J. Agr. Sci. Tech. (2012) Vol. 14: 225-239 Micromorphology of Pedogenic Carbonate Features in Soils of Kohgilouye, Southwestern Iran 1 H. R. Owliaie ABSTRACT The micromorphology of pedogenic carbonate features in calcareous soils of arid and semiarid regions of Kohgilouye Province, Southwestern Iran, has been studied to determine their genesis and distribution in different climatic regions. Eight representative pedons (from a total number of 24 pedons) were studied in aridic-ustic (minimum rainfall), ustic and xeric (maximum rainfall) soil moisture regimes. Micromorphological studies indicated that the frequency of secondary calcite as pedogenic nodules, coating or infilling in voids or channels increase from aridic-ustic to xeric soil moisture regimes. The presence of pedogenic calcite coating superimposed on clay coatings in pedons of more humid regions probably suggests a complex history of carbonate leaching, deposition of secondary calcite and clay illuviation. Pendants of calcite were observed in soils with coarser texture in aridic-ustic region as a common pedofeature. Pedogenic nodules in more developed soils of xeric regions were harder containing denser and more contiguous micritic calcite. Degree of impregnation of calcite nodules with Fe/Mn oxides as well as calcite depletion pedofeatures increase in areas with higher rainfall. Needle shaped calcite and cytomorphic calcite were observed in the near surface horizons of the regions with higher rainfall and denser vegetation growth of ustic and xeric soil moisture regimes. Keywords : Arid zone, Calcite, Micromorphology, Soil, Southwestern Iran. INTRODUCTION differences among estimates are a result, at least in part, of the difficulty of differentiating between primary carbonates (lithogenic Arid and semi-arid soils cover over a third of carbonates) and carbonates of secondary origin the world landmasses and are becoming or pedogenic carbonates. The occurrence of increasingly more important worldwide calcitic pedofeatures in calcareous soils of because of increased human populations and southern and southwestern Iran has often been consumption of natural resources (Buol et al., reported in the literature (Khormali et al. , 1997). Calcite is one of the best studied, but 2006; Owliaie et al. , 2006). Downloaded from jast.modares.ac.ir at 8:07 IRST on Saturday October 9th 2021 also one of the most variable pedogenic According to Wright (1987) the type of minerals. It shows a wide variety of shapes pedogenic calcium carbonate is controlled and habits, and occurs as well in arid and mainly by the parent material, climate, and semiarid soils as in temperate and tropical soils vegetation. The formation of pedogenic with restrained drainage (Stoops and Delvigne, carbonates involves complex processes of 1990). dissolution, translocation and precipitation. Secondary or pedogenic carbonates are those Four models have commonly been used to that are formed through the processes describe pedogenic carbonate formation responsible for the soil development. They are namely, the per decensum, per ascensum, in found in relatively dry soils and develop under situ, and biogenic models (Monger, 2002). In natural good drainage and vegetation most micromorphological descriptions, a comprising grass and shrubs mixture. The distinction is made only between _____________________________________________________________________________ 1 Department of Soil Science, Faculty of Agriculture, Yasouj University, Yasouj, Islamic Republic of Iran. e-mail: [email protected] or [email protected] 225 ______________________________________________________________________________ Owliaie microcrystalline (sometimes called micritic), level (a.s.l.) in the southwest to 4,400 m crystalline (sparitic) and acicular calcite a.s.l. in the northeast. The mean annual (Stoops and Delvigne, 1990). Lithogenic precipitation ranges from about 350 mm in carbonate dissolves under ambient moisture southwestern parts to about 1,000 mm in and a relatively high soil CO 2 partial pressure southeastern regions of the Province. 2+ 2+ 2− and the dissolved Ca , Mg and CO 3 ions General data and a location map of the move downward with the percolating soil regions studied are presented in Table 1 and water. As the moisture content decreases, Figure 1. The soil parent material is calcite precipitates (Wang and Anderson, moderately to highly calcareous, although 1998). Development of various pedogenic parent material of southwestern sectors is features, such as channels, cutans, pedotubules dominated by a combination of calcareous and secondary carbonates in soils of arid and gypsiferous materials (pedon 8). Based regions is due to major processes of on previous soil surveys, eight dissolution, leaching and recrystallization representative pedons (from a total number (Chen, 1997). of 24 pedons) with calcic horizons were Pedogenic carbonate has been studied with selected for this investigation in different respect to its importance for plant nutrition, climatic regions of the Province. Soils were carbon sequestration, landscape age, described and classified according to the paleoclimatology, paleoecology, and as a phenomenon per se (Monger, 2002). Soil Survey Manual (Soil Survey Staff, Micromorphological analysis of carbonate 1993) and Keys to Soil Taxonomy (Soil nodules and nodular fabrics in different soil Survey Staff, 2010), respectively. materials shows that their development is a function of several factors: nature of the host MATERIALS AND METHODS matrix (texture, porosity), carbonate and non- carbonate clay distribution, bulk density and the interactions among them (Wiede and Chemical and Physical Analyses Yaalon, 1982). Distinction between pedogenic and Particle-size distribution was determined lithogenic carbonate is of great importance, using sodium hexametaphosphate for the e.g., in soil classification, and has been determination of sand, silt and clay fractions discussed several times in the literature by the pipette method (Day, 1965). (Monger and Gallegos, 2000; Nordt et al., Alkaline-earth carbonate was measured by 2000). acid neutralization (Salinity Laboratory No previous studies have been performed on Staff, 1954). Organic carbon was measured the genesis and micromorphology of the well by wet oxidation with chromic acid and back Downloaded from jast.modares.ac.ir at 8:07 IRST on Saturday October 9th 2021 developed and diverse calcitic pedofeatures in titration with ferrous ammonium sulphate soils of Kohgilouye Province. The main (Nelson, 1982). Determination of the soil pH objective of this study is therefore to was in a saturation paste and electrical investigate the genesis of different calcitic conductivity (EC) in a saturation extract features in the calcareous soils of the different (Salinity Laboratory Staff, 1954). climatic regions of southwestern Iran, using micromorphological studies. Micromorphological Analyses Background of the Studied Area Thin sections with an area about 30 cm 2 were prepared using standard techniques Kohgilouye Province with a land area of (Murphy, 1986). Air-dried oriented clods about 1.6 million ha, is located in were impregnated under vacuum with an southwestern Iran. The elevation of the acetone diluted polyester resin (50/50 ratio), province varies from 300 m above the sea 226 Pedogenic Carbonate Features of Kohgilouye Soils ________________________________ Figure 1. Location map of the study area. and cured for a minimum of one week. Oriented sections were cut with a diamond saw and cemented onto glass microscope slides. The slides were ground and polished to a thickness of 30 µm and described following the guidelines of Stoops (2003). SEM Studies Undisturbed dried samples were mounted on aluminum stubs using double-sided tape and carbon paste, then coated with gold and examined using a Cambridge scanning electron microscope (SEM). Intact soil samples were also observed using a Dino- Lite digital microscope at 60-230X Downloaded from jast.modares.ac.ir at 8:07 IRST on Saturday October 9th 2021 magnification with 1.3M pixel resolution. RESULTS AND DISCUSSION General Results Soil Texture of the studied soils varied from silty loam in pedon 8 to clay in pedons 1, 2 and 4 (Table 2). All soils were calcareous throughout the profile and in most pedons calcium carbonate increased with depth (Table 2). Pedogenic gypsum in pedon 8 occurred as medium to large needle shaped crystals as well as mycelium. The 227 ______________________________________________________________________________ Owliaie Table 2. Some physico -chemical and morphological properties of the pedons studied. a b c Horizon Lower Sand Silt Clay pH OC CCE EC Color depth (%) (%) dSm -1 (moist) (cm) Pedon 1 - Chromic Calcixerert Ap 20 14 42 44 7.8 0.5 11 0.51 10YR4/3 Btss 65 18 29 53 8.0 0.6 18 0.42 10 YR 4/3 Bk 125 22 27 51 8.0 0.7 23 0.48 10 YR 4.5 / 3 Pedon 2 - Calcic Haploxeralf Ap 25 18 38 44 7.5 0.5 29 0.30 7.5 YR 4/4 Btk 1 75 22 26 52 7.4 0.4 38 0.23 7.5 YR 4/4 Btk2 115 20 30 50 7.5 0.4 46 0.27 7.5YR4/4 Btk3 150 28 25 47 7.7 0.2 49 0.38 7.5YR5/4 Pedon 3 - Calcic Argixeroll Ap 25 25 36 39 7.5 1.5 20 0.57 10 YR 3/1 Bt 50 19 32 49 7.7 0.7 24 0.31 7.5YR4/4 Btk 100 40 26 34 7.8 0.2 55 0.24 10YR4/4 Pedon 4 - Calcic Rhodxeralf Ap 25 18 38 44 7.5 0.5 29 0.60 5R4/3 Btk1 75 22 26 52 7.4 0.4 38 0.28 10R3/5 Btk2 115 20 30 50 7.5 0.4 46 0.24 10R3/6 Btk 3 150 28 25 47 7.7 0.2 49 0.21 10 R3/6 Pedon 5 - Calcic Haplustalf Ap 20 30 44 26 7.5 1.4 30 0.53 7.5YR4/4 Btk 1 45 25 36 39 7.6 0.7 32 0.47 10 YR 4/4 Btk 2 100 23 42 35 7.6 0.3 35 0.75 10 YR 4/4 Ck 150 44 36 20 7.5 0.2 79 0.91 7.5YR8/ 1 Pedon 6 - Calcic Haplustept Ap 30 26 46 28 7.4 1.6 42 0.73 10 YR 4/ 4 Bw 55 20 46 34 7.5 1.1 37 0.74 7.5YR4/ 4 Bk 1 95 22 44 34 7.7 0.2 47 0.31 7.5 YR 4/ 4 Bk2 150 22 48 30 7.7 0.1 48 0.30 7.5YR4/ Downloaded from jast.modares.ac.ir at 8:07 IRST on Saturday October 9th 2021 4 Pedon 7 - Typic Calciustept Ap 20 24 52 24 7.4 1.1 55 0.78 10YR4/ 4 Bk 1 65 26 45 29 7.5 0.3 58 0.54 10 YR 4/ 4 Bk 2 90 24 49 27 7.9 0.3 60 0.31 10 YR 4/ overall microstructure of the studied soils (Table 3).