Cd accumulation and human health risk assessment of rice in high background areas of heavy metals: A case study of Nagu Town, Huize County, Yunnan Province
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摘要:
调查发现，重金属高背景区存在土壤中的镉含量超标，而农作物中的镉含量并未超标的现象，但对其原因知之甚少。开展区内农作物镉积累研究并探寻制约农作物镉吸收的因素，对土壤环境质量评价和污染土地生态修复都具有重要意义。以云南省会泽县娜姑镇娜姑坝子为研究区，采集了水稻籽实和配套土壤样品各41件，采用ICP-MS方法检测了Cd含量。通过偏相关、R型聚类、逐步线性回归等分析了水稻镉与土壤元素（指标）之间的相关关系。结果显示，水田土壤镉（Cd）含量为 0.467~1.87 mg/kg，平均为 0.78 mg/kg，是全国土壤背景值的 5.69 倍，58.5% 的样品超过农用地土壤污染风险筛选值。水稻（糙米）镉含量为 0.007~0.062 mg/kg，平均 0.016 mg/kg，均低于食品安全国家标准限值。水稻镉与土壤铀（U）、钼（Mo）等呈显著正相关，与土壤有机碳（TOC）、硫（S）等呈显著负相关，与土壤 Cd 总量和酸碱度（pH）无显著相关性。土壤镉超标而水稻镉不超标的现象可能与土壤富含有机碳有关，该地区土壤有机碳（TOC）含量达全国背景值的5.57倍。当 TOC > 3% 时，水稻镉含量迅速下降。据此提出土壤镉环境等级的有机碳修正方法，将研究区3.42 km2（
5129 亩）Ⅱ等水田修正为Ⅰ等。此外，还提出了一些土壤养护和修复的建议。Abstract:A study comparing cadmium (Cd) accumulation in soil and rice in an area of high background heavy metal concentrations in Huize County, Yunnan, showed that, occasionally, the cadmium content in soil exceeds the standard, whereas the cadmium content in crops remains within the standard. However, our understanding of the reasons for this phenomenon is limited. It is of great significance for soil environmental quality evaluation and ecological restoration to study cadmium accumulation in crops and its control factors in these areas. In this study, 41 rice seed samples and 41 corresponding soil samples were collected from the Nagubazi, Nagu Town, Huize County, Yunnan Province, and their cadmium content was determined by ICP-MS. The correlation between rice cadmium and soil elements (indices) was analyzed by partial correlation, R-type clustering, and stepwise linear regression. The results showed that the soil cadmium content ranged from 0.467 to 1.87 mg/kg, with an average of 0.78 mg/kg, which is 5.69 times that of the national soil background value. Moreover, 58.5% of the soil samples exceeded the soil pollution risk threshold for agricultural land. In contrast, the cadmium content of rice (brown rice) ranged from 0.007 to 0.062 mg/kg, with an average of 0.016 mg/kg, which is lower than the national threshold for food safety.The content of cadmium in rice was significantly positively correlated with the content of U and Mo , and significantly negatively correlated with soil organic carbon (TOC) and the content of S in soil, but not significantly correlated with soil cadmium content and pH value. The phenomenon that soil cadmium exceeds the standard but rice cadmium does not exceed the standard may be related to the enrichment of organic matter in the soil. The content of soil TOC is 5.57 times that of the national background value. When TOC is greater than 3%, cadmium content in rice decreases rapidly. Therefore, the organic matter correction method for soil cadmium environmental grade evaluation was proposed, and the 3.42 km2 (5129 Mu) of paddy field originally classified as Class Ⅱ in the study area was revised to Class Ⅰ. In addition, some suggestions on soil protection and remediation were put forward.
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Key words:
- cadmium /
- soil-rice system /
- soil organic carbon /
- high background of heavy metals /
- Huize, Yunnan
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表 1 水稻和土壤分析测试指标、方法及检出限
Table 1. Analysis items, methods and detection limits of the rice and soil
样品类型 分析指标 指标代号 分析方法 单位 检出限 水稻 镉 Cd 等离子质谱法（ICP-MS） mg/kg 0.002 土壤 镉 Cd 等离子质谱法（ICP-MS） mg/kg 0.02 银 Ag 发射光谱法（ES） mg/kg 0.02 铅 Pb X射线荧光光谱法（XRF） mg/kg 2 铀 U 等离子质谱法（ICP-MS） mg/kg 0.05 钼 Mo 等离子质谱法（ICP-MS） mg/kg 0.1 有机碳 TOC 重铬酸钾容量法 % 0.05 全碳 TC 高频红外碳硫仪法 % 0.1 氮 N 酸碱滴定容量法 mg/kg 15 硫 S 容量法 mg/kg 20 氯 Cl X射线荧光光谱法（XRF） mg/kg 15 溴 Br X射线荧光光谱法（XRF） mg/kg 0.8 碘 I 催化比色法（COL） mg/kg 0.3 三氧化二铝 Al2O3 X射线荧光光谱法（XRF） % 0.01 酸碱度 pH pH计电极法（ISE） 1 
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表 2 研究区土壤地球化学特征统计参数
Table 2. Statistical parameters of soil geochemistry in the study area
指标名称 指标代号 单位 含量值范围 平均值（ 
）中位值 标准差 变异系数 全国土壤背景值 （X0A） 与全国土壤
背景值之比（
/X0A）镉 Cd mg/kg 0.467~1.87 0.78 0.71 0.30 0.42 0.137 5.69 银 Ag mg/kg 0.057~0.457 0.12 0.10 0.08 0.80 0.077 1.56 铅 Pb mg/kg 18.9~298 57.5 37.6 54.9 1.46 22 2.62 锌 Zn mg/kg 72.7~441 166 146 72.49 0.50 66 2.52 铀 U mg/kg 1.59~4.53 2.88 2.69 0.62 0.23 2.5 1.15 钼 Mo mg/kg 0.434~2.13 1.10 1.05 0.34 0.32 0.7 1.57 三氧化二铝 Al2O3 % 8.04~17.75 13.9 13.9 1.72 0.12 11.9 1.17 碘 I mg/kg 0.79~2.83 1.48 1.39 0.45 0.33 1.1 1.35 有机碳 TOC % 1.95~6.18 3.34 3.08 0.95 0.31 0.6 5.57 全碳 TC % 2.66~11.33 4.53 4.07 1.76 0.43 1.3 3.48 氮 N mg/kg 1902~ 6567 3393 3219 960 0.30 707 4.80 硫 S mg/kg 317~ 2464 669 653 336 0.51 245 2.73 氯 Cl mg/kg 62~154 90.5 82.9 25.2 0.30 78 1.16 溴 Br mg/kg 1.86~5.27 3.61 3.47 0.95 0.27 2.2 1.64 酸碱度 pH 5.84~8.13 7.26 7.44 0.68 0.09 8 0.91 注：全国土壤背景值数据来源于王学求等（2016）。
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表 3 研究区水稻镉含量特征统计表（含量单位：mg/kg）
Table 3. Statistical parameters of rice cadmium in the study area (content unit: mg/kg)
样品数 水稻 Cd 土壤 Cd
平均值水稻 Cd /土壤 Cd 含量值范围 平均值 中位值 标准差 变异系数 41 0.007~0.062 0.016 0.013 0.012 0.75 0.78 0.21
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表 4 研究区水稻镉含量与土壤元素（指标）相关、偏相关系数
Table 4. Correlation and partial correlation coefficients between rice cadmium and soil elements (indices) in the study area
控制
变量U Mo Ag Pb Zn Al2O3 Cd pH I N S TC Cl Br TOC 无 0.52** 0.48** 0.44** 0.42* 0.35* 0.14 0.29 －0.04 －0.22 －0.23 －0.29 －0.29 －0.30 －0.37* －0.39* U 0.24 0.18 0.20 0.22 －0.21 －0.11 －0.12 －0.43** －0.05 －0.41** －0.11 －0.21 －0.21 －0.18 TOC 0.35* 0.42** 0.35* 0.37* 0.37* －0.09 0.30 －0.04 －0.20 0.10 －0.26 －0.03 －0.11 －0.23 注：“**”表示显著水平（< 0.01）；“*”表示显著水平（< 0.05）；当无控制变量时，为Pearson相关系数。
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表 5 逐步线性回归过程及参数
Table 5. Stepwise linear regression process and parameters
步骤 变量/常量 非标准化系数 标准化系数 τ 值 显著性* 系数 标准误差 1 常量 －2.362 0.138 －17.132 <0.001 Usoil 0.176 0.047 0.515 3.754 0.001 2 常量 －0.796 0.468 －1.700 0.097 Usoil 0.164 0.041 0.480 3.946 <0.001 lgSsoil －0.549 0.158 －0.422 －3.466 0.001 3 常量 0.472 0.578 0.817 0.419 Usoil 0.256 0.047 0.751 5.432 <0.001 lgSsoil －0.800 0.162 －0.615 －4.927 <0.001 Al2O3soil －0.060 0.019 －0.490 －3.193 0.003 4 常量 0.048 0.560 0.085 0.932 Usoil 0.272 0.044 0.799 6.173 <0.001 lgSsoil －0.682 0.157 －0.524 －4.336 <0.001 Al2O3soil －0.051 0.018 －0.421 －2.909 0.006 lgIsoil －0.472 0.178 －0.290 －2.644 0.012 注：“*”表示显著水平 < 0.05 时，变量入选模型。
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表 6 土壤镉环境等级修正方法
Table 6. Correction method of soil cadmium environmental grade
修正条件 修正前等级 修正后等级 土壤污染风险 土壤养护及修复建议 土壤有机碳（TOC）含量
> 3%Ⅲ Ⅱ 污染风险可控 通过施用农家肥、秸秆还田等方法，确保土壤有机碳含量不降低。 Ⅱ Ⅰ 污染风险低 Ⅰ Ⅰ 污染风险低 土壤有机碳（TOC）含量
≤ 3%维持原有评级 污染风险等级
不变增加农家肥、有机肥等施用量，以提高土壤有机碳含量，同时监测农作物镉含量。
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