Source of Cadmium (Cd) in Soils and Its Transfer to Rice and Vegetables: Karotia Union, Tangail

Cadmium is a trace element which is not essential for human being. High cadmium concentration in some rice samples in Bangladesh was reported than that of other countries. This study was carried out to find out the Cd concentration fertilizers, soils, rice and vegetables and along with other chemical properties of Karotia union, Tangail sadar upazila, Tangail. A total of 29 samples were collected among the 14 soils from 7 stations at 0-15 cm and 16-30 cm depth respectively, 5 vegetables, 5 rice and also 4 different countries TSP fertilizers from local market. The Cd concentration values of all soil samples were found between 0.97 to 1.73 mg/kg (0-15 cm) and 0.53 to 0.83 mg/kg (16-30 cm), respectively and the vegetable sample values were found between 0.053 mg/kg to 0.123 mg/kg (d.w.).The rice sample values were found between 0.05 mg/kg to 0.096 mg/kg (d.w.). The fertilizer sample values were found between 20.67 to 92.33 mg/kg. The soil pH values obtained 7.06 to 7.70 (0-15 cm) and 7.48 to 7.88 (16-30 cm) which indicated that the study area soils were neutral to moderately alkaline. The EC values of all soil samples were between 47.67 to 82.67dSm-1 (0-15cm) and 33.33 to 58.33dSm-1 (16-30 cm). The organic matter content of all soils ranged from 0.789 to 0.905% and 0.351 to 0.869% at (0-15 cm and 16-30 cm), respectively. The available sodium (Na) values of soils were found between 1.84 to 1.92 ppm (0-15 cm) and 1.83 to 1.90 ppm (16-30 cm), respectively. The available potassium values ranged from 1.81 to 1.96 ppm (0-15 cm) and 1.84 to 1.97 ppm (16-30cm), respectively. The available calcium values of all soil samples were 1.92 to 1.97 ppm (0-15 cm) and 1.91 to 1.96 ppm (16-30 cm), respectively.


Introduction
With the rapid progression of industry and increasing release of agrochemicals into the environment, the potential accumulation of heavy metals in agricultural soils has caused a growing public concern in worldwide about food security (Wong et al., 2002).The agricultural areas soil often serve as the major sink for heavy metals and that may release into the environment from different anthropogenic sources (Alloway, 1999).

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View Article Online phosphate fertilizers (Tirado and Allsop, 2012). Rice (Oryza sativa L.) contains protein and is ranked as the second highly consumed cereal in the world (Rabbani et al., 2015). In Bangladesh rice contains Cd concentration much higher than the international guideline standards (Meharg et al., 2013). It is alarming that the rice intake in Bangladesh as per capita is much higher compared to the most of the countries of the worldwide. Day by day in Bangladesh, kidney patients are increasing at an alarming rate which has been linked with the Cd accumulation in soils. Vegetables are the important food of daily diet because they are the good source of Vitamin A and C, iron, calcium, folic acid and fiber (Garg et al., 2014). Some reports showed low birth weight and severe mental retardation of newborn children during pregnancy when the mother was exposed to toxic amounts of a heavy metal through direct or indirect consumption of vegetables (Sharma et al., 2008).Keeping all these views in mind, we aimed to analyze the cadmium concentration in soils, vegetables, rice and fertilizers at Karotia union in Tangail, Bangladesh.

Material and Methods
Total 14 soil samples from 7 points were collected from 0-15 cm (topsoil) and 15-30 cm (subsoil) depth respectively by using soil core sampler at Karotia union in Tangail, Bangladesh. Five vegetables, rice and four TSP fertilizers sample were collected from different locations at karotia union in Tangail. The collected soil and fertilizer samples were dried in an oven at 70ᵒC until a constant weight is observed. After drying each sample, they were mixed thoroughly, crushed, sieved with a 2 mm nylon sieve and preserved in plastic pots for subsequent laboratory analysis.
The soil pH was measured by Digital pH meter (Hannah). The soil EC was measured by Digital EC meter. The total organic carbon (TOC) and percentage of organic matter (OM) content was analyzed titrimetrically according to Walkley and Black's wet oxidation method (Walkley and Black, 1934). The available Sodium (Na) was analyzed by Ammonium acetate extraction method by using Flame Photometer 766 nm (Huq and Alam, 2005). The available Potassium (k) and Calcium (Ca) were analyzed by Ammonium acetate extraction method by using Flame Photometer at 589 nm (Huq and alam, 2005). The total Cd in soils, vegetables, rice and fertilizers were analyzed by perchloric-nitric acid digestion methods by using AAS Shimadzu 7000 (Huq and Islam, 2005).

Soil pH
The soil pH depends on kinds of basic rock or parent materials. The pH values of all soil samples were found to be ranged in between 7.06 to 7.70 (0-15 cm) and 7.48 to 7.88 (16-30 cm)which indicate that study area soils are neutral and moderate basic or alkaline. The mean values of soil pH were recorded 7.36 and 7.69 at the top soils and subsoils, respectively. Figure 1(a) shows the highest value of pH 7.88 in agricultural (Tomato) land was found in the station 6 and the lowest pH value 7.06 in agricultural (Amaranths) was found in the station 1. Hence it can be speculated that soil of this zone is neutral to moderately basic. Bhuiyan (1988) reported that the pH of different soil series of Bangladesh ranged from 4.4 to 8.0 due to the variation of soil ecosystem mainly.The pH of Tejgaon, Sonatola and Khadimnagar were 5.2, 6.5, 5.8 and 5.9, respectively (Debnath, 2002).Thus the study depicts that the pH range is suitable for crop production and cultivation.

Soil EC
Soil Electrical Conductivity (EC) is an important indicator of soil health that affects crop yields, crop suitability, plant nutrient availability, and activity of soil microorganisms (Smith and Doran, 1996).The EC values of all soil samples were found to be ranged in between 47.67 to 82.67 dSm-1(0-15 cm), 33.33 to 58.33 dSm-1(16-30 cm) which indicates that study area soils are very high electrical conductivity. The mean values of soil EC were recorded 66.57dSm-1 and 48.80dSm-1at the top soils and subsoils, respectively. Figure 1(b) shows the highest value of soil EC 82.67 dSm-1 in agricultural (Amaranths) land was found in the station 1 and the lowest value of soil EC 33.33dSm-1 in agricultural (Radish) land was found in the station 2. The electrical conductivity of the Madhupur sal forest soil ranged from 6 to 57µS/cm (Gomes, 2005).

Total Organic Carbon (TOC)
Large amounts of soil organic carbon will enhance water retention, prevent leaching of soluble nutrients and maintain soil fertility (Fullen et al., 2007). The total organic carbonvalues of the samples were found to be ranged in between 0.458% to 0.525% in the agricultural soils (0-15 cm) and 0.204% to 0.504% in the soils (16-30 cm) depth. The mean values of total organic carbon were recorded at 0.499% and 0.409% at the topsoils and subsoils, respectively. Figure 1(c) shows the highest value of TOC 0.525% in agricultural (Potato) land was found in the station 5 and the lowest value of TOC 0.204% in agricultural (Rice) land was found in the station 4. The most agricultural soils of Bangladesh have low carbon content (Aliet al., 1997).

Soil Organic Matter (SOM)
A good soil should have at least 2.5% organic matter, but in Bangladesh, most of the soil has less than 1.5% and some

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View Article Online soil possess even less than 1% organic matter (BARC, 2005). The Soil Organic Matter values of the samples were found to be ranged in between 0.789 to 0.905% in the agricultural soils (0-15 cm) and 0.351 to 0.869 % in the soils (16-30 cm). The mean values of soil organic matter were recorded at 0.860 % and 0.705% at the topsoils and subsoils, respectively. Figure 1(d) shows the highest value of SOM 0.905% in agricultural (Potato) land was found in the station 5 and the lowest value of SOM 0.351% in agricultural (Rice) land was found in the station 4. The level of SOM accumulation depends basically on tillage methods and residue management practices (Kong etal., 2009).

Available Sodium (Na)
Sodium is an essential element for plant growth and also high levels of sodium cause plant and microbial problems and very high levels cause soil sterilization and plant damage or death (Cantisano, 1997).The available sodium (Na) values of all soil samples were found to be ranged in between 1.84 to 1.92 ppm (0-15 cm) and 1.83 to 1.90 ppm (16-30 cm). The mean values of available sodium were recorded 1.91 and 1.92 ppm at the top soils and subsoils, respectively. Figure 2(a) shows the highest value of available sodium 1.97 in agricultural (Spinach) land was found in the station 7 and the lowest value of available sodium 1.83 in agricultural (Rice) land was found in the station 4.The available sodium (Na) content of Chorfasion 6.25 meq/100g soil and Asasuni 6.14 PAPER

Available Potassium (K)
With increasing depth, the potassium percentage usually remains about the same or increases in soils of initially uniform the parent material (Black, 1957).The available potassium (K) values of all soil samples were found to be ranged in between1.81 to 1.96 ppm (0-15 cm) and 1.84 to 1.97 ppm (16-30 cm). The mean values of available potassium (K) were recorded 1.91 and 1.92 ppm at the top soils and subsoils, respectively. Figure  2(b) shows the highest value of available potassium 1.97 in agricultural (Potato) land was found in the station 5 and the lowest value of available potassium 1.81 in agricultural (Cauliflower) land was found in the station 3. The available potassium(K) content of highland, medium highland,and medium lowland under the Madhupur upazila values 0.21ppm, 0.13 ppm,and 0.16 ppm, respectively (SRDI, 2001a).The available potassium (K) level 0.20% to 1.04% represents the considering agricultural fields were fertile (Rahman et al., 2014).

Available Calcium (Ca)
Calcium is a highly mobile element in slight acid soils and it quickly leaches out of soils low in organic matter and microbial activity (Cantisano, 1997). The available calcium (Ca) values of all soil samples were found to be ranged in between 1.92 to 1.97 ppm (0-15 cm) and 1.91 to 1.96 ppm (16-30 cm). The mean values of available calcium (Ca) were recorded at 1.94 and 1.93 ppm at the top soils and subsoils, respectively. Figure 2(c) shows the highest value of available calcium 1.97 in agricultural (Rice) land was found in the station 4 and the lowest value of available calcium 1.91 in agricultural (Amaranths) land was found in the station 1. Ideal limit of the percentage of total calcium (Ca) is four categories such as low (<3.0), medium (3.1-6.0), high (6.1-7.5) and very high (>7.5) (BARC, 2005).

Cadmium Concentration in Soil
Trace elements can be mobilized from arid soils through plant uptake and erosion/leaching processes, but these soils usually contain higher contents of trace elements than other soils (Kabata-Pendias and Pendias, 2010).The Cd concentration values of all soil samples were found to be ranged in between 0.97 to 1.73 mg/kg (0-15 cm), 0.533 to 0.833 mg/kg (16-30 cm). The mean values of Cd concentration were recorded 1.285 and 0.695 mg/kg at the top soils and subsoils, respectively. Figure 3 shows the highest value of Cd (1.73 mg/kg) in agricultural (Spinach) land was found in the station 7 and the lowest value of Cd (0.53 mg/kg) in agricultural (Potato) land was found in the station 5. The total soil Cd concentration of about 0.20 mg/kg in the topsoils (0-7.5 cm) of 86 non-agricultural sites in New Zealand (Roberts et al., 1994).

Cadmium Concentration in Vegetables
The concentrations of trace minerals in vegetables may vary depending on the inherent and environmental conditions of plants and animals (Pennington and Calloway, 1973). The Cd concentration values of all vegetable samples were found to be ranged in between 0.053 to 0.123 mg/kg. The mean values of Cd concentration in vegetables were recorded at0.075 mg/kg. Figure 4 shows the highest value of Cd concentrations in vegetables 0.123 mg/kg in (Spinach) land which was found in the station 2 and the lowest value of Cd concentration in vegetables were 0.053 mg/kg in (Amaranths and Cauliflower) land which was found in the station 1. In a farmland of Tianjin, more than 60% of the vegetable samples had Cd concentrations above the standard level 0.05 mg/kg (Wang, 2012).

Cadmium Concentration in Rice
Heavy metal contamination in a rice field is an important problem directly posing a serious health risk for human beings (Cao et al., 2010). The Cd concentration values of all rice samples were found to be ranged in between 0.053 to 0.096 mg/kg. The mean values of Cd concentration in rice were recorded at0.065 mg/kg. Figure 5 shows the highest value of Cd concentrations in rice 0.096 mg/kgwhich was found in the station 5 and the lowest value of Cd concentration in rice were 0.053 mg/kg which was found in the station 1 and 3.The Cd levels over 260 rice samples across 12 districts of Bangladesh in the range of <0.005-1.31 mg/kg, with the mean of 0.099 mg/kg (Meharg et al., 2013).

Cadmium Concentration in TSP Fertilizer
Excessive and continuous uses of phosphorous fertilizers for decades have converted the agricultural soils into virtual chemical time bombs (Dissanayake et al., 2009).The Cd concentration values of all TSP fertilizer were found to be ranged in between 20.67 to 92.33 mg/kg. The mean values of Cd concentration in fertilizers were recorded 53.46 mg/kg. Figure 6 showsthe highest value of Cd concentrations in fertilizer 92.33 mg/kg which were imported from Tunisia and the lowest value of Cd concentration in fertilizer were 20.67 mg/kg which produce in Bangladesh.The Cd concentration in Triple Superphosphate (TSP) fertilizer was 6.74 mg/kg (Atafar et al., 2010).

Conclusion
The Cd concentration in phosphate fertilizers imported from Tunisia was found the highest among the phosphate fertilizers sold in market Tangail. Among the plant species, spinach was observed to contain high Cd concentration which even exceeded the permissible limits of WHO. The cadmium concentration in rice was observed 0.053 mg/kg d.w. to 0.096 mg/kg d.w. which is below the standard level of FAO/WHO. As the people in Bangladesh generally eat large amount of rice daily, thus this might cause higher daily Cd intake by individuals. The Cd concentration values of all soil samples were 0.97 to 1.73 mg/kg (0-15 cm) and 0.533 to 0.833 mg/kg (16-30 cm) which is above the standard level of EPA.The Cd concentration values of all vegetable samples were found between 0.053 to 0.123 mg/kg. The Cd concentration values of all fertilizer samples were found 20.67 to 92.33 mg/kg. If we consider a single crop in a year and consider 80 kg/ha application rate of Phosphate fertilizer and calculate which shows that 15.70 to 70.17 tons Cadmium is applied in agricultural lands in every year in Bangladesh. As continuous application of phosphate fertilizers increase the Cd concentration in soils and eventually to plants, we need to find out the alternative source of commercial phosphate fertilizers.