General characteristics of soil in different sampling sites
A table for the general soil characteristics at different times in the sampling sites was given in Table 2 where pH and EC were almost statistically similar for most of the sampling sites (p<0.05) before cultivation and after harvesting. pH ranged from 7.04 to 7.93 which indicated almost neutral to alkaline in character and EC ranged from 4.05 to 4.40 dS/m that represented the moderate saline soil. However, organic carbon was comparatively lower in every soil sample and ranged from 0.34% to 1.15% but a significant difference was observed among the variables (p<0.05). In most cases, SOC was improved after harvesting.
Heavy metal analysis
According to fig. 2, the highest concentration of Ni was found in Gutudia (94.52±8.04 mg/kg) for the initial soil before cultivation. On the other hand, the soil after harvesting, the highest concentration was observed in Mechaghona (93.71±8.17 mg/kg) and Dhamalia showed the lowest concentration (37.76±2.87 mg/kg). This may due to irrigation water contained more Ni concentration (0.13 mg/L) in Koiya rather than other sites (Fig. 5). According to Barałkiewicz et al., 1999, the solubility of nickel in soils increases with increasing Ni concentration in irrigation water that supports the study [15]. Following fig. 2, the highest concentration of Cu was reported in Koiya (37.5±4.08 mg/kg) for the initial soil before cultivation. In soil after harvesting, the highest concentration was also notified in Koiya (36.47±2.17 mg/kg). This may due to irrigation water contained more Cu concentration (0.144 mg/L) in Koiya rather than other sites (fig. 5). According to the World Health Organization [4], soil containing Cu more than 100 mg/kg can be exposed to contaminate the soil. But in Bangladesh, no permissible limit has yet been set for Cu in agricultural soil [5]. However, in the investigated areas, irrigated soil loaded more Cu concentration than reference soil which is a matter of rising concern.
As per fig. 3, the highest concentration of As was seen in Koiya (8.64±0.20 mg/kg) for the initial soil before cultivation, and again, the soil after harvesting, the highest concentration was also noticed in Koiya field (8.47±0.18 mg/kg). This may due to irrigation water contained more As concentration (0.032 mg/L) in Koiya rather than other sites (fig. 5). Most important thing is that the World Health Organization [4] reports that soil containing As more than 20 mg/kg can be exposed to contaminate the soil. But in Bangladesh, no permissible limit has yet been set for As in agricultural soil [5]. However, in the investigated areas, most irrigated soils loaded less As concentration than reference soil which is a good indication for farming. As load may happen due to the use of As contaminated water and As-enriched fertilizers, as well as pesticides, for irrigation in the agricultural land [16, 17]. From Fig. 3, the highest concentration of Pb was found in Mechaghona (19.33±1.70 mg/kg) for the initial soil before cultivation. On contrary, the soil after harvesting, the highest concentration was observed in Gutudia (19.73±1.84 mg/kg). This may due to irrigation water contained more Pb concentration (0.032 mg/L) in Gutudia rather than other sites (fig. 5). According to the World Health Organization [4], soil containing Pb more than 100 mg/kg can be exposed to contaminate the soil. But in Bangladesh, no permissible limit has yet been set for Pb in agricultural soil [5]. However, in the investigated areas, most irrigated soils loaded more Pb concentration than reference soil which is a matter of rising concern.
Fig. 4. explained that the highest concentration of Mn was pointed in Koiya (372±16.34 mg/kg) for the initial soil before cultivation but in the soil after harvesting, the highest concentration was reported in Mechaghona (478.45±16.33 mg/kg) This may due to irrigation water contained more Mn concentration (0.42 mg/L) in Mechaghona rather than other sites (Fig. 4). According to the World Health Organization [4], soil containing Mn more than 2000 mg/kg can be exposed to contaminate the soil. But in Bangladesh, no permissible limit has yet been set for Mn in agricultural soil [5]. However, in the investigated areas, some irrigated soils loaded more Mn concentration than reference soil which is also a rising concern.
Heavy metal status in irrigation water samples
Fig. 5 stated that irrigation water contained the highest amount of Mn (0.1 to 0.4 mg/L) rather than any other metals. According to WHO/FAO [4], Mn content should be less than 0.2 mg/L for using water in irrigation. In that sense, irrigation water in Mechaghona sampling sites exceeded the recommended value. Though the Pb contents of different sampling sites were not higher (0.03 to 0.65 mg/L), Gutudia and Koiya sampling sites lived in borderline of risk according to WHO/FAO [4], guideline (0.065 mg/L). On the other hand, Cu values (0.06 to 0.14 mg/L) exceeded WHO/FAO [4], recommendation value (0.017 mg/L) in all sampling sites that is another concern. In terms of Ni and As, all sampling sites represented the optimal values as compared with recommended values by WHO/FAO [4].
Pearson correlation coefficient among heavy metals and physiochemical properties of soil
Pearson correlation coefficient among heavy metals and physiochemical properties of soil are presented in Table 3. Significant positive correlation was observed between pH-As (0.86*), pH- Pb (0.97**), Ni-As (0.19*) and As-Pb (0.70*). In addition, significant negative correlation was also noticed between pH-EC (-0.89*), EC-Mn (-0.91**), EC-Cu (-0.97**). Other’s correlations were found to be non-significant which means concentration of one element may not influence other elements concentrations in the studied area.
Soil pollution and risk index of heavy metal assessment
Soil pollution index (SPI) and Ecological risk (ER) status were presented in Table 4. According to the table, the SPI values of Mn before cultivation varied from 4.77 to 6.19 which indicated moderate to high soil pollution but the SPI values of Mn after harvesting ranged from 5.95 to 7.10 which indicated high soil pollution. The ER value of Mn was the highest in Dhamalia soil after harvesting (84.07) which recommended considerable ecological risk. It was also noticeable that all other sampling sites were presented the moderate ecological risk. This may due to irrigation water contained more Mn than other metals. The SPI values of Cu before cultivation varied from 1.78 to 3.62 which pointed slightly to moderate soil pollution but the SPI values of Mn after harvesting ranged from 2.37 to 3.53 which represented mild to moderate soil pollution. So, soil pollution is critically enhanced after cultivation and irrigation. The ER value of Mn was the highest in Koiya soil before cultivation (18.12) which indicated low ecological risk. However, all other sites also presented a low ecological risk. The SPI values of As before cultivation varied from 4.03 to 4.68 and the SPI values of As after harvesting ranged from 4.01 to 4.92 which specified moderate soil pollution. So, soil pollution is critically remained the same before and after cultivation and irrigation. The ER value of As was the highest in Mechaghona soil before cultivation (49.20) which indicated a moderate ecological risk. However, all other sites also presented the moderate ecological risk. This may due to sampling sites contained As bearing minerals. The SPI values of Ni before cultivation varied from 2.51 to 3.85 which showed mild to moderate soil pollution and the SPI values of Ni after harvesting ranged from 3.17 to 3.54 which symbolized moderate soil pollution. So, the soil pollution was increased after cultivation and harvesting especially in Dhamalia and Mechaghona soil because before cultivation SPI was mild in both fields which converted to moderate SPI after harvesting. The ER value of Ni was the highest in Gutudia soil after harvesting (21.22) because Ni content in irrigation water of Koiya field was higher than other fields. In terms of Pb, the SPI values were lower than other metals except Koiya field. ER values also indicated that all sampling sites presented a low ecological risk. This may due to irrigation water contained the lowest Pb concentration. According to Table 5, all studied area represented moderate risk of pollution in both stages (before cultivation and after harvesting). This is a great matter of concern because agricultural soils are loaded by heavy metals day-by-day thus can be a potential cause for human health deterioration due to carcinogenic effect through food ingestion, grown this soils.
Heavy metal assessment in the grain samples
As per Table 6, As concentration in all samples were crossed the permissible limit [18]. The highest As content was observed in Gutudia grains (0.53±0.05 mg/kg). Transfer factors of As were ranged from 0.037 to 0.115. According to Hojsak et al [19], the permissible limit of As in rice grains is 0.15 mg/kg. So, the sampling sites are now at great risk of As. Although the Mn content was still below the permissible limit, its transfer factor was quite higher (0.059 to 0.156). In terms of Ni, Cu, and Pb, metal concentrations in grain samples were not exceeded the permissible limit recommended by WHO/FAO [4]. But the transfer factor of Ni ranged from 0.071 to 0.148 and the TF of Cu ranged from 0.04 to 0.136. So, it might be a concern in the future. However, a higher transfer factor means a higher risk of metal exposure. The lowest TF was found for Pb that ranged from 0.002 to 0.008 which is a good indication for Pb accumulation in rice grain.