainly in areas with an ustic moisture regime and a denser vegetative growth, but rarely in xeric areas. Layered pendants of calcite are common beneath coarser fragments, but are considered as relicts of a more humid paleoclimate. Calcite depletion pedofeatures are observed in xeric–mesic areas with higher available soil moisture. Several researchers reported that clay coatings in soils of arid and semiarid regions may be destroyed by shrink/swell activity upon wetting and drying, by root activity, or by calcium carbonate crystallization (Blanco and Stoops, 2007; Gunal and Ransom, 2006a, 2006b; Kemp and Zarate, 2000; Khormali et al., 2003; Mermut and Arnaud, 1981; Nettleton et al., 1969; Rostad et al., 1976; Srivastava et al., 2002; Verheye and Stoops, 1973). The fragments of clay coatings (papules), visible under the optical microscope, testify to this destruction process. Hopkins and Franzen (2003) observed a strong relationship between the depths of clay accumulation and clay content. They demonstrated that thicker clay coatings occur in soils with deeper argillic horizons. In argillic horizons of loess-derived soils of southern Colorado, thick and continuous clay coatings were associated with parallel striated, porostriated, and granostriated b-fabrics (Nettleton et al., 1990). However, the occurrence of thick, laminated and continuous clay coatings along with granostriated b-fabric was related to stress in the micromass caused by high shrink/swell activity (Gunal and Ransom, 2006a, 2006b). Shohet (2008) proposed a simple method for estimating the horizontal effective stress in clay soils that has a particular relevance with respect to the effects of moisture abstraction on shrinkable clay soils by vegetation, or simply by seasonal variations. Hurst (1977) suggested several physicochemical and mineralogical indices to evaluate argillic horizon development such as free Fe2O3, the color indices, the clay illuviation index, and the smectite/(chlorite + illite) ratio. To find a relationship between age and clay accumulation in argillic horizons, Levine and Ciolkosz (1983) suggested a clay accumulation index. This index showed a favorable correlation in terms of soil development in soils of northern Pennsylvania. Several researches have carried out quantitative analysis on the degree of argillic horizon development, using micromorphological characteristics, such as MISODI and MISECA index (Khormali et al., 2003; Magaldi and Tallini, 2000). In these indices, each feature was quantified by the application of a simple rating expressing its degree of development. The usefulness of the micromorphological indices depends on the type of characteristics used. The aims of the present study were to: 1) document the variation in soil properties across a climatic gradient; 2) elucidate the soil forming processes resulting in clay coatings in the presence of carbonates and 3) study the micromorphological properties of soil horizons in loess derived soils in humid and sub-humid regions of northern Iran, in order to establish the stage of soil development. 2. Materials and methods 2.1. Site setting The area studied is located in northern Iran between 36° 30′ and 37° 30′N latitude and 54° 15′ and 56° 00′E longitude, Golestan
province (Fig. 1). Six pedons, located in the foreland and foothill zone of Alborz Mountains, were selected at xeric–thermic (pedons 1, 2, and 3) and udic–mesic (pedons 4, 5, and 6) SMR-STR along a precipitation and temperature gradient ranging from 580 mm to 900 mm and 13.8 °C to 17 °C, respectively (Table 1). According to the De-Martine method (Mather, 1974), the climate in the study area ranges from sub-humid to humid. Precipitation occurs dominantly as rain. The mean annual reference crop evapotranspiration is between 900 and 1150 mm (Table 1). The parent material of all the studied soils in the area is loess. According to pedostratigraphic correlation and results of luminescence age estimates, the uppermost layers of the loess deposits covering the Alborz Mountain foothills accumulated during the last glacial under dry and probably colder climatic conditions than today (Kehl et al., 2005). Samples from loess layers immediately below the modern soil gave IRSL ages of 17900 ± 1800 years before present (ybp) to 20500 ± 1800 ybp (Fig. 3, Fig. 6 in Frechen et al., 2009). The formation of the modern soil did not start before dust accumulation was considerably reduced after a change to more humid and warmer climatic conditions during the Late Pleistocene glacial or Early Holocene. Although the onset of soil formation cannot yet be determined precisely, it can be assumed that the duration of soil development was comparable for all soils studied here given the close proximity of the selected pedons. The elevation ranges between 215 m and 584 m above sea level (Table 1). All soils were well drained and soil formation was not influenced by groundwater. The natural vegetation is characterized by large trees consisting mainly of Fagus orientalis and Quercus castaneafolia getting denser with increasing rainfall. 2.2. Sampling and analyses Soil color was determined using a Munsell Soil Color Chart. The soils were classified according to Soil Taxonomy (Soil Survey Staff, 2010). Six representative pedons with argillic horizons were recognized using the methodology outlined by Soil Survey Staff (1999) and the characteristics of each pedon and its individual horizons were described. After dissolution of CaCO3 with 2 N HCl, decomposition of organic matter with 30% H2O2 and removing salts by washing with distilled water, samples were prepared for particle size analyses with the hydrometer method (Bouyoucos, 1962). Soil pH was measured in a saturated paste (Salinity Laboratory Staff, 1954). Organic carbon (OC) was determined by wet oxidation with chromic acid and back titration with ferrous ammonium sulphate (Nelson and Sommers, 1982). Electrical conductivity (EC) of the soil was measured in 1:2.5 soil:water suspension using a Digital Conductivity Meter-CC 601 (Jackson, 1975). Cation exchange capacity (CEC) was determined using sodium acetate (NaOAc) at pH 8.2 (Chapman, 1965). The calcium carbonate equivalent (CCE) was determined by acid neutralization (Salinity Laboratory Staff, 1954). Bulk density and water saturation percentage (SP) values were measured using the procedure described by Soil Survey Staff (1999). Clay fractions were separated based on methodology outlined by Kittrick and Hope (1963) and Jackson (1975). Iron- and carbonatefree samples were centrifuged at 750 rpm for 5.4 min to separate total clay (b2 mm). The same concentration of clay suspensions was used for all samples to give reliable comparisons between relative peak intensities. Clay minerals were estimated semi-quantitatively from the relative x-ray peak areas of glycol-treated samples (Johns et al., 1954). For micromorphological analysis, 3 undisturbed samples were collected from each soil horizon (total of 72) with modified Kubiena tins and thin sections of about 60 and 40 cm 2 were prepared from air-dried, undisturbed clods using standard techniques described by Murphy (1986). Micromorphological descriptions were made according to Bullock et al. (1985) and Stoops (2003). The MISECA index was used to evaluate the degree of argillic horizon development (Khormali et al., 2003). Argillic horizons according to this index can
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Fig. 1. Location map of the study area and the pedons studied.
be classified from weakly to well-developed stages. Micromorphological criteria for the degree of soil development, based on MISECA index, are microstructure, b-Fabric, clay coating, decalcified zone, Fe/Mn oxides and alteration degree of silicate minerals. With increasing degree of soil development, the MISECA values can range between 0 and 24. 3. Results and discussion 3.1. Morphological and physicochemical properties Based on the WRB classification system, most of the studied soils are classified as Luvisols and one as Chernozems (WRB, 2006). According to Soil Taxonomy (Soil Survey Staff, 2010), the strongly developed soils of the udic–mesic SMR-STR are classified as Hapludalfs and those of the xeric–thermic SMR-STR as Haploxeralfs and Argixerolls (Table 2). All of the pedons show argillic horizons overlying calcic horizons. Relatively high degree of leaching, stability of the geomorphic surface, dense vegetation, and favorable drainage have provided appropriate conditions for translocation of clay through the profile and the formation of argillic horizon. Munsell colors vary from yellowish brown (10YR4/6) to yellowish red (5YR4/6) when moist, and a distinct difference in color was observed between the
argillic horizons and surface horizons (A) (10YR in surface horizons vs. 7.5YR or 5YR in argillic horizons) (Table 2). This phenomenon can be attributed to the increase in iron oxides and illuviation of clay in the argillic horizons. Argillic horizons show angular to subangular blocky structure and are slightly hard to hard (dry) with friable consistence (moist) in all horizons. Samples from A and Bt horizons did not effervesce with 10% HCl but the effervescence was strong in the Bk and C horizons. Irregularly shaped nodules of calcium carbonate were observed in the Bk horizons. The pores varied from fine to very fine in size and common to many in abundance. Clay pedofeatures and calcium carbonate depletion zones were common in the soils studied and are very pronounced in the udic SMR which is in accordance with the findings of Reheis (1987). The specific environmental condition resulting from the relatively high rainfall (P/ETO > 0.6), dense vegetation (Wright, 1987), high soil CO2 concentration, higher available soil moisture (Treadwell-Steitz and McFadden, 2000), and well-drained conditions (Khormali and Ajami, 2011) facilitated carbonate removal from the upper horizon and translocation and precipitation to lower horizons. According to Hopkins and Franzen (2003), a considerable increase in the amount of total clay in deeper argillic horizons corresponds to a strongly developed soil.
Table 1 Climatological data and site characteristics of the studied regions. Pedon
Location UTM (Zone 40)
Elv m
P mm
T °C
SMR-STR
ETO mm
P/ETo
Climate*
1 2 3 4 5 6
371113 341544 263063 347175 264567 263280
526 310 215 500 320 584
580 610 660 700 810 900
17 17 16.6 13.9 14 13.8
xeric–thermic xeric–thermic xeric–thermic udic–mesic udic–mesic udic–mesic
1130 1150 1100 900 941 910
0.56 0.56 0.62 0.78 0.86 0.99
subhumid subhumid subhumid humid humid humid
4137523 4108820 4075847 4097687 4072412 4068386
Elv: Elevation Above Sea Level; P: Mean Annual Precipitation; T: Mean Annual Temperature; SMR-STR: Soil Moisture and Temperature Regime; ETO: Mean Reference Crop Evapotranspiration; P/ETO = the ratio of Mean Annual Precipitation to Mean Reference Crop Evapotranspiration; *Based on De- Martine Method (Mather, 1974).
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Table 2 Morphological and physicochemical characteristics of the selected pedons. Pedon
Morphological properties
Horizon
Depth (cm)
Color (moist)
Physico-chemical properties Calcite nodules
pH
OC (%)
SP (%)
CCE (%)
CEC cmolkg− 1
Particle size distribution % Clay
Silt
Sand
– – c2sc f1sc
7.3 7.3 7.4 7.5
2.0 1.1 0.4 0.2
55 54 45 44
6 8 19 21
30 33 26 20
32 40 39 23
55 53 50 65
13 7 11 13
P2. Calcic Argixeroll (Luvi-calcic Chernozem (siltic)) A 0–30 10YR2/2 3c sbk Bt 30–60 7.5YR4/4 2 m abk Bk1 60–105 10YR6/4 2 m abk Bk2 105–145 10YR6/4 1f abk C 145–190 10YR6/5 m
– – c2sc m2sc –
7.0 7.5 7.9 7.9 7.8
2.1 1.1 0.2 0.2 0.1
57 60 44 46 46
8 10 30 33 28
35 32 21 20 20
33 42 26 26 26
49 50 61 61 63
18 8 13 13 11
P3. Calcic Haploxeralf ( Calcic A 0–10 Bt 10–50 Bk1 50–80 Bk2 80–107 Ck 107–150
1fabk 2mabk 2mabk 1mabk m
– – c2sc c2sc f1sc
6.9 7.0 7.8 7.8 7.7
2.6 1.0 0.5 0.4 0.2
59 55 50 44 42
7 5 34 35 34
42 21 19 18 15
30 44 30 23 18
57 51 54 63 69
13 5 16 14 13
P4. Mollic Hapludalf (Haplic Luvisol (siltic)) A 0–18 10YR3/3 Bt1 18–45 7.5YR3/4 Bt2 45–90 7.5YR4/6 Btk 90–125 10YR4/6 C 125–170 10YR4/6
1mgr 3msbk 2msbk 1msbk m
– – – m2sc –
6.7 6.7 6.7 7.5 7.6
3.4 1.3 1.1 0.6 0.3
55 59 63 44 42
3 7 8 25 21
60 49 45 34 24
22 33 46 27 23
62 59 48 60 63
16 8 6 13 14
P5. Typic Hapludalf ( Haplic Luvisol (siltic)) A 0–7 10YR2/2 Bt1 7–45 7.5YR4/6 Bt2 45–85 5YR4/6 Bk 85–125 10YR4/6 C 125–150 10YR4/6
2mgr 3mabk 2mabk 1csbk m
– – – c2sc –
6.8 6.7 7.3 7.5 7.7
3.5 0.7 1.3 0.4 0.2
72 65 61 46 43
4 7 9 27 23
35 33 40 36 20
30 48 53 26 23
57 50 41 61 64
13 2 14 13 13
P6. - Typic Hapludalf (Haplic Luvisol (siltic)) A 0–13 10YR2/2 Bw 13–30 7.5YR4/6 Bt1 30–61 5YR4/6 Bt2 61–103 5YR4/5 Bk 103–125 10YR4/5 Ck 125–165 10YR5/6
2mgr 3msbk 3mabk 2mabk 1msbk m
– – – – m2sc f1sc
6.4 6.4 7.4 7.2 7.4 7.6
4.8 0.6 1.4 0.2 0.1 0.1
70 67 53 58 46 40
7 9 6 8 24 21
33 21 31 19 37 22
32 33 52 38 28 21
58 53 37 51 60 65
10 14 11 11 12 14
P1. Calcic Haploxeralf (Calcic Luvisol (siltic)) A 0–16 10YR4/4 Bt 16–45 7.5YR4/4 Bk 45–90 10YR4/6 Ck 90–125 10YR4/6
Luvisol (siltic)) 10YR3/3 7.5YR3/4 10YR4/6 10YR4/6 10YR4/6
Structure
1fgr 2mabk 2mabk m
The thickest occurrence of Bt horizon was observed in soils of the humid part of the climate gradient indicating the greater development compared to other regions. In pedons 1, 2 and 3 in the xeric SMR, considerable amounts of CaCO3 have accumulated in Bk horizons beginning at 40 to 75 cm indicating the lower intensity of decalcification compared to pedons of udic SMR, which exhibited carbonate accumulations deeper than 90 cm (Table 2). All the forested soils representative of the udic SMR were slightly acid, particularly in the surface horizons (pH 6.4 to 6.8). The pH of the studied soils varies between 6.4 and 7.9. Clay content generally increases with depth, mainly due to the illuviation of clay particles in subsurface horizons (Fig. 2a). The illuviation of clay is more pronounced in the udic SMR, particularly pedons 5 and 6. The silt content shows the opposite trend compared to clay (Table 2, Fig. 2b). Clay content ranges from as low as 18% in C horizon of pedon 3 to as high as 53% in Bt2 horizon of pedon 5. As seen in Fig. 3, there is an increasing trend both in the Bt horizon clay content and its clay difference relative to the surface mineral horizon with higher precipitation accompanied by the lower temperature. The silt content shows an inverse trend. Moreover depth to the upper boundary of the Bk horizon increased with rainfall showing the higher leaching conditions provided in the more humid parts. The lower temperature along with higher precipitation increased the water available for leaching (Table 1, P/ET index). The soil organic
carbon in the A horizon increased while its pH decreased from xeric–thermic to udic–mesic areas. 3.2. Mineralogical properties As shown in Table 3, chlorite, illite, smectite, kaolinite, vermiculite, and hydroxy-interlayered vermiculite (HIV) were found in the clay fraction of the soils. The dominant clay minerals are illite and smectite in soils of the xeric SMR (pedons 1, 2, and 3), and illite and vermiculite, in the more humid part of the climosequence (pedons 4, 5, and 6). Hydroxy-interlayered vermiculite was found in A and Bt horizons of the soils of udic SMR. In the well drained soils with deep groundwater and enough precipitation, the weathering process provided favorable conditions for K + release and transformation of illite into vermiculite and smectite. Illite and chlorite were the dominant clay minerals of the parent material loess with smectite and kaolinite occurring in lower amounts (Table 3). The increase in vermiculite in soils with higher precipitation in the forested area is most probably related to higher leaching, increasing Al activity, and relatively lower pH of the soils (Douglas and Thompson, 1985). According to Boettinger and Southard (1995), limited moisture availability for chemical weathering favored the formation and stability of vermiculite. In contrast, in high chemical weathering conditions, hydroxy-
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Fig. 2. Variation of clay (a) and silt fraction (b) with depth (cm) in the soils studied.
Fig. 3. Relationship between precipitation and (a) clay content and absolute clay increase in Bt horizon; (b) thickness of Bt horizon; (c) silt content in the Bt horizon; (d) OC content in A horizon; (e) pH of the surface soil; and (f) depth to Bk horizon.
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Table 3 The relative abundance of minerals in the clay fraction based on X-ray diffraction in soils studied. Order of occurrence
Mineralogical composition of clay fraction %
Pedon Horizon
HIV
vermiculite
chlorite
kaolinite
illite
smectite
1- Calcic Haploxeralf I>S>C>k S>I>K>C I>C>K>S
− − −
− + −
++ ++ ++
+ + +
+++ +++ +++
++ ++++ +
A Bt Ck
2- Calcic Argixeroll I>S>K>C>V S>I>C>K I>C>K>S
− − −
+ − −
+ ++ ++
+ + ++
++++ +++ +++
++ +++ +
A Bt C
3- Calcic Haploxeralf I>S>C>K I>S>C>K I>C>K>S
− − −
− − −
++ ++ ++
+ + +
++++ ++++ +++
+++ +++ ++
A Bt Ck
4- Mollic Hapludalf I>V>S>C>K I > V > S > C > HIV > K I>C>K>S
− ++ −
+++ +++ −
++ + ++
+ + +
+++ +++ +++
+++ ++ +
A Bt1 C
5- Typic Hapludalf I > V > C > S > K > HIV V > I > HIV > S > K > C I>C>K>S>V
+ ++ −
+++ ++++ +
++ + ++
+ + ++
+++ +++ ++++
++ ++ +
A Bt1 C
6- Typic Hapludalf V > I > S > K > HIV > C V > I > S > HIV > K > C I>C>K>S
+ ++ −
++++ ++++ −
+ + ++
+ + +
+++ +++ +++
++ ++ +
A Bt1 Ck
+ b10%; ++ 10–20%; +++ 20–30%; ++++ 30–50%; - not present. I = Illite, S = Smectite, V = Vermiculite, HIV = Hydroxy -interlayer vermiculite, K = Kaolinite, C = Chlorite.
interlayered vermiculite can be formed mainly through weathering of chlorite (Douglas, 1989). In the pedons studied, the occurrence of illite and vermiculite, notwithstanding the considerable clay content (about 50%), has limited the shrink–swell potential of the soils (Tables 2 and 3).
Fig. 4 shows the X-ray diffraction pattern of the clay fraction in Bt1 horizon of pedon 6. Jackson et al. (1952) indicated that weathering of mica leads to the formation of illite, vermiculite, interstratified 2:1 minerals, and smectite. The broad peaks observed from 10 to 14 A° in K 25 °C treatment (Fig. 4) shifting to 10A° following heating up
Fig. 4. (a) X-ray diffraction pattern of clay fraction in Bt1 horizon of pedon 6, (b) magnified XRD pattern showing presence of smectite and (c) considerable amount of vermiculite, (S = Smectite, K = Kaolinite, V = Vermiculite, I = Illite).
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to 550 °C corresponds to the presence of vermiculite and interstratified 2:1 minerals, which give a sharp and intense 10 A° peak in K 550 °C treatment (Moore and Reynolds, 1997). 3.3. Micromorphological observations The observed voids consist of channels, chambers, vughs (50–350 μm), and planes with random distribution (Table 4). Voids in all horizons are dominated by channels (500–700 μm). Calcite nodules, brownish excrements in chambers and the tissue root residues in channels were also observed. The microstructure varies from weakly separated subangular blocky in the soils with a lower degree of development (pedons 1, 2, 3, Bk), to well separated subangular blocky (pedons 4, 5, 6, Bt) (Table 4). The argillic horizons have moderately to well separated subangular blocky microstructure associated with planes, channels and vughs. According to Fanning and Fanning (1989), the occurrence of well developed subangular blocky microstructure is related to the clay content, the type of dominant clay mineral, and the shrink–swell properties. The c/f-related distribution is open porphyric in all the argillic horizons, with quartz (10–60 μm) as the major coarse component. Open porphyric distribution provides the free translocation and orientation of clay particles from upper horizons around voids. Clay skins on the void walls, observed in argillic horizons, imply the translocation and accumulation of clay particles (Brewer, 1976; Emadi et al., 2008; Gile and Grossman, 1968; Gunal and Ransom, 2006b). In general, the voids were affected by the major pedogenic processes including the clay illuviation and calcite accumulation. The planar voids are the only types which increase in abundance with depth.
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This phenomenon can be attributed to the shrink–swell activity during the wetting and drying periods. The dominant b-fabric of the Bt horizons varies from speckled and partly crystallitic in xeric–thermic soils to speckled and striated in the udic–mesic soils (Table 4 and Fig. 5). Diffuse distribution of iron hydroxides (100–300 μm) and the occurrence of moderately to well impregnated typical ferruginous nodules (40–600 μm) are signs of well-developed hydromorphism (Stoops, 2003). Most of the nodules have well-defined boundaries and classify as orthic (Lindbo et al., 2010). According to Stoops (2003), the typical ferruginous nodules have a pedogenic origin. The high amount of calcium carbonate in Bk horizons (Table 2) can be attributed to the later recalcification processes (Khormali and Ajami, 2011; Khormali et al., 2003). According to Fig. 6 (a) this suggestion is confirmed by observing calcite coatings and hypocoatings along chambers and channels. A distinct change is observed from the mainly speckled b-fabric in the Bt horizons to the crystallitic b-fabric (Fig. 6b) in the Bk horizons indicating the translocation of calcite (Bullock et al., 1985) from the upper horizons (Bt) and recalcification in the deeper horizons (Bk) (Table 4). Illuvial clay pedofeatures were identified mainly as clay coatings occurring in voids and thick and laminated clay coatings associated with calcite coatings and nodules (pedon 6) (Fig. 7a). It is noted that the internal boundaries of these features were generally distinct. Oriented and thick clay coatings (100–300 μm) were observed in the pedons of udic–mesic soils where vermiculite and illite were also dominant. Occurrence of vermiculite and illite, notwithstanding the considerable clay content (~ 50%), limited the shrink–swell properties, and thus hindered the disturbance of clay coatings.
Table 4 Micromorphological description and the degree of soil development. Pedon, horizon, depth, cm
Microstructure
Void
B-fabric
Clay coating
Non-calcareous zone %
Secondary carbonate
Alteration degree
Fe/Mn oxides
MISECA
Degree of soil development
1. Calcic Haploxeralf A, 15 cm crumb Bt, 30 cm mod sep abk Bk, 60 cm weak sep sbk Ck, 100 cm m
ch > cm > vu ch > pl > cm ch > vu > cm vu
spk > cry spk > cry cry cry
– few – –
60 70 – –
– – common very few
1 1 1 0
few few few –
10 12 11 9
moderately developed
2. Calcic Argixeroll A, 15 cm vughy Bt , 45 cm mod sep abk Bk1,75 cm weak sep sbk C, 120 cm m
ch > cm > vu ch > pl > cm ch > vu > cm vu > pl
spk > cry spk > cry cry cry
– few – –
60 70 – –
– – common –
1 1 1 0
few few few –
11 13 12 7
moderately developed
3. Calcic Haploxeralf A, 10 cm crumb Bt, 35 cm mod sep abk Bk1, 90 cm weak sep sbk Ck, 120 cm m
ch > vu > cm ch > pl > cm ch > vu > pl ch > vu
spk > cry spk > cry cry cry
– common – –
70 70 – –
– – common very few
1 1 1 0
few few few –
13 15 14 10
moderately developed
4. Mollic Hapludalf A, 15 cm channel Bt1, 60 cm well sep abk Bk, 110 cm mod sep sbk C, 150 cm m
ch > cm > vu ch > cm > pl ch > vu > cm vu
spk = und spk cry > spk cry
– common – –
> 90 > 90 30 –
– – common –
2 2 1 0
few common few few
14 18 15 9
well developed
5. Typic Hapludalf A, 5 cm Bt1, 60 cm Bk, 120 cm C, 150 cm
crumb well sep abk mod sep sbk m
ch > cm > vu ch > cm > pl ch > vu > cm vu
und = spk spk > str cry > spk cry
– common – –
> 90 > 90 30 –
– – common –
2 2 1 0
few common few –
15 19 17 10
well developed
6. Typic Hapludalf A, 10 cm Bt1, 75 cm Bk, 110 cm Ck, 150 cm
channel well sep abk mod sep sbk m
ch > cm > vu ch > cm > pl ch > vu > cm vu
und > spk spk > str cry > spk cry
– common – –
> 90 > 90 40 –
– – common very few
2 2 1 0
few common few –
15 19 16 12
well developed
Mod sep abk: moderately separated angular blocky microstructure; m: massive; ch: channel; vu: vugh; cm: chamber; pl: plane; spk: speckled; cry: crystallitic; und: undifferentiated; str: striated; note: MISECA is calculated for the most developed horizon, i.e. Bt., –:not observed.
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Fig. 5. (a) Speckled and partly crystallitic b-fabric in Bt horizon of pedon 3, and (b) speckled and partly striated b-fabric in Bt1 horizon of pedon 5, (crossed-polarized light).
Fig. 6. (a) Calcite coatings and hypocoatings along chambers and channels, and (b) crystallitic b-fabric as a result of recalcification in Bk1 horizon of pedon 3, (crossed-polarized light).
Calcite depletion zones are one of the most characteristic pedofeatures .The decalcified zone area, which is characterized with a speckled b-fabric, varies from 70% in Bt horizons of xeric–thermic soils to >90% in Bt horizons of udic–mesic soils (Table 4). In the Bt horizons studied, the translocation of clay particles from the upper horizons and shrink/ swell activity were the pedogenic processes causing clay orientation. With increases in P/ET O ratio, the clay orientation increases as well.
MISECA value in Bt horizons varies from 12 in pedon 1, to 19 in pedons 5 and 6. With increasing the degree of soil development, as indicated by the MISECA index, the soil structure improves from massive to blocky-prismatic structure (Kemp and Zarate, 2000). The average MISECA in all horizons also increases with increasing precipitation and decrease in temperature and varies from 10.5, as in pedon 1, to 15.5 in pedon 6. The decalcified zones (depletion pedofeatures) covered almost the whole area of the thin sections of argillic horizons (> 90% in the Bt horizons) of the udic–mesic SMR-STR. With increasing degree of soil development, the soil b-fabric proceeds from crystallitic to speckled and striated (Gunal and Ransom, 2006a). In the argillic horizons of the xeric–thermic soils (pedons 1, 2, and 3) the b-fabric of the Bt horizons was speckled and partly crystallitic indicating the ongoing and incomplete process of decalcification while in the udic–mesic soils (pedons 4, 5, and 6), decalcification almost completely occurred and speckled or striated b-fabric are dominant. In general, the major pedogenic processes in the soils include illuviation of clay and decalcification concomitant with little addition of organic matter. According to the MISECA values the following classes were obtained:
3.4. Properties affecting the occurrence and type of clay pedofeatures Clay coatings were best expressed in soils formed under udic SMR and mesic STR that had the speckled b-fabric. From the xeric–thermic to the udic–mesic soil climates, the thickness of the clay coating and the area covered by clay coatings increased. Thicker oriented clay coatings (50–300 μm) were associated with relatively higher clay content and higher relative abundance the vermiculite. Dominance of vermiculite and illite in udic–mesic soils (as opposed to smectite) limited the shrink/swell potential and thus helped reduce the disruption of clay coatings. The type of the clay minerals present also affected the thickness and orientation of clay skins. Gunal and Ransom (2006b) reported similar results in different soils of the precipitation gradient of 540–715 mm. They showed thicker clay coatings were related to the more humid regions. 3.5. Degree of soil development The degree of the soil development as indicated by MISECA index is strongly related to the climate gradient and shows a significant increasing trend with increasing precipitation and decrease in temperature. Based on the MISECA index the horizons studied are categorized into moderately and strongly developed groups (Table 4). The
(1) well developed soils with thick, continuous and strongly illuvial-oriented clay coatings (5–10%) with the presence of the developed calcitic pedofeatures (MISECA = 18–19). (2) moderately developed soils with thin and striated clay coatings around voids (b5%) (MISECA = 12–15). 3.5.1. Well developed soils (pedons, 4, 5 and 6) Theses soils are mainly formed in more humid parts of the climate gradient (i.e. udic–mesic areas). They contain higher amount of clay
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soils formed in udic–mesic regions (Fig. 3). Presence of the Chernozems (pedon 2) in this region and dominance of Luvisols in the more humid region suggest the evolution of the Chernozems into Luvisols by increase in precipitation and decrease in temperature which provide suitable conditions for clay illuviation under forest vegetation. Unlike the first group in which vermiculite is the dominant clay mineral, smectite and illite prevails in these soils and vermiculite and hydroxyl interlayered minerals are rarely observed. In this category the clay coatings are thin (20–50 μm) with little orientation (Fig. 7b). The abundance of clay coatings was lower than 5% of thin sections. The lower occurrence of clay pedofeatures in the xeric–thermic SMR-STR is partly related to the lower precipitation and partly to the dominance of smectite and the subsequent higher shrink–swell properties and the disruption of the clay coating. Compared to the soils of the udic–mesic SMR-STR, in spite of presence of large areas with calcite depletion pedofeatures, the decalcification of the upper horizons is still not complete in soils of the xeric–thermic SMR-STR.
4. Conclusion
Fig. 7. (a) Thick oriented clay coating around voids in Bt1 horizon of pedon 6, and (b) thin clay coatings in Bt horizon of pedon 3, (crossed-polarized light).
formed in their Bt horizons (Fig. 3). Moreover their horizons are associated with higher organic carbon and leaching of calcium carbonate compared to the soils formed in xeric–thermic regions (Fig. 3). Higher precipitation plus lower temperatures should be more effective at dissolving carbonates. Vermiculite is mostly dominant in the clay fraction of these soils. The Bt horizons of pedons of humid regions are included in the “well developed” classes. The maximum amount of yellowish- to reddish-brown clay coatings, were observed as 5–10%, with the 20 to 300 μm thickness in these horizons. Clay coatings were thick, laminated and oriented along channels (Fig. 7a). The b-fabric is dominantly speckled in Bt horizons with more than 90% depletion pedofeatures. The orientation and high amounts of clay coatings were attributed to high precipitation (>600 mm), the dominance of vermiculite clay minerals, which has allowed preservation of clay coatings under shrink/swell conditions, and the stability of the geomorphic surface. The largest calcite and Fe/Mn nodules observed in these soils were 600 μm to 3 mm and 600 to 800 μm thick, respectively. In the argillic horizons of udic SMR, few calcite nodules observed have been subsequently covered by the laminated yellowish- to reddish-brown clay coatings, indicating decalcification of the surface horizons (A) and downward movement of the CaCO3 occurred prior to the formation of clay coatings. 3.5.2. Moderately developed soils (pedons, 1,2 and 3) Theses soils are mainly formed in the subhumid regions of the climate gradient (i.e. xeric–thermic areas). They contain lower amount of clay formed in their Bt horizons compared to the previous group (Fig. 3). In addition, their horizons are associated with lower organic carbon and leaching of calcium carbonate compared to the
The presence of clay pedofeatures and calcium carbonate were common in the soils in humid and subhumid climates of the area studied. Our study suggests that the formation of clay coatings and carbonate pedofeatures were two basic pedogenic processes occurring in the soils studied. Examination of the MISECA development index showed that clay coatings and calcite depletion pedofeatures were the main factors for rating of the soil development. The degree of development of the soil horizons varies from well developed, with thick and strong clay coatings, to moderately developed, thin clay coatings and strong decalcified zones. Orientation of clay coating ascribed to rainfall (higher than 600 mm), the occurrence of vermiculite clay minerals allowed the preservation of clay skins despite a mild shrink/swell conditions. The results of this study revealed that the MISECA development index, in addition to the arid and semiarid conditions, can be applied as a reliable index to monitor the development of soils in humid and subhumid climate.
Acknowledgements We would like to acknowledge Gorgan University of Agricultural Sciences and Natural Resources, Iran, for providing financial support for this research. The technical assistance of Eng. Mohammad Ajami in XRD analyses is appreciated. The authors acknowledge the constructive comments of the anonymous reviewers.
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Soil formation in loess-derived soils along a subhumid to humid climate gradient, Northeastern Iran Farhad Khormali a,⁎, Shadi Ghergherechi a, Martin Kehl b, Shamsollah Ayoubi c a b c
Department of Soil Science, Faculty of Water and Soil Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran Institutes for Geography, University of Cologne, Albertus-Magnus-Platz, 50932 Cologne, Germany Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2011 Received in revised form 2 February 2012 Accepted 4 February 2012 Available online 22 March 2012 Keywords: Micromorphology Loess Clay illuviation Decalcification Climate gradient
a b s t r a c t In order to contribute to the understanding of carbonate enrichment and clay illuviation in loess-derived soils of subhumid to humid regions, the development of soils was studied along a climate gradient with xeric and udic soil moisture regimes (SMR) and thermic and mesic soil temperature regimes (STR), respectively, in the Golestan Province, Northeastern Iran. Six representative pedons along a climate gradient were investigated. Soils were classified mainly as Hapludalfs and Haploxeralfs. Stability of the geomorphic surface under forest vegetation associated with high leaching conditions has provided appropriate conditions for decalcification followed by clay migration through the profile and formation of argillic horizons in all the studied soils. Clay content of the Bt horizons, soil organic carbon concentration of the A horizons, and depth of the Bk horizons increased significantly with increasing precipitation and decreasing temperature. There was a considerable decrease in silt content with soil development. The main pedofeatures observed in the Bt horizons were clay coatings and decalcified zones. Nodules, coatings and hypocoatings were the main calcitic pedofeatures observed in the Bk horizons. Occurrence and preservation of clay coatings were more pronounced in the udic regions with illite and vermiculite as the dominant clay minerals. Type of clay minerals, shrink/swell properties, and precipitation rate are factors affecting the abundance and preservation of clay coatings. In the strongly developed horizons of the udic SMR, the occurrence of vermiculite clay minerals could reduce the shrink/swell potential and increase the amount of clay coatings. The presence of crystallitic b-fabrics and the high carbonate contents (CaCO3) in the lower horizons (Bk) were mainly related to decalcification processes under descending water flow in the overlying horizons. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Most of the physico-chemical properties of the modern soils are related to climate. Clay movement and illuviation are the major processes occurring in subhumid and humid loess derived soils. Clay increase and orientation, however, cannot always be attributed to a single genetic process. Illuviation, in-situ weathering of mica or feldspar, and shrink/swell activity of clay minerals affect the orientation and content of clay particles (Mermut and Pape, 1971; Ransom and Bidwell, 1990). The results of the study along a loess climosequence in Northern Iran showed that with increasing precipitation soil pH and calcium carbonate contents decreased, whereas soil organic carbon, clay content, and cation exchange capacity increased. From north to south representing a precipitation gradient from arid to humid, the relative proportion of smectite increases reaching almost dominance
⁎ Corresponding author. Tel.: + 98 171 4426523; fax: + 98 171 4420981. E-mail addresses: [email protected], [email protected] (F. Khormali). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2012.02.002
in regions with typic xeric soil moisture regime. In the more humid areas vermiculite is dominant (Khormali and Kehl, 2011). The degree of argillic horizon development strongly depends on the amount of calcium carbonate in parent material. Since Ca 2 + facilitates the flocculation of clay particles, leaching of calcium carbonate is often taken as a prerequisite for dispersion and vertical translocation of clay particles in soil (Fanning and Fanning, 1989). Argillic horizons in calcareous soils of arid to semiarid regions characterized by limited leaching may indicate periods of more humid climatic conditions in the past. Pietsch and Kuhn (2009) reported that calcium carbonate found at a depth of up to 30 cm in most of the layered soils is very typical of an increasing aridity in parts of the semiarid tropics, particularly in areas with high groundwater levels and interflow. In fact, clay pedofeatures in the presence of calcium carbonate (CaCO3) are common in soils of arid and semiarid climates (Abtahi, 1977; Pal et al., 1994, and Sehgal et al., 1975) and often related to a fluctuation in climate (Emadi et al., 2008; Gile, 1975; Reheis, 1987; Reynders, 1972). Khormali et al. (2003) illustrated that decalcification was the dominant process in the evolution of calcareous soils with argillic horizons in semiarid southern Iran and related both
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decalcification and clay illuviation to a former period of increased precipitation. On the other hand, Holliday (1985) indicated that abundant calcium carbonate may prevent argillic horizon development, but if pores are available and precipitation is high the clay particles can be translocated into the lower horizons of the soil, despite the tendency of clay to flocculate in the presence of Ca2+ ions. Available soil moisture, soil temperature, soil texture, vegetation and availability of calcium carbonate are among the main factors determining the type and morphology of calcitic features (Khormali et al., 2006). Size and frequency of microcrystalline calcite nodules and coatings increase from aridic to xeric, but decrease again towards the areas with an ustic moisture regime. Cytomorphic and acicular calcite are observed mainly in areas with an ustic moisture regime and a denser vegetative growth, but rarely in xeric areas. Layered pendants of calcite are common beneath coarser fragments, but are considered as relicts of a more humid paleoclimate. Calcite depletion pedofeatures are observed in xeric–mesic areas with higher available soil moisture. Several researchers reported that clay coatings in soils of arid and semiarid regions may be destroyed by shrink/swell activity upon wetting and drying, by root activity, or by calcium carbonate crystallization (Blanco and Stoops, 2007; Gunal and Ransom, 2006a, 2006b; Kemp and Zarate, 2000; Khormali et al., 2003; Mermut and Arnaud, 1981; Nettleton et al., 1969; Rostad et al., 1976; Srivastava et al., 2002; Verheye and Stoops, 1973). The fragments of clay coatings (papules), visible under the optical microscope, testify to this destruction process. Hopkins and Franzen (2003) observed a strong relationship between the depths of clay accumulation and clay content. They demonstrated that thicker clay coatings occur in soils with deeper argillic horizons. In argillic horizons of loess-derived soils of southern Colorado, thick and continuous clay coatings were associated with parallel striated, porostriated, and granostriated b-fabrics (Nettleton et al., 1990). However, the occurrence of thick, laminated and continuous clay coatings along with granostriated b-fabric was related to stress in the micromass caused by high shrink/swell activity (Gunal and Ransom, 2006a, 2006b). Shohet (2008) proposed a simple method for estimating the horizontal effective stress in clay soils that has a particular relevance with respect to the effects of moisture abstraction on shrinkable clay soils by vegetation, or simply by seasonal variations. Hurst (1977) suggested several physicochemical and mineralogical indices to evaluate argillic horizon development such as free Fe2O3, the color indices, the clay illuviation index, and the smectite/(chlorite + illite) ratio. To find a relationship between age and clay accumulation in argillic horizons, Levine and Ciolkosz (1983) suggested a clay accumulation index. This index showed a favorable correlation in terms of soil development in soils of northern Pennsylvania. Several researches have carried out quantitative analysis on the degree of argillic horizon development, using micromorphological characteristics, such as MISODI and MISECA index (Khormali et al., 2003; Magaldi and Tallini, 2000). In these indices, each feature was quantified by the application of a simple rating expressing its degree of development. The usefulness of the micromorphological indices depends on the type of characteristics used. The aims of the present study were to: 1) document the variation in soil properties across a climatic gradient; 2) elucidate the soil forming processes resulting in clay coatings in the presence of carbonates and 3) study the micromorphological properties of soil horizons in loess derived soils in humid and sub-humid regions of northern Iran, in order to establish the stage of soil development. 2. Materials and methods 2.1. Site setting The area studied is located in northern Iran between 36° 30′ and 37° 30′N latitude and 54° 15′ and 56° 00′E longitude, Golestan
province (Fig. 1). Six pedons, located in the foreland and foothill zone of Alborz Mountains, were selected at xeric–thermic (pedons 1, 2, and 3) and udic–mesic (pedons 4, 5, and 6) SMR-STR along a precipitation and temperature gradient ranging from 580 mm to 900 mm and 13.8 °C to 17 °C, respectively (Table 1). According to the De-Martine method (Mather, 1974), the climate in the study area ranges from sub-humid to humid. Precipitation occurs dominantly as rain. The mean annual reference crop evapotranspiration is between 900 and 1150 mm (Table 1). The parent material of all the studied soils in the area is loess. According to pedostratigraphic correlation and results of luminescence age estimates, the uppermost layers of the loess deposits covering the Alborz Mountain foothills accumulated during the last glacial under dry and probably colder climatic conditions than today (Kehl et al., 2005). Samples from loess layers immediately below the modern soil gave IRSL ages of 17900 ± 1800 years before present (ybp) to 20500 ± 1800 ybp (Fig. 3, Fig. 6 in Frechen et al., 2009). The formation of the modern soil did not start before dust accumulation was considerably reduced after a change to more humid and warmer climatic conditions during the Late Pleistocene glacial or Early Holocene. Although the onset of soil formation cannot yet be determined precisely, it can be assumed that the duration of soil development was comparable for all soils studied here given the close proximity of the selected pedons. The elevation ranges between 215 m and 584 m above sea level (Table 1). All soils were well drained and soil formation was not influenced by groundwater. The natural vegetation is characterized by large trees consisting mainly of Fagus orientalis and Quercus castaneafolia getting denser with increasing rainfall. 2.2. Sampling and analyses Soil color was determined using a Munsell Soil Color Chart. The soils were classified according to Soil Taxonomy (Soil Survey Staff, 2010). Six representative pedons with argillic horizons were recognized using the methodology outlined by Soil Survey Staff (1999) and the characteristics of each pedon and its individual horizons were described. After dissolution of CaCO3 with 2 N HCl, decomposition of organic matter with 30% H2O2 and removing salts by washing with distilled water, samples were prepared for particle size analyses with the hydrometer method (Bouyoucos, 1962). Soil pH was measured in a saturated paste (Salinity Laboratory Staff, 1954). Organic carbon (OC) was determined by wet oxidation with chromic acid and back titration with ferrous ammonium sulphate (Nelson and Sommers, 1982). Electrical conductivity (EC) of the soil was measured in 1:2.5 soil:water suspension using a Digital Conductivity Meter-CC 601 (Jackson, 1975). Cation exchange capacity (CEC) was determined using sodium acetate (NaOAc) at pH 8.2 (Chapman, 1965). The calcium carbonate equivalent (CCE) was determined by acid neutralization (Salinity Laboratory Staff, 1954). Bulk density and water saturation percentage (SP) values were measured using the procedure described by Soil Survey Staff (1999). Clay fractions were separated based on methodology outlined by Kittrick and Hope (1963) and Jackson (1975). Iron- and carbonatefree samples were centrifuged at 750 rpm for 5.4 min to separate total clay (b2 mm). The same concentration of clay suspensions was used for all samples to give reliable comparisons between relative peak intensities. Clay minerals were estimated semi-quantitatively from the relative x-ray peak areas of glycol-treated samples (Johns et al., 1954). For micromorphological analysis, 3 undisturbed samples were collected from each soil horizon (total of 72) with modified Kubiena tins and thin sections of about 60 and 40 cm 2 were prepared from air-dried, undisturbed clods using standard techniques described by Murphy (1986). Micromorphological descriptions were made according to Bullock et al. (1985) and Stoops (2003). The MISECA index was used to evaluate the degree of argillic horizon development (Khormali et al., 2003). Argillic horizons according to this index can
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Fig. 1. Location map of the study area and the pedons studied.
be classified from weakly to well-developed stages. Micromorphological criteria for the degree of soil development, based on MISECA index, are microstructure, b-Fabric, clay coating, decalcified zone, Fe/Mn oxides and alteration degree of silicate minerals. With increasing degree of soil development, the MISECA values can range between 0 and 24. 3. Results and discussion 3.1. Morphological and physicochemical properties Based on the WRB classification system, most of the studied soils are classified as Luvisols and one as Chernozems (WRB, 2006). According to Soil Taxonomy (Soil Survey Staff, 2010), the strongly developed soils of the udic–mesic SMR-STR are classified as Hapludalfs and those of the xeric–thermic SMR-STR as Haploxeralfs and Argixerolls (Table 2). All of the pedons show argillic horizons overlying calcic horizons. Relatively high degree of leaching, stability of the geomorphic surface, dense vegetation, and favorable drainage have provided appropriate conditions for translocation of clay through the profile and the formation of argillic horizon. Munsell colors vary from yellowish brown (10YR4/6) to yellowish red (5YR4/6) when moist, and a distinct difference in color was observed between the
argillic horizons and surface horizons (A) (10YR in surface horizons vs. 7.5YR or 5YR in argillic horizons) (Table 2). This phenomenon can be attributed to the increase in iron oxides and illuviation of clay in the argillic horizons. Argillic horizons show angular to subangular blocky structure and are slightly hard to hard (dry) with friable consistence (moist) in all horizons. Samples from A and Bt horizons did not effervesce with 10% HCl but the effervescence was strong in the Bk and C horizons. Irregularly shaped nodules of calcium carbonate were observed in the Bk horizons. The pores varied from fine to very fine in size and common to many in abundance. Clay pedofeatures and calcium carbonate depletion zones were common in the soils studied and are very pronounced in the udic SMR which is in accordance with the findings of Reheis (1987). The specific environmental condition resulting from the relatively high rainfall (P/ETO > 0.6), dense vegetation (Wright, 1987), high soil CO2 concentration, higher available soil moisture (Treadwell-Steitz and McFadden, 2000), and well-drained conditions (Khormali and Ajami, 2011) facilitated carbonate removal from the upper horizon and translocation and precipitation to lower horizons. According to Hopkins and Franzen (2003), a considerable increase in the amount of total clay in deeper argillic horizons corresponds to a strongly developed soil.
Table 1 Climatological data and site characteristics of the studied regions. Pedon
Location UTM (Zone 40)
Elv m
P mm
T °C
SMR-STR
ETO mm
P/ETo
Climate*
1 2 3 4 5 6
371113 341544 263063 347175 264567 263280
526 310 215 500 320 584
580 610 660 700 810 900
17 17 16.6 13.9 14 13.8
xeric–thermic xeric–thermic xeric–thermic udic–mesic udic–mesic udic–mesic
1130 1150 1100 900 941 910
0.56 0.56 0.62 0.78 0.86 0.99
subhumid subhumid subhumid humid humid humid
4137523 4108820 4075847 4097687 4072412 4068386
Elv: Elevation Above Sea Level; P: Mean Annual Precipitation; T: Mean Annual Temperature; SMR-STR: Soil Moisture and Temperature Regime; ETO: Mean Reference Crop Evapotranspiration; P/ETO = the ratio of Mean Annual Precipitation to Mean Reference Crop Evapotranspiration; *Based on De- Martine Method (Mather, 1974).
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Table 2 Morphological and physicochemical characteristics of the selected pedons. Pedon
Morphological properties
Horizon
Depth (cm)
Color (moist)
Physico-chemical properties Calcite nodules
pH
OC (%)
SP (%)
CCE (%)
CEC cmolkg− 1
Particle size distribution % Clay
Silt
Sand
– – c2sc f1sc
7.3 7.3 7.4 7.5
2.0 1.1 0.4 0.2
55 54 45 44
6 8 19 21
30 33 26 20
32 40 39 23
55 53 50 65
13 7 11 13
P2. Calcic Argixeroll (Luvi-calcic Chernozem (siltic)) A 0–30 10YR2/2 3c sbk Bt 30–60 7.5YR4/4 2 m abk Bk1 60–105 10YR6/4 2 m abk Bk2 105–145 10YR6/4 1f abk C 145–190 10YR6/5 m
– – c2sc m2sc –
7.0 7.5 7.9 7.9 7.8
2.1 1.1 0.2 0.2 0.1
57 60 44 46 46
8 10 30 33 28
35 32 21 20 20
33 42 26 26 26
49 50 61 61 63
18 8 13 13 11
P3. Calcic Haploxeralf ( Calcic A 0–10 Bt 10–50 Bk1 50–80 Bk2 80–107 Ck 107–150
1fabk 2mabk 2mabk 1mabk m
– – c2sc c2sc f1sc
6.9 7.0 7.8 7.8 7.7
2.6 1.0 0.5 0.4 0.2
59 55 50 44 42
7 5 34 35 34
42 21 19 18 15
30 44 30 23 18
57 51 54 63 69
13 5 16 14 13
P4. Mollic Hapludalf (Haplic Luvisol (siltic)) A 0–18 10YR3/3 Bt1 18–45 7.5YR3/4 Bt2 45–90 7.5YR4/6 Btk 90–125 10YR4/6 C 125–170 10YR4/6
1mgr 3msbk 2msbk 1msbk m
– – – m2sc –
6.7 6.7 6.7 7.5 7.6
3.4 1.3 1.1 0.6 0.3
55 59 63 44 42
3 7 8 25 21
60 49 45 34 24
22 33 46 27 23
62 59 48 60 63
16 8 6 13 14
P5. Typic Hapludalf ( Haplic Luvisol (siltic)) A 0–7 10YR2/2 Bt1 7–45 7.5YR4/6 Bt2 45–85 5YR4/6 Bk 85–125 10YR4/6 C 125–150 10YR4/6
2mgr 3mabk 2mabk 1csbk m
– – – c2sc –
6.8 6.7 7.3 7.5 7.7
3.5 0.7 1.3 0.4 0.2
72 65 61 46 43
4 7 9 27 23
35 33 40 36 20
30 48 53 26 23
57 50 41 61 64
13 2 14 13 13
P6. - Typic Hapludalf (Haplic Luvisol (siltic)) A 0–13 10YR2/2 Bw 13–30 7.5YR4/6 Bt1 30–61 5YR4/6 Bt2 61–103 5YR4/5 Bk 103–125 10YR4/5 Ck 125–165 10YR5/6
2mgr 3msbk 3mabk 2mabk 1msbk m
– – – – m2sc f1sc
6.4 6.4 7.4 7.2 7.4 7.6
4.8 0.6 1.4 0.2 0.1 0.1
70 67 53 58 46 40
7 9 6 8 24 21
33 21 31 19 37 22
32 33 52 38 28 21
58 53 37 51 60 65
10 14 11 11 12 14
P1. Calcic Haploxeralf (Calcic Luvisol (siltic)) A 0–16 10YR4/4 Bt 16–45 7.5YR4/4 Bk 45–90 10YR4/6 Ck 90–125 10YR4/6
Luvisol (siltic)) 10YR3/3 7.5YR3/4 10YR4/6 10YR4/6 10YR4/6
Structure
1fgr 2mabk 2mabk m
The thickest occurrence of Bt horizon was observed in soils of the humid part of the climate gradient indicating the greater development compared to other regions. In pedons 1, 2 and 3 in the xeric SMR, considerable amounts of CaCO3 have accumulated in Bk horizons beginning at 40 to 75 cm indicating the lower intensity of decalcification compared to pedons of udic SMR, which exhibited carbonate accumulations deeper than 90 cm (Table 2). All the forested soils representative of the udic SMR were slightly acid, particularly in the surface horizons (pH 6.4 to 6.8). The pH of the studied soils varies between 6.4 and 7.9. Clay content generally increases with depth, mainly due to the illuviation of clay particles in subsurface horizons (Fig. 2a). The illuviation of clay is more pronounced in the udic SMR, particularly pedons 5 and 6. The silt content shows the opposite trend compared to clay (Table 2, Fig. 2b). Clay content ranges from as low as 18% in C horizon of pedon 3 to as high as 53% in Bt2 horizon of pedon 5. As seen in Fig. 3, there is an increasing trend both in the Bt horizon clay content and its clay difference relative to the surface mineral horizon with higher precipitation accompanied by the lower temperature. The silt content shows an inverse trend. Moreover depth to the upper boundary of the Bk horizon increased with rainfall showing the higher leaching conditions provided in the more humid parts. The lower temperature along with higher precipitation increased the water available for leaching (Table 1, P/ET index). The soil organic
carbon in the A horizon increased while its pH decreased from xeric–thermic to udic–mesic areas. 3.2. Mineralogical properties As shown in Table 3, chlorite, illite, smectite, kaolinite, vermiculite, and hydroxy-interlayered vermiculite (HIV) were found in the clay fraction of the soils. The dominant clay minerals are illite and smectite in soils of the xeric SMR (pedons 1, 2, and 3), and illite and vermiculite, in the more humid part of the climosequence (pedons 4, 5, and 6). Hydroxy-interlayered vermiculite was found in A and Bt horizons of the soils of udic SMR. In the well drained soils with deep groundwater and enough precipitation, the weathering process provided favorable conditions for K + release and transformation of illite into vermiculite and smectite. Illite and chlorite were the dominant clay minerals of the parent material loess with smectite and kaolinite occurring in lower amounts (Table 3). The increase in vermiculite in soils with higher precipitation in the forested area is most probably related to higher leaching, increasing Al activity, and relatively lower pH of the soils (Douglas and Thompson, 1985). According to Boettinger and Southard (1995), limited moisture availability for chemical weathering favored the formation and stability of vermiculite. In contrast, in high chemical weathering conditions, hydroxy-
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Fig. 2. Variation of clay (a) and silt fraction (b) with depth (cm) in the soils studied.
Fig. 3. Relationship between precipitation and (a) clay content and absolute clay increase in Bt horizon; (b) thickness of Bt horizon; (c) silt content in the Bt horizon; (d) OC content in A horizon; (e) pH of the surface soil; and (f) depth to Bk horizon.
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Table 3 The relative abundance of minerals in the clay fraction based on X-ray diffraction in soils studied. Order of occurrence
Mineralogical composition of clay fraction %
Pedon Horizon
HIV
vermiculite
chlorite
kaolinite
illite
smectite
1- Calcic Haploxeralf I>S>C>k S>I>K>C I>C>K>S
− − −
− + −
++ ++ ++
+ + +
+++ +++ +++
++ ++++ +
A Bt Ck
2- Calcic Argixeroll I>S>K>C>V S>I>C>K I>C>K>S
− − −
+ − −
+ ++ ++
+ + ++
++++ +++ +++
++ +++ +
A Bt C
3- Calcic Haploxeralf I>S>C>K I>S>C>K I>C>K>S
− − −
− − −
++ ++ ++
+ + +
++++ ++++ +++
+++ +++ ++
A Bt Ck
4- Mollic Hapludalf I>V>S>C>K I > V > S > C > HIV > K I>C>K>S
− ++ −
+++ +++ −
++ + ++
+ + +
+++ +++ +++
+++ ++ +
A Bt1 C
5- Typic Hapludalf I > V > C > S > K > HIV V > I > HIV > S > K > C I>C>K>S>V
+ ++ −
+++ ++++ +
++ + ++
+ + ++
+++ +++ ++++
++ ++ +
A Bt1 C
6- Typic Hapludalf V > I > S > K > HIV > C V > I > S > HIV > K > C I>C>K>S
+ ++ −
++++ ++++ −
+ + ++
+ + +
+++ +++ +++
++ ++ +
A Bt1 Ck
+ b10%; ++ 10–20%; +++ 20–30%; ++++ 30–50%; - not present. I = Illite, S = Smectite, V = Vermiculite, HIV = Hydroxy -interlayer vermiculite, K = Kaolinite, C = Chlorite.
interlayered vermiculite can be formed mainly through weathering of chlorite (Douglas, 1989). In the pedons studied, the occurrence of illite and vermiculite, notwithstanding the considerable clay content (about 50%), has limited the shrink–swell potential of the soils (Tables 2 and 3).
Fig. 4 shows the X-ray diffraction pattern of the clay fraction in Bt1 horizon of pedon 6. Jackson et al. (1952) indicated that weathering of mica leads to the formation of illite, vermiculite, interstratified 2:1 minerals, and smectite. The broad peaks observed from 10 to 14 A° in K 25 °C treatment (Fig. 4) shifting to 10A° following heating up
Fig. 4. (a) X-ray diffraction pattern of clay fraction in Bt1 horizon of pedon 6, (b) magnified XRD pattern showing presence of smectite and (c) considerable amount of vermiculite, (S = Smectite, K = Kaolinite, V = Vermiculite, I = Illite).
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to 550 °C corresponds to the presence of vermiculite and interstratified 2:1 minerals, which give a sharp and intense 10 A° peak in K 550 °C treatment (Moore and Reynolds, 1997). 3.3. Micromorphological observations The observed voids consist of channels, chambers, vughs (50–350 μm), and planes with random distribution (Table 4). Voids in all horizons are dominated by channels (500–700 μm). Calcite nodules, brownish excrements in chambers and the tissue root residues in channels were also observed. The microstructure varies from weakly separated subangular blocky in the soils with a lower degree of development (pedons 1, 2, 3, Bk), to well separated subangular blocky (pedons 4, 5, 6, Bt) (Table 4). The argillic horizons have moderately to well separated subangular blocky microstructure associated with planes, channels and vughs. According to Fanning and Fanning (1989), the occurrence of well developed subangular blocky microstructure is related to the clay content, the type of dominant clay mineral, and the shrink–swell properties. The c/f-related distribution is open porphyric in all the argillic horizons, with quartz (10–60 μm) as the major coarse component. Open porphyric distribution provides the free translocation and orientation of clay particles from upper horizons around voids. Clay skins on the void walls, observed in argillic horizons, imply the translocation and accumulation of clay particles (Brewer, 1976; Emadi et al., 2008; Gile and Grossman, 1968; Gunal and Ransom, 2006b). In general, the voids were affected by the major pedogenic processes including the clay illuviation and calcite accumulation. The planar voids are the only types which increase in abundance with depth.
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This phenomenon can be attributed to the shrink–swell activity during the wetting and drying periods. The dominant b-fabric of the Bt horizons varies from speckled and partly crystallitic in xeric–thermic soils to speckled and striated in the udic–mesic soils (Table 4 and Fig. 5). Diffuse distribution of iron hydroxides (100–300 μm) and the occurrence of moderately to well impregnated typical ferruginous nodules (40–600 μm) are signs of well-developed hydromorphism (Stoops, 2003). Most of the nodules have well-defined boundaries and classify as orthic (Lindbo et al., 2010). According to Stoops (2003), the typical ferruginous nodules have a pedogenic origin. The high amount of calcium carbonate in Bk horizons (Table 2) can be attributed to the later recalcification processes (Khormali and Ajami, 2011; Khormali et al., 2003). According to Fig. 6 (a) this suggestion is confirmed by observing calcite coatings and hypocoatings along chambers and channels. A distinct change is observed from the mainly speckled b-fabric in the Bt horizons to the crystallitic b-fabric (Fig. 6b) in the Bk horizons indicating the translocation of calcite (Bullock et al., 1985) from the upper horizons (Bt) and recalcification in the deeper horizons (Bk) (Table 4). Illuvial clay pedofeatures were identified mainly as clay coatings occurring in voids and thick and laminated clay coatings associated with calcite coatings and nodules (pedon 6) (Fig. 7a). It is noted that the internal boundaries of these features were generally distinct. Oriented and thick clay coatings (100–300 μm) were observed in the pedons of udic–mesic soils where vermiculite and illite were also dominant. Occurrence of vermiculite and illite, notwithstanding the considerable clay content (~ 50%), limited the shrink–swell properties, and thus hindered the disturbance of clay coatings.
Table 4 Micromorphological description and the degree of soil development. Pedon, horizon, depth, cm
Microstructure
Void
B-fabric
Clay coating
Non-calcareous zone %
Secondary carbonate
Alteration degree
Fe/Mn oxides
MISECA
Degree of soil development
1. Calcic Haploxeralf A, 15 cm crumb Bt, 30 cm mod sep abk Bk, 60 cm weak sep sbk Ck, 100 cm m
ch > cm > vu ch > pl > cm ch > vu > cm vu
spk > cry spk > cry cry cry
– few – –
60 70 – –
– – common very few
1 1 1 0
few few few –
10 12 11 9
moderately developed
2. Calcic Argixeroll A, 15 cm vughy Bt , 45 cm mod sep abk Bk1,75 cm weak sep sbk C, 120 cm m
ch > cm > vu ch > pl > cm ch > vu > cm vu > pl
spk > cry spk > cry cry cry
– few – –
60 70 – –
– – common –
1 1 1 0
few few few –
11 13 12 7
moderately developed
3. Calcic Haploxeralf A, 10 cm crumb Bt, 35 cm mod sep abk Bk1, 90 cm weak sep sbk Ck, 120 cm m
ch > vu > cm ch > pl > cm ch > vu > pl ch > vu
spk > cry spk > cry cry cry
– common – –
70 70 – –
– – common very few
1 1 1 0
few few few –
13 15 14 10
moderately developed
4. Mollic Hapludalf A, 15 cm channel Bt1, 60 cm well sep abk Bk, 110 cm mod sep sbk C, 150 cm m
ch > cm > vu ch > cm > pl ch > vu > cm vu
spk = und spk cry > spk cry
– common – –
> 90 > 90 30 –
– – common –
2 2 1 0
few common few few
14 18 15 9
well developed
5. Typic Hapludalf A, 5 cm Bt1, 60 cm Bk, 120 cm C, 150 cm
crumb well sep abk mod sep sbk m
ch > cm > vu ch > cm > pl ch > vu > cm vu
und = spk spk > str cry > spk cry
– common – –
> 90 > 90 30 –
– – common –
2 2 1 0
few common few –
15 19 17 10
well developed
6. Typic Hapludalf A, 10 cm Bt1, 75 cm Bk, 110 cm Ck, 150 cm
channel well sep abk mod sep sbk m
ch > cm > vu ch > cm > pl ch > vu > cm vu
und > spk spk > str cry > spk cry
– common – –
> 90 > 90 40 –
– – common very few
2 2 1 0
few common few –
15 19 16 12
well developed
Mod sep abk: moderately separated angular blocky microstructure; m: massive; ch: channel; vu: vugh; cm: chamber; pl: plane; spk: speckled; cry: crystallitic; und: undifferentiated; str: striated; note: MISECA is calculated for the most developed horizon, i.e. Bt., –:not observed.
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Fig. 5. (a) Speckled and partly crystallitic b-fabric in Bt horizon of pedon 3, and (b) speckled and partly striated b-fabric in Bt1 horizon of pedon 5, (crossed-polarized light).
Fig. 6. (a) Calcite coatings and hypocoatings along chambers and channels, and (b) crystallitic b-fabric as a result of recalcification in Bk1 horizon of pedon 3, (crossed-polarized light).
Calcite depletion zones are one of the most characteristic pedofeatures .The decalcified zone area, which is characterized with a speckled b-fabric, varies from 70% in Bt horizons of xeric–thermic soils to >90% in Bt horizons of udic–mesic soils (Table 4). In the Bt horizons studied, the translocation of clay particles from the upper horizons and shrink/ swell activity were the pedogenic processes causing clay orientation. With increases in P/ET O ratio, the clay orientation increases as well.
MISECA value in Bt horizons varies from 12 in pedon 1, to 19 in pedons 5 and 6. With increasing the degree of soil development, as indicated by the MISECA index, the soil structure improves from massive to blocky-prismatic structure (Kemp and Zarate, 2000). The average MISECA in all horizons also increases with increasing precipitation and decrease in temperature and varies from 10.5, as in pedon 1, to 15.5 in pedon 6. The decalcified zones (depletion pedofeatures) covered almost the whole area of the thin sections of argillic horizons (> 90% in the Bt horizons) of the udic–mesic SMR-STR. With increasing degree of soil development, the soil b-fabric proceeds from crystallitic to speckled and striated (Gunal and Ransom, 2006a). In the argillic horizons of the xeric–thermic soils (pedons 1, 2, and 3) the b-fabric of the Bt horizons was speckled and partly crystallitic indicating the ongoing and incomplete process of decalcification while in the udic–mesic soils (pedons 4, 5, and 6), decalcification almost completely occurred and speckled or striated b-fabric are dominant. In general, the major pedogenic processes in the soils include illuviation of clay and decalcification concomitant with little addition of organic matter. According to the MISECA values the following classes were obtained:
3.4. Properties affecting the occurrence and type of clay pedofeatures Clay coatings were best expressed in soils formed under udic SMR and mesic STR that had the speckled b-fabric. From the xeric–thermic to the udic–mesic soil climates, the thickness of the clay coating and the area covered by clay coatings increased. Thicker oriented clay coatings (50–300 μm) were associated with relatively higher clay content and higher relative abundance the vermiculite. Dominance of vermiculite and illite in udic–mesic soils (as opposed to smectite) limited the shrink/swell potential and thus helped reduce the disruption of clay coatings. The type of the clay minerals present also affected the thickness and orientation of clay skins. Gunal and Ransom (2006b) reported similar results in different soils of the precipitation gradient of 540–715 mm. They showed thicker clay coatings were related to the more humid regions. 3.5. Degree of soil development The degree of the soil development as indicated by MISECA index is strongly related to the climate gradient and shows a significant increasing trend with increasing precipitation and decrease in temperature. Based on the MISECA index the horizons studied are categorized into moderately and strongly developed groups (Table 4). The
(1) well developed soils with thick, continuous and strongly illuvial-oriented clay coatings (5–10%) with the presence of the developed calcitic pedofeatures (MISECA = 18–19). (2) moderately developed soils with thin and striated clay coatings around voids (b5%) (MISECA = 12–15). 3.5.1. Well developed soils (pedons, 4, 5 and 6) Theses soils are mainly formed in more humid parts of the climate gradient (i.e. udic–mesic areas). They contain higher amount of clay
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soils formed in udic–mesic regions (Fig. 3). Presence of the Chernozems (pedon 2) in this region and dominance of Luvisols in the more humid region suggest the evolution of the Chernozems into Luvisols by increase in precipitation and decrease in temperature which provide suitable conditions for clay illuviation under forest vegetation. Unlike the first group in which vermiculite is the dominant clay mineral, smectite and illite prevails in these soils and vermiculite and hydroxyl interlayered minerals are rarely observed. In this category the clay coatings are thin (20–50 μm) with little orientation (Fig. 7b). The abundance of clay coatings was lower than 5% of thin sections. The lower occurrence of clay pedofeatures in the xeric–thermic SMR-STR is partly related to the lower precipitation and partly to the dominance of smectite and the subsequent higher shrink–swell properties and the disruption of the clay coating. Compared to the soils of the udic–mesic SMR-STR, in spite of presence of large areas with calcite depletion pedofeatures, the decalcification of the upper horizons is still not complete in soils of the xeric–thermic SMR-STR.
4. Conclusion
Fig. 7. (a) Thick oriented clay coating around voids in Bt1 horizon of pedon 6, and (b) thin clay coatings in Bt horizon of pedon 3, (crossed-polarized light).
formed in their Bt horizons (Fig. 3). Moreover their horizons are associated with higher organic carbon and leaching of calcium carbonate compared to the soils formed in xeric–thermic regions (Fig. 3). Higher precipitation plus lower temperatures should be more effective at dissolving carbonates. Vermiculite is mostly dominant in the clay fraction of these soils. The Bt horizons of pedons of humid regions are included in the “well developed” classes. The maximum amount of yellowish- to reddish-brown clay coatings, were observed as 5–10%, with the 20 to 300 μm thickness in these horizons. Clay coatings were thick, laminated and oriented along channels (Fig. 7a). The b-fabric is dominantly speckled in Bt horizons with more than 90% depletion pedofeatures. The orientation and high amounts of clay coatings were attributed to high precipitation (>600 mm), the dominance of vermiculite clay minerals, which has allowed preservation of clay coatings under shrink/swell conditions, and the stability of the geomorphic surface. The largest calcite and Fe/Mn nodules observed in these soils were 600 μm to 3 mm and 600 to 800 μm thick, respectively. In the argillic horizons of udic SMR, few calcite nodules observed have been subsequently covered by the laminated yellowish- to reddish-brown clay coatings, indicating decalcification of the surface horizons (A) and downward movement of the CaCO3 occurred prior to the formation of clay coatings. 3.5.2. Moderately developed soils (pedons, 1,2 and 3) Theses soils are mainly formed in the subhumid regions of the climate gradient (i.e. xeric–thermic areas). They contain lower amount of clay formed in their Bt horizons compared to the previous group (Fig. 3). In addition, their horizons are associated with lower organic carbon and leaching of calcium carbonate compared to the
The presence of clay pedofeatures and calcium carbonate were common in the soils in humid and subhumid climates of the area studied. Our study suggests that the formation of clay coatings and carbonate pedofeatures were two basic pedogenic processes occurring in the soils studied. Examination of the MISECA development index showed that clay coatings and calcite depletion pedofeatures were the main factors for rating of the soil development. The degree of development of the soil horizons varies from well developed, with thick and strong clay coatings, to moderately developed, thin clay coatings and strong decalcified zones. Orientation of clay coating ascribed to rainfall (higher than 600 mm), the occurrence of vermiculite clay minerals allowed the preservation of clay skins despite a mild shrink/swell conditions. The results of this study revealed that the MISECA development index, in addition to the arid and semiarid conditions, can be applied as a reliable index to monitor the development of soils in humid and subhumid climate.
Acknowledgements We would like to acknowledge Gorgan University of Agricultural Sciences and Natural Resources, Iran, for providing financial support for this research. The technical assistance of Eng. Mohammad Ajami in XRD analyses is appreciated. The authors acknowledge the constructive comments of the anonymous reviewers.
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