Contents of cadmium in soil and rice in Heilongjiang region
The cadmium content in 110 soil samples collected is shown in Table 2-4, between 0.061~0.225mg/kg, the average value was 0.122 mg/kg, and the coefficient of variation was 0.280% <11%. Within the range of the coefficient of variation allowed, and according to the Chinese national standard GB 15618-2018 (soil environmental quality - agricultural land risk control standards), the cadmium content in the soil in the five regions did not exceed the acceptable range. Hence, the risk of cadmium pollution in agricultural land in the study area was low. The content of cadmium in the soil was in the order: Chahayang> Xiangshui> Wuchang> Fangzheng> Jiansanjiang.
The content of cadmium in brown rice and polished rice in 110 rice samples collected was between 0.0003-0.0610 mg/kg and 3.96×10-6 to 0.056, with the average values of 0.007 mg/kg and 0.004 mg/kg, respectively. The coefficients of variation were 1.470% (brown rice) and 2.009% (polished rice), both within the range allowed by GBT27404-2008 "Laboratory Quality Control Specification Food Physical and Chemical Testing".In addition, according to GB 2762-2017 "Food Safety National Standards for Contaminants in Foods" the standard content of cadmium is below 0.2 mg/kg. None of the 110 samples of rice samples determined in this experiment exceeded this limit. The content of cadmium in brown rice ranged in the order: Chahayang> Xiangshui> Fangzheng> Jiansanjiang> Wuchang. The order of cadmium content in polished rice was consistent with that in brown rice.
The average value of cadmium in soil in this study was 0.122 mg/kg, which does not exceed the limit specified in the China's soil environmental quality standard. It is higher than the content of cadmium in soil of Heilongjiang area (0.096mg/kg) reported in 2012[34]. The content of cadmium in the soil of Wuchang area was 0.124, which was lower than the cadmium content in the soil of Harbin area reported by Wang (2011) [35]. The cadmium content was much higher in Chahayang than the other four regions, but the cadmium content in the soil was not much different in the five regions.
Studies have shown that the absorption and accumulation of heavy metals in rice is greatly affected by genetic background, cultivar type and heavy metal interaction [36] (Lin 2018). The varieties with high Se accumulation showed a tendency to inhibit the accumulation of heavy metal Cd [37] (Li 2003). In addition, some studies found that the cadmium content in rice was decreased by the optimal zinc content in soil [38] ( Zhang 2015).
The single rice variety (rice flower) and soil background in Wuchang area are the main factors leading to the difference between Wuchang and other regions. The cadmium content in rice in the two areas of Jianshuanjiang is also different by the cadmium content in paddy soil. In the process of absorption and accumulation of heavy metals, rice is not only affected by heavy metal content in soil, but also affected by other factors. Such as rice varieties, soil microbial content, precipitation, air quality and so on.
Analysis of the significance of differences between different regions
The variance analysis of five areas of polished rice, brown rice and soil was carried out by SPSS statistics 12.0. The results are shown in the figure below.
It can be seen from Fig. 1 (a) that the content of cadmium in soils in Wuchang, Chahayang and Jiansanjiang was significantly different. In addition, there was a significant difference in the content of cadmium in soils in Chahayang and Fangzheng areas and also in soils in Fangzheng and Xiangshui areas and between Xiangshui and Jiansanjiang. The differences between Wuchang and Fangzheng and between Chahayang and Xiangshui were not significant.
It can be seen from Fig. 1 (b) that the content of cadmium in brown rice was significantly different between Wuchang and Chahayang, Fangzheng, Xiangshui, and Jiansanjiang as well as between Chahayang and Fangzheng, Xiangshui and Jiansanjiang. There was no significant difference between Xiangshui and Jiansanjiang in the content of cadmium in brown rice.
It can be seen from Fig. 1 (c).There were significant differences in the content of cadmium in polished rice between Chahayang and Wuchang, Fangzheng, Xiangshui, and Jiansanjiang. The differences in cadmium content in polished rice between Wuchang and Fangzheng, Xiangshui and Jiansanjiang were not significant.
In summary, the differences in cadmium content in soil, brown rice and polished rice in the study area were not consistent. The rice varieties have an effect on the absorption and accumulation of cadmium by rice, and some agricultural factors such as pesticides, fertilization and irrigation may also greatly affect absorption of cadmium by rice. Some studies have shown that natural conditions such as precipitation and CO2 concentration [39] also have an impact on absorption and accumulation of cadmium in rice.
Soil-rice system migration model of Cd element
The absorption and accumulation of heavy metals in rice is not only affected by the total metal content in the soil, but also by the physical, chemical and biological properties of the soil. Many researchers have studied the factors affecting the absorption of metal elements in rice, including soil pH [40,41], organic matter [42,43], redox potential [44], salinity [45], and phosphorus content [46]. Soil pH is an important factor in controlling heavy metal absorption [47,48]. Table 5 shows the physical and chemical properties of soils in the study area.
The present paper studied the factors affecting absorption of cadmium in the “soil-rice system” and proposed the best fitting model for predicting the content of cadmium in rice. A multivariate regression model of arsenic content in rice was established by using soil pH and organic matter. The multiple regression equation was shown in Table 6. There was a significant negative correlation between the content of cadmium in rice and the concentration of cadmium in soil (P<0.05). The content of cadmium in Chahayang rice was positively correlated with soil pH (P<0.05), and it was significantly related to soil cadmium concentration The negative correlation (P<0.05); the content of cadmium in Xiangshui rice was significantly positively correlated with the concentration of cadmium in soil (P<0.05). The partial coefficient between cadmium content and soil pH in Jiansanjiang rice was not significant. Therefore, the best prediction model for the Jiansanjiang area was based on the concentration of cadmium in soil.
The content of cadmium in rice in Chahayang, Wuchang and Xiangshui areas could be predicted well by the concentration of cadmium in soil. The pH value of soil could predict the content of arsenic in rice in Chahayang area. However, in the established regression model, the content of cadmium in rice in Fangzheng and Jiansanjiang areas was not significantly correlated with soil cadmium content, pH and organic matter. Therefore, the prediction models of these two regions have yet to be worked out. Dudka [49] etal. (1996) reported that the relationship between heavy metals in rice and soil could be described by three models: linear (constant distribution model), plateau model (saturated) and Langmuir model. The metal adsorption also followed a linear model in the range of low metal concentrations in the soil.
In the present study, soil samples collected from paddy fields contained relatively low levels of cadmium. By fitting and comparing the three models, it was found that the linear model was the best fitting model. Therefore, the linear model is used for fitting. The R2 value of the fitted model was between 0.256 and 0.468 (Table 5). The D-W index was close to 2, the autocorrelation of the independent variables was not obvious, and the model design was good . Dudka etal.[49] reported R2 values of 0.94 and 0.92 for the correlation between, respectively, Cd and Zn contents in barley grains and Cd and Zn contents in soil. McBride [50] found similar correlation coefficients. In the present study, the correlation coefficient was lower than the correlation coefficients (<0.9) of the previous studies. These above-mentioned studies were carried out in pots or small experimental field; so, the soil properties changed little, if at all, during the modeling process. Hence, the model established under these conditions had a higher degree of fit. In the present study, the rice and soil samples were collected under natural field conditions. Paddy soils are a complex system. In addition to the variability in total metal content and pH value of soil, other soil properties may also affect the availability of heavy metals, potentially weakening the model fit between metal accumulation by rice and the soil metal content and pH.
Evaluation of cadmium pollution in rice grains and health risk assessment of intake
According to formulae (1) and (2), the results of heavy metal pollution assessment in the study area were obtained (Tables7-8). The single factor pollution index evaluation was less than 1 (Table 7), indicating that the five areas of the study were not polluted by Cd (the proportion of pollution-free soil in each region was 100%). The comprehensive rice pollution index in the study area was 0.153 (Table 8), which was non-polluting; the comprehensive pollution index of each region was in the order Chahayang> Fangzheng> Xiangshui> Jiansanjiang> Wuchang.
Due to the different geographical locations, and the differences in economic development level and industrial structure distribution, there are expected differences in the content of Cd in rice in different regions. The areas studied in this paper represent the geographically-protected rice products in Heilongjiang Province. The results reported in this paper did not exceed the values specified in the health risk index, and 100% of rice in the five regions was non-contaminated. This is despite extensive economic development in the five regions studied in recent years, including the construction of farm towns, usage of pesticides and fertilizers, agricultural irrigation, and automobile exhaust gases, representing the main sources of heavy metals.
According to the results of heavy metal carcinogenic risk assessment (Table 9), the average intake (ADD) of Cd in adults and children was lower than the reference exposure dose (RfD). Hence, the individual health risk index was less than 1, indicating the amounts of daily intake of cadmium would not be considered a health risk to humans. The order of rice intake influencing the health risks to adults and children was Chahayang> Fangzheng> Xiangshui> Jiansanjiang> Wuchang. Comparing the results of individual pollution assessment of rice in the study area, the evaluation results of Wuchang, Jiansanjiang and Chahayang were completely consistent. The pollution index of Wuchang was the smallest of all areas, indicating Wuchang was not affected by Cd.
The average Cd carcinogenic risk index of adults in different regions of each region was Wuchang 0.34×10-4, Chahayang 5.61×10-4, Fangzheng 1.08×10-4, Xiangshui 1.26×10-4, Jiansanjiang 0.56×10-4; the average health risk index of children was Wuchang 0.52×10-4, Chahayang 8.69×10-4, Fangzheng 1.67×10-4, Xiangshui 1.95×10-4,Jiansanjiang 0.87×10-4. The risk index values of Chahayang, Fangzheng and Xiangshui were higher than the USEPA recommendation. The maximum acceptable level was 1×10-4,and there is a risk of cancer; The risk index values of both Wuchang and Jiansanjiang were lower than the maximum acceptable level recommended by USEPA (1×10-4). The health risk of Fangzheng and Xiangshui was on the edge of the acceptable risk range for humans, and the risk of cancer was low.
There was a certain deviation between the Cd carcinogenic risk index values and the evaluation results of rice single factor pollution in the study area. In the single factor pollution assessment, the pollution index of the Fangzheng area was higher than that of Xiangshui, and the opposite was true in the cancer risk assessment, which is mainly related to the original level of heavy metals. Cadmium is a harmful element upon accumulation. It has obvious toxic effects on human nervous and reproductive systems, and is a heavy metal element with a high carcinogenic risk.