Effect of Irrigation Regime on Mineralogical Properties of Clay Fraction in some Iraqi Rice Soils

Five soil pedons were chosen from two sites in Najaf and Diwaniyah, governorates, represent some of the soils of Middle Euphrates area. The soils are exploited by rice crop, and irrigated by two methods (flooded and dry) during the growing season. The mineralogical composition of < 2μm fractions for two depths (0-30 and 31-60 cm) in all studied soils were examined by X-ray diffractions. Results showed the presence of Mica, Smectite, Real chlorite and Kaolinite minerals in clay fraction of soils irrigated by the wet method. Results also, showed the precipitation of Brucite layer into Montmorillonite interlayers and caused to shifting Montmorillonite towards Chlorite mineral in soils irrigated by the dry method, and this was attributed to creation a suitable conditions from continuous wet-dry cycling, which encouraged a chloritization phenomenon to occur in these soils.. While, the continuous waterlogging process did not affect much on the weathering of mica minerals, and transformed it towards of expandable 2:1 minerals in these soils.


Introduction
Agriculture is now facing a big challenge of providing suffisant food for the rapidly growing population.Rice is one of the most important cereal crops, feeding more than 75% of the Iraqi population and 40% of the global population .Iraq has scarce water resources, and rice farming uses nearly 40% of Iraqis freshwater resources [1,2].As a result of the changing climate, and water shortage, Alternate wetting and drying irrigation has been implemented widely in Iraq [3].Also at same time some Iraqi governorates still using the old flooded method in irrigation of rice soils.In flooded rice soil, soil swelling is dominant because clay absorbs water.In addition, continuous flooding irrigation adjusts soil properties in advantageous ways, such as creation of soft tilth for easier root penetration.This technique also leads to lower nutrient losses than alternate wetting and drying irrigation [4,5].[6], showed that the dominant clay minerals are palygorskite and illite, followed by kaolinite and the mixed layer of montmorillonite-chlorite.[7], indicate that the heaviest minerals are opaque, amphiboles, pyroxenes, chlorite, epidotes, biotite garnet, muscovite, zircon, kyanite, staurolite, and rutile.These sediment are typically formedby sedimentary rocks (single or many cycles, low and high-rank metamorphic rocks, acidic and basic igneous rocks, and pegmatite rocks .In alternate wetting and drying irrigation , the soil is allowed to dry out before irrigation is applied again [3] as such, water will removal from within and between clay microstructures.Thus, the alternate wetting and drying irrigation tends to result in a rather dramatic change in the soil physical and chemical environment.[8], showed that Iraqi clays like bentonite are profitably effective to remove 1259 (2023) 012007 IOP Publishing doi:10.1088/1755-1315/1259/1/012007 2 toxic environmental contaminants.This environment controls water and nutrient availability and plant growth.The nutrient uptake of rice in alternate wetting and drying irrigation method differs from that in continuous flooded due to the physiological responses of rice to water stress and the nutrient availability in the system of alternating wet and dry [9].Also, the difference in the irrigation regimes used in rice soils, it may affect on many mineralogical properties of these soils.Therefore, the current study was conducted to find out the effect of variation in the used irrigation systems on the mineralogical properties of clay fraction in some Iraqi rice soils.

Materials and Methods
Five soil pedons were chosen from two sites in Najaf and Diwaniyah, governorates, represent some of the soils of Middle Euphrates area .The soils are exploited by rice crop, and irrigated by two methods (Flooded and Dry) during the growing season.The flooded method is the traditional method used to grow rice crop, in which the soil is flooded throughout the growing season of the rice crop, while the dry method has been applied recently in Iraq to irrigate the rice crop, in order to water consumption, so that the soil is flooded for three days, and followed by three days drying till mid of season, and then soil flooded till end of season.Soil samples were taken from two depths (0-30 and 31-60 cm) in each site, and air dried, crashed and passed through 2 mm sieve.The particle size distribution was determined according to international pipette method as described in [10].The electrical conductivity EC and pH were measured according to [11].While total calcium carbonate was determined by acid neutralization method of Pansu and Gautheyrou, 2006 .Soil organic matter was determined according to Wakley-Black method as described in [12].The mineralogical composition of the clay-sized fraction was determined by X-ray on parallel oriented clay of the Mg 2+ and K + saturated clay samples.The Mg 2+ saturated clay was treated with the X-ray in the air dry and ethylene glycol salvation.The K + saturated clay was heated to 350 0 C and 550 0 C. X-ray analyses were made with a Philips-P.W1840 diffractometer equipped with a graphite diffracted beam monochromatot using Cukα radiation.

Chemical and Physical Properties of Studied Soils
Results in Table (1) show some chemical and physical properties of studied soils.Results indicated that the range of pH values was between 7.00 -7.60 reflecting that the reaction of all studied soils was moderate alkali, and this attributed to effect of calcareous parent material of these soils (Essa, 2001) .Values of Electrical conductivity (EC) was ranged between 1.00 -20.00 dS.m -1 which indicated that the studied soils were low to moderate saline.The total amounts of CaCO 3 equivalent was ranged between 87.91 -263.74 g.Kg -1 indicated a calcareous nature of studied soils.Results show that amounts of total CaCO 3 was increased with depth especially in soils irrigated by flooded method, reflect the effect of many factors, such as high leaching rat during growth season, calcification process, and calcareous parent material of these soils.[13], indicated, whether calcium carbonate present in parent material or transferred from other sources, the pedogenic processes of solution, translocation and deposition are responsible for sedimentation and accumulation of CaCO 3 and the depth of accumulation depend upon the depth of water which carried the Ca 2+ and HCO 3 1-ions.Also the results of Table (1) show that the total contend of O.M was ranged between 3.00 -9.60 g.Kg -1 , and the highest values were obtained in soil surface due to the accumulation of high amount of plant residuals at the upper part of soil.In general the low amount of organic matter recorded in studied soils is due to the effect of high temperatures during summer season, which causes the decomposition of organic matter in these soils, and this agrees with results obtained by many studies have been done on Iraqi soils [14,15].The values of cation exchange capacity CEC were ranged between 24.40 -34.00 Cmol.Kg -1 , in general the CEC values were low and varied in the studied soils, this attributed to the variation in the content and type of clay minerals in these soils.Results of particle size distribution in Table (1) show that the amount of clay, silt, and sand were ranged between (286.00 -412.00),(378.00-630.00), and (64.00 -231.00)g.Kg -1 respectively.In general the results showed that there are no significant differences in the values of particle size distribution in all the studied soils, and the evidence for this is that the textures of these soils were very close (clay lom -silt clay loam) .This convergence in the textures can be attributed to the fact that these soils are sedimentary soils, and were formed from the same sources, and underwent to similar sedimentation cycles [16][17][18].
Table 1.Some chemical and physical properties of studied soils.

Mineralogical Properties of Studied Soils
The mineralogical composition of ˂2µ fractions for two depths (0-30 and 31-60 cm) in all studied soils (Figs. 1 -5 ) were examined by X-ray diffraction.In Al-Najaf (control) soil at depth of 0 -30 cm , results in Fig. (1A) show the presence of 14.98 A 0 reflection, with his third diffraction at d-spacing of 4.81 A 0 in air-dried, Mg-saturated treatment, and continued presence without changed in all treatments, reflecting the presence of real chlorite in this soil.The presence of 10.57A 0 reflection in air-dried, Mg-saturated treatment and remaining without change in all treatments reflecting the presence of mica minerals in this soil, also the appearance of second order of mica with a low intensity at d-spacing of 5.11 A 0 confirmed the presence of Biotite in this soil.This agrees with results obtained by many studies have been done on Iraqi soils [19][20], who have shown that the distinguish between two types of mica minerals (Biotite and Muscovite) depending on the second order of mica at 5 A 0 , in Muscovite the peak of second order is always strong and high, while it is disappear or very weak in Biotite .Also, the six order of mica was used for distinguish between mica minerals, so the six order of Muscovite appear at 1.5 A 0 , while it is appear at 1.54 A 0 in case of Biotite.In Fig. (1A) the Kaolinite is identified by 7.33 A 0 reflection and remains in air-dried, Mg-saturated, ethylene glycol salvation, and K-saturated with heat treatment to 350 0 C , and disappear in K-saturated with heat treatment to 550 0 C. Results also, showed that the X-ray examination for clay sample at depth of 31-60 cm in Al-Najaf control soil (Fig. 1B), which reveals almost the same mineralogical composition, as in the surface depth of this pedon, which was the presence of real Chlorite, Biotite, and Kaolinite.Results of X-ray examination for 0-30 cm depth of Al-Najaf (dry) soil, which exploited by the cultivation of rice crop, and irrigated by the dry method, are shown in Fig. (2A) which reveals the presence of 15.73 A 0 in air-dried, Mg-saturated treatment and shifts to an 18.24A 0 reflection following ethylene-glycol salvation, and then shifts to an 15.12A 0 reflection in K-saturated and heating to 350 and 550 0 C, which confirmed to presence of Mg-hydroxyl-Montmorillonite, due to precipitation of Mg-hydroxide into Montmorillonite interlayers.This agrees with results obtained by many studies [21][22][23], who have shown that the appearance the Montmorillionite at d-spacing of 15.20 A 0 in airdried, Mg-saturated treatment, and expands to 18.60 A o reflection following ethylene-glycol salvation, and then collapsed to 15 A 0 reflections in K-saturated and heating to 350 0 C and 550 0 C , which confirming the precipitation of Brucite layer into Montmorillonite interlayers and shifts towards Chlorite mineral. .Also, results of Fig. (2A) show the presence of 21.52 A 0 reflection in K-saturated and heated to 550 0 C treatment, which explains its existence in this soil in two ways: The first, this reflection is belong to (S-Ch) interstratified mineral due to precipitat of Mg-hydroxyl-interlayer in Montmorillonite interlayers.And the seconed explanation is that the presence of (S-Ch) interstratified mineral in this soil, may be due to the effect of subjected the soil horizon to continuous cycles of wetdry process.According to our point of view, the precipitation of Mg-hydroxide into Montmorillonite interlayers at depth of 0-30 cm in AL-Najaf (dry) soil, which exploited by the cultivation of rice crop, and irrigated by the dry method, and this will created a suitable conditions from continuous wet-dry cycling, which encouraged a chloritization phenomenon to occur in that soil.These results are agree with finding of [24], through his study of the Iraqi soils, that the process of wet-dry cycle is considered one of the most important factors encouraging the precipitation of Mg-hydroxide into Montmorillonite interlayers.Also, results of Fig. (2A) show the presence of 10.62 A 0 reflection in air-dried, Mgsaturated treatment, and remaining without change in all treatments, reflecting the presence of Mica minerals in the clay fraction of this soil, while the disappeared of second order of Mica at 5 A 0 confirmed the presence of Biotite in this soil.Also, Kaolinite was diagnosed through presence of 7.40 A 0 reflection in treatments of air-dried, Mg-saturated , ethylene glycol salvation , and K-saturated with heat treatment to 350 0 C , and disappeared in K-saturated with heat treatment to 550 0 C .In the depth of 31-60 cm of Al-Najaf (dry) soil, results in Fig. (2B) show the presence of 15.64 A 0 reflection in air-dried, Mg-saturated treatment which expanded to 17.67 A 0 reflection following ethylene-glycol salvation, and then shifts to an 10.36A 0 and 15.64 A 0 reflections in K-saturated and heating to 350 0 C and 550 0 C respectively, indicated the presence of Mg-hydroxide-Montmorillonite in that soil.[25] also, during their work on Iraqi rice soils have been diagnosed Mg-hydroxide-Montmorillionite in soils exploited by the cultivation of rice crop, and irrigated by the dry method, they attributed the formation of this mineral to the fact that the soil was subjected to continuous wetdry cycling.Also, the variance of d-spacing intensity and collapsing of Montmorillonite reflection in treatments of K-saturated with heating to the 350 0 C and 550 0 C within clays of 0-30 and 31-60 cm depths in Al-Najaf (dry) soil, even though they both been subjected to the same wet-dry cycling , it may attributes to the degree of filling of Mg-hydroxyl interlayer within Montmorillonite interlayers .[26], indicated that the degree of filling and integration of Mg-hydroxide into Montmorillonite interlayers was highly effects on the intensity and stability of Montmorillonite refection, when exposed to high temperatures in K-saturated and heating to 350 0 C and 550 0 C .The appearance of 10.36 A 0 reflection in air-dried, Mg-saturated treatment and remaining without change in all treatments (Fig. 2B), reflecting the presence of mica minerals in this soil, also the appearance of second order of mica with medium intensity, indicates the presence of two types of Mica minerals (Biotite and Muscovite) in this soil., Kaolinite was diagnosed through presence of 7.50 A 0 reflection in treatments of air-dried, Mg-saturated , ethylene glycol salvation , and K-saturated with heat treatment to 350 0 C , and disappeared in K-saturated with heat treatment to 550 0 C .Results of Fig. (3A) represented the X-ray examination for clay sample of 0-30 cm depth in Al-Najaf (wet) soils, exploited by the cultivation of rice crop, and irrigated by the wet method, show the presence of 14.85 A 0 reflection in air-dried, Mg-saturated treatment and shifts to an 18.14 A 0 reflection following ethylene-glycol salvation, while the treatments of K-saturated with heating to 350 0 C and 550 0 C, led to collapse the reflection to d-spacing of 10.58A 0 and 15.74A 0 respectively, which confirmed to presence of Mg-hydroxyl-Montmorillonite [27], due to precipitation of Mg-hydroxide into Montmorillonite interlayers in this soil.Whereas, the appearance of this mineral under the waterlogged conditions is not surprising, Georgiadis et al., 2020 show that the formation of hydroxide layer into the Montmorillonite, often not subject to ideal conditions, such as the availability of suitable content of organic matter, and subjected the soil horizons to continuous cycles of wet-dry process, while the studies of [28,29], were indicated the formation of Mg-hydroxyl-Montmorillonite in waterlogged soils.Also, Inoue and Yoshida, 1990 and through their study of waterlogged soils, they showed that a white gelatinous layer formed within the interlayers of expendable 2:1 clay minerals, and they interpreted it as a linking of Al 3+ ion to humic organic compounds, and they return the reason for the formation of that layer to precipitate Al-hydroxide-interlayer into these minerals by the effect of chloritization process.Results of X-ray examination for 0-30 cm depth of Al-Diwaniya (dry) soil, which exploited by the cultivation of rice crop, and irrigated by the dry method, are shown in Fig. (4A) which reveals the presence of 14.98 A 0 in air-dried, Mg-saturated treatment and shifts to an 17.66 A 0 reflection following ethylene-glycol salvation, and then shifts to an 15.21 and 15.08 A 0 reflections in K-saturated and heating to 350 and 550 0 C respectively.The continuous retention the diffraction in his d-spacing at 15 A 0 in heating treatments confirmed to presence of Mg -hydroxyl-Montmorillonite [27], and did not collapsing the diffraction to d-spacing of 14 A 0 in k-saturated and heat to 550 0 C treatment reveal that the degree of Mg-hydroxide interlayer filling was high.This agrees with results obtained by many studies [22,27], who have shown that the degree of Mg-hydroxide interlayer filling within interlayers of Montmorillonite is affects the properties of the new mineral, as they pointed out that with increasing the degree of Mg-hydroxide interlayer filling the properties of the new mineral were closer to chlorite mineral, and shows higher resistance to high temperatures .While the properties of the new mineral are close to the Montmorillonite at a low degree of filling of Mg-hydroxide interlayer, and its first order collapses at a temperature below 300 0 C .Also, results of

Conclusion
Many studies have been conducted to investigated the chemical and physical properties of rice soils, but few of them have been studied the mineralogical changes under different water regimes.In the current study we found that the irrigation methods used in irrigating the studied rice soils were affected on the transformations of clay minerals in these soils.Where the dry method of irrigation, through which occurs the process of successive wetting and drying cycles, was created a suitable conditions encouraged a chloritization phenomenon to occur in these soils.So, during these conditions, we assumed that Mg-hydroxide precipitated first and subsequently adsorbed in the interlayer space of montmorillonite through hydrogen bonding, and the whole process is happening by encouraging of subjected these soils to continuous wet and dry cycles .This agree with finding of [22], who hypothesized that the hydroxyl-Mg interlayers in montmorillonite were formed in slightly alkaline conditions.This would suggest that MgOH +1 as an intermediate step toward hydroxide formation can be important in the formation of interlayer, and this happened by effect of the exposure the soils to successive wetting and drying cycles.The differences in d-spacing values for diffraction of Montmorillonite mineral which have Mg-hydroxyl interlayer, may attributes to the degree of filling of Mg-hydroxyl interlayer within Montmorillonite interlayers.[26], indicated that the degree of filling and integration of Mg-hydroxide into Montmorillonite interlayers was highly effects on the intensity, stability, and d-spacing of Montmorillonite diffraction, when exposed to high temperatures in Ksaturated and heating to 350 0 C and 550 0 C.
Results in Fig. (3A) also, show the presence of 10.58A 0 reflection in airdried, Mg-saturated treatment and remaining without change in all treatments reflecting the presence of Mica minerals in this soil.Results in Fig. (3B) show the X-ray examination for clay sample at depth of 31-60 cm in Al-Najaf (wet) soil, which reveals almost the same mineralogical composition, as in the surface depth of this pedon, which was the presence of Mica, and Kaolinite minerals in clay fraction of this horizon .

BFigure 4 .BFigure 5 .
Fig. (4A) show the presence of 10.31A 0 reflection in air-dried, Mg-saturated treatment, and remaining without change in all treatments, reflecting the presence of Mica minerals in the clay fraction of this soil, while the appearance of second order of Mica at 5 A 0 with medium intensity confirmed the presence of Biotite and Muscovite minerals in this soil.Also, Kaolinite was diagnosed through presence of 7.32 A 0 IOP Publishing doi:10.1088/1755-1315/1259/1/0120077 reflection in treatments of air-dried, Mg-saturated , ethylene glycol salvation , and K-saturated with heat treatment to 350 0 C , and disappeared in K-saturated with heat treatment to 550 0 C .A X-ray diffraction peaks of clay fraction for A. Al -Diwaniyah (dry) 0 -30 cm and B. Al -Diwaniyah (dry) 31 -60 cm.Results of X-ray examination for 31-60 cm depth of Al-Diwaniya (dry) soil, are shown in Fig.(4B) which reveals the presence of 15.36 A 0 in air-dried, Mg-saturated treatment and shifts to an 17.80 A 0 reflection following ethylene-glycol salvation, and then collapsed to an 14.74 and 14.81A 0 reflections in K-saturated and heating to 350 and 550 0 C respectively.The discontinuous presence of the 15.36A 0 diffraction and collapsed to 14 A 0 in heating treatments confirmed to presence of incomplete Mghydroxyl-interlayer in Montmorillonite[27].Also, results of Fig.(4B)show the presence of 10.46 A 0 reflection in air-dried, Mg-saturated treatment, and remaining without change in all treatments, reflecting the presence of Mica minerals in the clay fraction of this soil, while the appearance of second order of Mica at 5 A 0 with low intensity confirmed the presence of Biotite mineral in this soil.The accompany diffraction of 15.15A 0 to the 17.80 A 0 diffraction in ethylene-glycol treatment, and then collapsed to 14.81 A 0 in K-saturated and heating to 550 0 C, confirmed to presence of real chlorite in this soil.Kaolinite was diagnosed through presence of 7.35 A 0 reflection in treatments of air-dried, Mg-saturated , ethylene glycol salvation, and K-saturated with heat treatment to 350 0 C , and disappeared in K-saturated with heat treatment to 550 0 C .Results of X-ray examination for 0-30 cm depth of Al-Diwaniya (wet) soil, which exploited by the cultivation of rice crop, and irrigated by the wet method, are shown in Fig.(5A) which reveals the presence of 15.08 A 0 in air-dried, Mg-saturated treatment and shifts to an 18.22 A 0 reflection following ethylene-glycol salvation, and then collapsed to an 15.07 and 14.61A 0 reflections in K-saturated and heating to 350 and 550 0 C respectively, confirmed to presence of Mg-hydroxyl-Montmorillonite.The presence of 10.42 A 0 and remaining without change in all treatments, reflecting the presence of Mica minerals in the clay fraction of this soil, while the disappeared of second order of Mica at 5 A 0 confirmed the presence of Biotite in this soil.Also, Kaolinite was diagnosed through presence of 7.31 A 0 reflection in treatments of air-dried, Mg-saturated , ethylene glycol salvation , and K-saturated with heat treatment to 350 0 C , and disappeared in K-saturated with heat treatment to 550 0 C .In the depth of 31-60 cm of Al-Diwaniya (wet) soil, results in Fig.(5B)show the presence of 15.62 A 0 in air-dried, Mg-saturated treatment and shifts to an 17.67 A 0 reflection following ethylene-glycol salvation, and then collapsed to an 15.07 and 14.15A 0 reflections in K-saturated and heating to 350 and 550 0 C respectively, and confirmed to presence of Mg-hydroxyl-interlayer in Montmorillonite[27].Also, results of Fig.(5B)show the presence of 21.34A 0 reflection in K-saturated and heated to 350 0 C treatment, which explains its existence in this soil in two ways: The first, this reflection is belong to (S-Ch) interstratified mineral due to precipitat of Mg-hydroxyl-interlayer in Montmorillonite interlayers.And the seconed explanation is that the presence of (S-Ch) interstratified mineral in this soil, may be due to the effect of the continuous waterlogging of this soil during the cultivation of the rice crop, which led to an effect on the structural composition of real chlorite and transformed it towards (S-Ch) interstratified mineral.Also, the presence of 10.71A 0 and remaining without change in all treatments, reflecting the presence of Mica minerals in the clay fraction of this soil, while the appearance of second order of Mica at 5 A 0 with low intensity confirmed the presence of Biotite mineral in this soil.Kaolinite was diagnosed through presence of 7.41 A 0 reflection in treatments of air-dried, Mg-saturated , ethylene glycol salvation, and K-saturated with heat treatment to 350 0 C , and disappeared in K-saturated with heat treatment to 550 0 C .A X-ray diffraction peaks of clay fraction for A. Al -Diwaniyah (wet) 0 -30 cm and B. Al -Diwaniyah (wet) 31 -60 cm.