Physicochemical Properties and Surface Charge Characteristics of Arid Soils in Southeastern Iran

The majority of previous studies on surface charge characteristics were done on tropical and subtropical soils. Information of such studies in the arid regions is limited. A study was conducted to investigate the relation between soil chemical and mineralogical properties and surface charge characteristics of an arid region in Southeastern Iran. Eight soil pedons, representing the alluvial and the colluvial deposits, were described, and their mineralogical and physicochemical properties were examined. The common clay minerals in the studied area are smectite, palygorskite, kaolinite, chlorite, and illite. The point of zero charge (pH 0 ) values are low (2.85–3.35) in all soils mostly affected by organic carbon (OC) and free iron oxide (Fed). pH0 has a significant negative correlation with pH under field conditions (r = −0.45, P < 0.05). The point of zero net charge (PZNC) levels for all the soils were <2, due to the excess negative charge in these soils. The estimated PZNC values were less than pH 0 in all soils because of the high permanent negative charge in these soils. The permanent negative charge (σ p ) of the soils studied is high and it has a significant positive correlation with pH, CEC, Na, Mg, SAR, clay content, palygorskite, OC, and Fed.


Introduction
Soil surface charges affect the chemical properties of soil by varying the quantity of electric and surface charge density.Surface charge properties have an important bearing on the migration of ions in soil, the formation of organomineral complexes, soil structure, plant nutrition, and the dispersion, flocculation, swelling, and shrinkage of the soil fractions [1].Based on differences in the surface properties, soils can be classified into two basic categories: permanent-charge soils and variable-charge soils [2,3].
Point of zero charge (PZC) often denoted as pH 0 is one of the most important parameters used to describe variablecharge surfaces [4].Uehara and Gillman [5] indicated that pH 0 is the pH where the amounts of negative and positive charge of variable charge components where these are equal.The pH 0 value for the temperate region of Iran was reported in the range of about 2.6 to 3.75 [6].Anda et al. [7] reported that the pH 0 of three Oxisols in Malaysia was about 3.9 to 5.7.The previous studies showed that the pH 0 values decrease with increasing organic matter content and increase with increase in sesquioxides [6][7][8].Taubaso et al. [9] reported that the pH 0 values of Argentinean provinces soils were smaller than the pH in water, indicating that these soils had negative charge under natural conditions.
Gonzales-Batista et al. [10] reported that for systems free of permanent charge, the point of zero net charge (PZNC) should coincide with pH 0 , but for systems where permanent and variable charges coexist, PZNC may differ from pH 0 .Usually in the young soils (Entisols or Inceptisols) PZNC is lower than its pH 0 and higher than pH 0 if the soils are highly weathered, such as Oxisols [9].In addition, the model of Uehara and Gillman [11] and Yu [2] predicts that in soils having permanent negative charge, the PZNC value is lower than the pH 0 and vice versa.
Most studies on soil surface charge properties were conducted in tropical and subtropical regions where the soils are poor and low in negative charge.In Iran, Sharami et al. [6] conducted similar study for temperate soils of the Northern region but no study was done on arid regions of Southern Iran or elsewhere in the world.The electrochemical properties of soils are useful fundamental knowledge in solving the fertility problems of soils of the arid regions.For example, in the arid regions, the NH 4 + fixed by soil particles and leached out as NO 3 − form due to high permanent negative charge in the surface of soil particles [12] in comparison with highly weathered soils where NO 3 − leaching because of high positive charge present in the soils [13].The objectives of this study were to determine the physicochemical and mineralogical properties, the pH 0 , PZNC, and permanent negative charge (  ) and to determine the relation between pH 0 and   with characteristics of arid soils in Southeastern Iran.

Description of the Study Area.
The studied area is located in the province of Hormozgan, Southeastern Iran, between latitude 28 ∘ 8  and 28 ∘ 15  North and longitudes 56 ∘ 5  and 56 ∘ 15  East (Figure 1).The climate is arid with an average annual precipitation of about 162 mm and evaporation of 4243 mm.Burdon [14] reported that the estimated potential evapotranspiration could be in excess of 50 times the mean annual precipitation.The temperature regime is hyperthermic with an aridic and ustic moisture regimes [15].The sedimentary rocks accompanying marine deposits of the Tertiary age formed the catchment area, which is predominantly composed of thick alluvial and colluvial materials occasionally deposited by the Dehshaykh River for the past thousands of years.The river is dry most of the time except during heavy downfall.The catchment is currently cultivated with wheat, maize, potato, and watermelon as important agricultural crops [16].
Eight pedons were selected among the dug profiles representing alluvial fan and colluvial deposits along the slope area of the catchment by geopedology approach and different soil horizons were sampled for laboratory analysis.The morphological properties of the soils were described in the field [17] and were classified according to the USDA Soil Taxonomy [18].

Physical and Chemical
Analyses.Field study and samples collection were carried out in the summer season of 2009 (August to September).The soil samples were air-dried and sieved with a 2 mm sieve and kept in plastic bags prior to analysis.

Determination of Point of Zero Charge.
Point of zero charge (PZC), denoted as pH 0 , is defined as the pH at which the net charge on the variable charge components is zero and was determined according to the method of Gillman and Sumner [27].Six portions of 2 g of 2 mm air-dried samples were weighed into 50 mL plastic centrifuge tubes and shaken with 20 mL of 0.1 M CaCl 2 for 2 h to saturate the soil with Ca.The suspension was centrifuged (3000 rpm, 10 min) and the supernatant was discarded.The Ca saturated soil was washed twice with 20 mL of 0.002 M CaCl 2 and after the third addition of 20 mL of 0.002 CaCl 2 , the pH was adjusted to six values of 0.1 M HCl in the range of 3.5 to 6 mL by dropwise addition.Equilibration of the pH took several days.When equilibrium occurred for 0.002 M CaCl 2 , the pH was recorded as pH 0.002 , then 0.5 mL of 2 M CaCl 2 was added to each sample and shaken for 3 h, and pH was recorded thereafter and designated as pH 0.05 .For each tube, pH was calculated as (pH 0.05 − pH 0.002 ).The pH 0 was obtained from the plot of pH against pH 0.002 .The pH 0 was identified as the point where pH was zero.

Determination of Point of Zero Net
Charge.The point of zero net charge (PZNC), the pH at which net surface charge of the whole system (taking into account both permanent and variable charge colloids) is zero, was obtained by interpolation of the data from the measurement of charge variation with pH.Charge variation with pH was determined by the fingerprint method of Gillman and Sumner [27].Two-gram air-dried soil samples in 50 mL preweighed plastic centrifuge tubes were shaken with 20 mL of 0.1 M CaCl 2 for 2 h to saturate the soil with Ca.The suspension was centrifuged (3000 rpm, 10 min) and the supernatant was discarded.The Ca saturated soil was washed twice with 20 mL of 0.002 M CaCl 2 and after the third addition of 20 mL of 0.002 M CaCl 2 soil suspension was recorded, and thereafter the pH was adjusted to different values by dropwise addition of 0.1 M HCl or saturated Ca(OH) 2 .The equilibration of the pH took several days.After the pH was stable, the suspension was centrifuged and the supernatant was collected to determine Ca, Al, and Cl in the entrained solution.The tubes were then weighed to estimate the volume of entrained 0.002 M CaCl 2 .
The soil was then extracted with 20 mL of 1 m NH 4 NO 3 , and Ca, Al, and Cl were determined in the extract.Allowing for entrained Ca, Al, and Cl, the amounts of cation (Ca 2+ + Al 3+ ) and anion (Cl − ) adsorbed were calculated and equated with total negative (CEC T ) and positive (AEC) charge, respectively.The amount of Ca adsorbed was equated with basic cation exchange capacity (CEC B ). Aluminum and Ca were determined by AAS and Cl measured by silver nitrate titration method [28].The PZNC was identified as the pH value where total negative and positive charges are equal.The permanent charge was estimated as the difference between AEC and CEC T values at pH 0 [29].

Mineralogical Analyses.
Removal of chemical cementing agents and separation of the different size fractions of soils were carried out according to Mehra and Jackson [21], Kittrick and Hope [30], and Jackson [31].Organic matter and carbonate were removed with 30% H 2 O 2 and 1 N HCl, respectively.X-ray diffraction (XRD) studies were carried out on both fine and coarse clay fractions, while equal concentrations of clay suspensions were used for all samples in order to allow for a more reliable comparison between relative peak intensities for different samples.Two drops of the prepared suspension were used on each glass slide.The clay samples were finally examined by XRD analysis using a Philips diffractometer with Co K radiation in order to allow for a more reliable comparison between the relative heights of peaks in the XRD data.The K-saturated samples were studied both after drying and heating at 500 ∘ C for 4 h to identify kaolinite in the presence of trioctahedral chlorite, and samples were also treated with 1 N HCl at 80 ∘ C, overnight.Semiquantitative estimation of clay minerals was made by directly converting the diffraction peak areas using the method of Karanthanasis and Hajek [32].Selected dried clay particles of the soils were studied under a transmission electron microscope (TEM, LEO 912 AB).The samples dispersed in 100% alcohol and 100 L of the suspension was dropped on formvar-coated Cu grids.
The soil chemical and mineralogical properties and their relationship with the surface charge characteristics were analyzed by multiple and simple linear regression using SAS and Excel softwares.

Field Observation and Soil Classification.
The soil chemical data are shown in Table 1.Pedons 1, 3, 7, and 4 are soils developed on the piedmont alluvial plains while pedon 2 and 6 are formed on river alluvial plains.Pedons 5 and 8 are formed on alluvial colluvial fans and sited on a higher slope area.All pedons were dug with a maximum depth of 150 cm and obvious textural changes throughout the profile were observed.Since the plain is part of an alluvial and colluvial fan, the deposition of materials from the alluvial and colluvial wash varies from time to time creating different textural layers.This was observed in all the soils studied where the soils textural changes were extreme, particularly for the silt and sand content.There were also variations in their structures, which is also associated with the amount of sand and silt in the soils.The soils tend to be massive when the silt content is high, otherwise the structures would be single grain when the sand content is high.The rounded nature of these gravels suggests that they were washed and deposited along part of the plain.All soils were weakly developed and evidenced by weak horizon formation in the profiles.Under the arid environment with annual precipitation value of less than 200 mm and evaporation of more than 4000 mm, one would surely expect to see slow rate of soil pedological development because of very low rate of P/ET 0 [33].
The eight representative profiles were classified using the USDA Soil Taxonomy [18].All soils are young and differentiated by their moisture regime.They were identified as Calcic Haplosalids, Aridic Ustorthents, and Aridic Ustifluvents (Table 1).

Physical Properties.
The soil physical data are presented in Table 2. None of the soil physical characteristics show any clear trend with depth.In pedon 1, the clay content distribution was irregular.In pedons 4, 5, 7, and 8, clay increases with depths, while in pedon 2, it increased only until the top 100 cm.In contrast to the above observations, pedon 6 exhibits a remarkable decrease in clay content with soil depth.In pedons 3 and 4, the silt content decreases with depth.The sand size particles are dominant in all pedons except for pedons 1 and 7, but their distribution throughout the profile was very irregular.The results are consistent with the nature of alluvial and colluvial deposition that varies from time to time.Khresat and Qudah [34] also observed a similar deposition trend on studying the Azraq Basin in Northeastern Jordan, which is also formed from alluvial and colluvial deposition.

Chemical Properties.
The soil chemical data of the representative profiles are presented in Table 3.The soils are alkaline and calcareous in nature, containing 19.83% to 61.45% CaCO 3 throughout the profiles.All soils have pH values above 7.0 showing that they are alkaline.The average electrical conductivity values exhibited some variations among the soils studied, placing these soils from slightly saline to saline level.In pedons 2, 4, 5, and 8, the soils were noted to be nonsaline with EC values ranging from 0.4 to 2.64 dS m −1 , while soils in pedons 3, 6, and 7 were slightly saline with EC values ranging from 2.1 to 7 dS m −1 .Pedon 1 soil was observed to be saline with EC values ranging from 18.11 to 30.5 dS m −1 .The Na content showed slight variability among the soils studied.In pedons 2, 5, and 8, the amount of Na was between 0.4 and 6.8 cmol c kg −1 as compared to pedons 1, 3, 4, 6, and 7, which Na ranges between 1.84 in 20 cmol c kg −1 .The soils, however, are nonsodic with SAR values ranging from 0.3 to 7.7.The cation exchange capacity ranges from 1.7 to 41.4 cmol c kg −1 .In pedons 1, 3, 6, and 7, the CEC was between 19.5 and 41.1 cmol c kg −1 as compared to pedons 2, 4, 5, and 8, with values of 11.7 to 21.5.The organic carbon content was low in all soils, which is common for soils of these regions where the vegetation is scarce.
The variability of the soil chemical properties again reflects the alluvial and colluvial nature of soil deposition and their rocks origin.According to Navai [35], the predominant rocks surrounding the study area are limestone, dolomite (Jahrom Formation), limestone, marl (Aghajari Formation), limestone, green marl (Mishan Formation), salt domes (Hormoze Formation), and conglomerate (Bakhtiari Formation).The weathered rock materials from these formations were washed to the catchment areas and these contributed to the variability in the soil physicochemical properties.
The common minerals in all samples of the studied area are smectite, palygorskite, illite, chlorite, and kaolinite.Kaolinite is found in all soils with the exception of pedon 8. Sepiolite exists in pedons 3, 4, 6, and 7.The dominance of these minerals in the arid regions agrees with the results obtained by Owliaie et al. [36] and Emadi et al. [37].Palygorskite and sepiolite are fibrous clays and are considered to be of authigenic origin, inherited from weathered parent materials of the surrounding hills and plateaus [33].The presence of kaolinite in most soils is probably due to inheritance from the weathering of coarse and fine materials transported from the surrounding upland zones.It may also suggest the occurrence of palygorskite and sepiolite kaolinization that apparently may be due to the effect of organic matter decomposition and fibrous clay destabilization caused by Mg uptake by plants [38].

Point of Zero Charge Characteristics.
The pH 0 values varied from 2.8 in subsurface horizon of pedon 2 to 3.35 in the topsoil horizon of pedon 1 (Table 4) and were positively correlated with the Fe d content ( = 0.87 * * ,  < 0.01) which was far more than its correlation with the OC percentage ( = −0.83* * ,  < 0.01).The pH 0 also has a negative significant correlation with pH H 2 O ( = −0.45* ,  < 0.05).Hence, the Fe d content had more effect on pH 0 values compared with the OC content in the soils studied.The respective pH 0 values of organic matter and sesquioxides have been reported to decrease and increase, respectively [8,39].
Li and Xu [40] reported that the layer silicate clays (2 : 1 clays) decreased the pH 0 values, while the 1 : 1 clay minerals as kaolinite increased the pH 0 .Besides, Uehara and Gillman [5] reported that the oxides of Fe and Al have high pH 0 values (pH 7-9), while silica (SiO 2 ) and organic matter have low pH 0 values.Therefore, the low levels of pH 0 in the present study were probably related to layer silicate clays (2 : 1 clays) and high pH in all the pedons.
The values of pH (pH 0 − pH CaCl 2 ) indicate the evolution of positive and negative charges on the variable charge components [11].The greatest values of pH (pH 0 − pH CaCl 2 ) were related to subsurface horizon of profile 2 (−5.10 units), which agreed with its strongly negative net charge in the field conditions (Table 4).The mineralogical data also confirmed the occurrence of significant amounts of layer silicate minerals (smectitic type) in this profile (Table 4).Generally, the variations of pH (pH 0 − pH CaCl 2 ) for all the studied horizons showed a rather high level (between −4.25 and −5.10 units) (Table 4), which was in accordance with their CEC levels (Table 3).However, because of a large quantity of layer silicate minerals (2 : 1 clays) and a scarcity of 1 : 1 clays and Fe d , this rising trend from surface to subsurface horizons was not so noticeable.

Point of Zero Net Charge (PZNC).
For all the studied soils, the PZNC levels were <2, which was probably due to the excess negative charge in these soils.Gonzales-Batista et al. [10] reported that for systems free of permanent charge, PZNC should coincide with pH 0 , but for systems where permanent and variable charges coexist, PZNC may differ from pH 0 .Usually, in the young soils (Entisols or Inceptisols) PZNC is lower than its pH 0 and higher than pH 0 if the soils are highly weathered, such as Oxisols [41].In addition, the model of Uehara and Gillman [11] and Yu [2] predict that in soils having constant negative charge, the PZNC value is lower than pH 0 and vice versa.None of the charge variation curves of the samples indicated the PZNC in pH between ranges of 2.6 and 6, which was due to the excessive amount of negative charge compared with the positive charge in these ranges of pH.Besides, the amount of estimated ZPNC in all the samples was lower than the pH 0 , confirming the presence of the permanent negative charge in these soils.These data also agree with their mineralogical information (Table 4).
Earlier, the effects of Fe d on the surface charge properties were discussed.Hamdan et al. [42] showed that charge characteristics were significantly correlated with soil organic matter, free iron oxides, and clay content.Sesquioxides have amphoteric properties; that is, their surface charge can bear either positive charge in an acid condition or negative charge in an alkaline condition [43].
The   has a positive significant correlation with clay content ( = 0.54 * * ,  < 0.01).The results agree well with those of Sharami et al. [6].At high pH, OH − ions interact with the edge of the clay particles, making them neutral or negatively charged [44].
The mineralogical compositions in these soils were mixed and each clay mineral plays an important role in the total permanent charge.Although all soils are a mixture of permanent and variable-charge components, whether one or the other dominants depends largely on its mineralogy [45].For instance, palygorskite has a positive significant correlation with   ( = 0.60 * * ,  < 0.01).Structurally, palygorskite includes alternation of blocks and tunnels that grow along the length of the fiber.Each structural block is composed of two discontinuous tetrahedral sheets of silica that a central octahedral sheet sandwiched between them.Because of the discontinuity of the silica sheets, silanol groups (Si-OH) are present on the surface of the fiber.
The   has a positive significant correlation with OC ( = 0.60 * * ,  < 0.01).Organic matter also acts as an important source of surface negative charge.Oades et al. [46] reported several relationships between surface charge characteristics and organic matter in an Oxisol under rainforest vegetation in Australia.Hydroxyl groups of organic matter can also contribute to negative charge [2].Shamshuddin and Anda [47] showed that organic carbon in soils is responsible for lowering the pH 0 because of the creation of negative surface charge and/or masking of positive surface charge.
The SAR, exchangeable Na and exchangeable Mg also have a positive significant correlation with   with -values of 0.84 * * , 0.92 * * , and 0.82 * * , respectively.Through alteration of the crystal lattice structure of soil colloids, permanent charge develops when ions of lower valence substitute for ions of higher valence [48].The mechanisms of the effect of electrolyte on surface charge of soils are rather complex.According to the theory of diffuse double layer, two mechanisms may exist.The surface of variable charge minerals is one kind of reversible constant potential surface.The charge density  on this type of surface is proportional to the square root of ion concentration  of the solution, that is,  = () 1/2 , where  is a constant [28].
The valence, ionic radius, and thickness of hydrated layer of cations and anions of various electrolytes are different.Morais et al. [49] showed that the nature and valence of the counterions also influenced the magnitude of the electric charges on the soil particles.For example, the quantities of negative surface charge and net surface charge in MgCl 2 , MgSO 4 , and K 2 SO 4 solutions are lower than those in KCl solution.
Furthermore, the   has a positive significant correlation with CEC ( = 0.96 * * ,  < 0.01).This demonstrates that the CEC has the highest effect on the   .Since organic matter is low in the soils, the CEC is affected by the expanding minerals (e.g., smectite) [50] and basic exchangeable cations (e.g., Na).Sollins et al. [45] reported that most of the permanent charge in the soils is on the surfaces of layer-silicate clays, which compose most of the surface area of p-c soils.These clays include the 2 : 1 layer-silicates (illite, vermiculite, and smectite) and the 2 : 2 clays (chlorite).The 1 : 1 group of kaolin clays has a small amount of permanent charge on their surface.
The pH H 2 O has a positive significant correlation with the permanent negative charge (  ) ( = 0.72 * * ,  < 0.01).The pH KCl also has a positive significant correlation with the permanent negative charge (  ) ( = 0.55 * * ,  < 0.01).Rashad and Dultz [51] indicated that at low pH the surface charge of both clay soil samples (original clay and the organic matter removed clay soil) had low negative values, which is due to the protonation of variable charge with increasing pH, where deprotonation of functional groups occurs, the surface charge becomes more negative.Shamshuddin et al. [52] reported that the negative charges on the soil surface were found to increase significantly with an increase in pH.Thus, when the pH is raised, more OH − are adsorbed onto the surface or H + are released into the solution, causing an increase in negative charges.
3.5.4.Charge Variation with pH.Charge (negative and positive) variations with pH (0.002 M CaCl 2 ) for different horizons of the soils are presented in Figure 2. The inverse relations between both negative and positive charge variation curves both in surface and between subsurface horizons (having small levels of variable charge components) were noticeable.The results showed that all of the samples showed large negative charge over the range of applied pH.The highest level of negative charge was evident in profile 1, which was probably due to high levels of clay content in its surface and subsurface horizons and the relation to 2 : 1 clay minerals.The charge properties did not show any clear trend with depth.Since the plain is part of an alluvial and colluvial fan, the deposition of materials from the alluvial and colluvial wash varies from time to time creating different textural layers.For instance, the level of negative charge in horizon Cz1 (−27.3 cmol c kg −1 ) of profile 1 (Calcic Haplosalids) was lower than that of horizon Cyz2 (35.7 cmol c kg −1 ).Hence, the soil electrochemical properties in profile 1 were different from the other studied pedons.
Profile 4 (horizon C 1 ) has the lowest negative charge (11.2 cmol c kg −1 ), which is probably due to low levels of clay content and the relatively low smectite in this layer.Profile 4 also shows the lowest levels of negative charge in both surface and subsurface horizons compared with the other profiles, due to its lower CEC, clay content, and smectite.
The negative charge of the soils in the present study is influenced by the 2 : 1 clay minerals where they are prevalent and organic matter accompanied by iron oxide is scarce (Tables 3 and 4).Consequently, pH variations could not have a great influence on the charge curve.Therefore, for soil pH (CaCl 2 ) of 7.55, the amount of negative charge in the subsurface horizon of profile 1 is more than the above horizon.Naidu et al. [53] studied the clay mineralogy and surface charge characteristics of four basaltic soils from Western Samoa.The level of trace to subordinate amounts of kaolin minerals was present in these soils.The surface charge-pH curves followed a constant potential model, indicating the presence of substantial amounts of pH-dependent charge.
In addition, they reported that negative charge was present at pH values as low as 3.0 and small quantities of positive charge were detected at pH values as high as 9.0.Values for PZC ranged from 2.2 to 3.9 and these were generally greater than the pH 0 determined by pH method in their study.The permanent negative charge of the soils is high because the majority of parent materials are limestone.The results agreed well with those obtained by Sharami et al. [6].
Generally, for all the soils examined, the ranges of charge variation with pH in the surface horizons were more noticeable than in the subsurface horizons.Most of these

Figure 1 :
Figure 1: Location of the study area.

Table 1 :
General information on the studied pedons.

Table 2 :
Some physical properties of the representative profiles.

Table 3 :
Chemical properties of the representative profiles.

Table 4 :
Relative abundance of clay mineral and some electrochemical characteristics of the soils studied.