sodium equally influence soil morphology (Brady and Weil 1999). Salt-affected soils occur in wide climatic zones, from the humid to arid areas, but they are most abundant in the arid and semiarid regions where evaporation is very greater than rainfall. The Islamic Republic of Iran is situated in one of the most arid and semi-arid regions of the world. The average annual precipitation is 252 mm (one-third of the world’s average precipitation), and this is under conditions in which 179 mm or 71 percent of rainfall is directly evaporated. Therefore, the climatic condition is suitable for the salt accumulation of in Iranian soils mainly Central Plateau, the Khuzestan and Southern Coastal Plains and the Caspian Coastal Plain.
Several geologic formations provide the basic parent materials involved in salt-affected soils development (QADIR et al., 2008). Parent materials from inheritance or weathering provide a large quantity of salts. Salt-affected soils generally occur at low elevations, but they also occur at higher elevations such as the North-west of Iran. The majority of the Salt-affected soils in Iran occur in lower physiographic areas, i.e., in the lower piedmont plains and valleys (QADIR et al., 2008) or in micro-depressions.
Material and methods
3.1 Environmental setting
The study was carried out in the Miandoab region, the catchments of the Urmia Lake, situated in the south of Urmia City from 45?40′ to 45 ? 58′ E longitude and 37? 0′ to 37? 16′ N latitude in western Azerbaijan Province, northwest Iran (Fig. 1). The area contains gently undulating plains with slopes 0- 2% with elevations ranging from 1240 to 1300 m above sea level (a.s.l.). Lowland is the major landform of the area and drainage is very poor.
Fig. 3.1 The situation of study area
The daily weather and overall climatic condition of the Miandoab region is very much a result of the geographic condition and physiographic features of western Azerbaijan Province. The present climate of the region is a semi-arid type, characterized by hot, dry summers, cold, wet winters and temperate autumn and spring with highly seasonal precipitation. The average annual precipitation is 320 mm, of which ~80% occurs during the five month period between December and April. Mean daily temperature maximum and minimum was 33 and -5.5 OC, respectively, and annual mean temperature was about 11.7 OC. Soil moisture and temperature regimes are xeric border to aridic and mesic, respectively.
This area that is part of Iranian plateau formed in the late period of Quaternary (10000 BP). Therefore, the geology of the region is mainly occupied by the alluvium and recent deposits belonging to the late Quaternary period mainly composed of limestone, shale, sandstone, and conglomerate.
Lowland is the major landforms of the studied region with slope of 0 to 2%. The maps of slope and aspect of area are presented in Figures 3 and 4, respectively.
Fig. 3.2 The slope map of the studied area
Fig. 3.3 The aspect map of the studied area
3.5 Vegetation and Land Use
In general, the studied area is covered by halophilous vegetation dominated by Salsola (SP), Salicornia (SP), and Atriplex (SP). Agricultural practices is not exists in the region mainly due to effects of soil salinity and alkalinity along with unsuitable drainage. The traditional land use pattern is for grazing purposes and wildlife.
3.6 Field Studies
Profiles location were identified before field work studies followed by air photo (1:55,000), topographic map (1: 50,000), and Google Earth image interpretations. Seven profiles with 1.5 m deep were dug by hand using the methodology outlined by Soil Survey Staff (1999) and the characteristics of each profile and its individual horizons were described (Fig.5). Soil samples were taken from each horizon transferred to the laboratory, air dried, and ground and separated into 2mm fraction using a 2 mm (fine earth) sieve.
Fig. 3.4 Some representative profiles in the studied area
3.7 Soil Drainage
Separation of soil drainage classes in the study area was based on the presence of reductiomorphic features in soil horizons at certain depths in the soil profiles (Soil Survey Staff, 2003). Reductiomorphic horizons have low chroma colors (moist chroma 2 or less, or moist chroma 3 with value 6 or more) that occupying 50% or more of the matrix exposed in a cut face of the horizon or are dominant on ped face. Based on field observations, one soil drainage class was identified in the study area which is poorly drained. The class was recognized by the presence of a reductiomorphic horizon within 30 cm of the mineral soil surface. Also, ground water was appeared in the depth of about 40 cm for most of the studied profiles. The chemical parameters of ground water sampled from the profiles of 4 and 6 are given in Table 1. The quality of waters was in the unsuitable categories (C4-S4) for irrigation based on FAO (Ayers and Westcot 1994) with respect to the values of EC (C) and SAR (S).
Table 3.1 The chemical composition of ground water sampled from some profiles
3.8 Laboratory Analysis
3.8.1 Physicochemical Analysis
Particle size distribution was performed by using the hydrometer method (Gee and Bauder, 1986). Soil pH and electrical conductivity were determined in saturated past and saturation extract, respectively. Organic carbon was measured using a rapid dichromate oxidation technique (Nelson and Sommer, 1996). Calcium carbonate equivalent was estimated volumetrically after treating the soil with HCl (Nelson, 1982). For the calculation of cation exchange capacity (CEC), 1N sodium acetate (NaOAc) was applied at a pH of 8.2 (Chapman, 1965). Exchangeable and soluble cations (Ca, Mg, Na, and K) were extracted by 1M NH4OAc buffered at pH 7 (Thomas 1982) and saturation extract, respectively. The content of Ca and Mg along with Na and K were determined by an atomic absorption spectrophotometer (Shimadzu AA-6300) and a flame photometer (Corning 400), respectively. Sodium absorption ration (SAR) was calculated using soil solution Na, Ca, Mg (Salinity Laboratory Staff, 1954). Exchangeable sodium percentage (ESP) was determined through exchangeable Na and CEC (Salinity Laboratory Staff, 1954).
3.8.2 Iron Oxides Analysis
Free or pedogenic Fe oxides (Fed) including crystalline, poorly crystalline, and organically bound Fe was extracted by dithionite-citrate-bicarbonate (DCB) method after 16-h shaking at 23 oC (Holmgren, 1976). Poorly crystalline and organically bound Fe (Feo) was extracted using 0.2M ammonium oxalate (AAO) following 4-h shaking at pH 3 in the darkness (McKeague & Day, 1966). All Fe oxides forms were determined using atomic absorption spectrometry. The difference between CBD-Fe and AAO-Fe was considered as an estimation of crystalline Fe oxides form.
3.8.3 Clay Minerals Analysis
Clay minerals were determined by X-ray diffraction (XRD) technique (Mehra and Jackson 1960; Kunze 1965) following some steps: (1) The removal of soluble salts and carbonates by washing and using 1N sodium acetate buffered at pH 5; (2) The removal of organic matter and iron oxides by 30% H2O2 and citrate-dithionite-bicarbonate (CDB), respectively; (3) The separation of clay fraction from sand and silt fraction by wet-sieving and centrifugation; (4) The saturation of clay fraction with ions of Mg and K; (5) The preparation of treatments on glass slides as Mg-saturated, Mg-saturated plus ethylene glycol salvation, K-saturated, and heat of K-saturated at 500 oC; (6) The scan of treatments from 3 to 30o 2? with speed of 1o 2? and a step size of 0.02° 2? at 1 s/step.
Results and Discussion
4.1 The Soil Morpho-Physical Properties
The studied region generally occurs in the level to depressional and basin type landforms under poor to somewhat poorly drained conditions. As shown in table 4-1, soil and redoximorphic features are different mainly due to the effect of ground water table. For instance, soil colors were in the range of 5Y and 5YR to 10YR for hue along with value and chroma of 2-4 and 1-3, respectively. For most of the examined soils, evidence of gleying and distinct mottles was observed in all horizons mainly in subsurface horizons (Table 4-1). The mottling characteristics associated with wetness could be ascribed to the presence of alternate oxidizing and reducing conditions (Rezapour et al., 2009). The removal of free iron under reducing conditions in low-lying topographical position imparts the soil mineral grain to appear gray (Boul et al., 2003).The soil color variation in subsurface horizons in contrast to surface horizons evidenced higher value and lower chroma which can be explained to the presence of high amount of free CaCO3 and to the saturation and reduction coupled with mixing of organic matter and soil matrix with carbonate (Rendall et al., 1996).
In all the studied soils, the accumulation of sodium led to the formation of platy to prismatic structure in topsoil while the subsoil structures was fine