农田镉（Cd）污染一直是我国农业生产面临的严峻环境问题之一，我国陆续颁布了三版《环境质量标准》用以规范农田重金属的安全阈值，但由于复杂的土壤环境以及不同植物镉富集能力的差异性，现行土壤镉阈值仍存在很大的局限性。本文针对来源于我国6个地区的83份原位农田镉污染土壤，研究了四种农作物的土壤镉阈值。同时，探究了镉固化菌株QX2阻控作物镉积累的效应，以期为安全合理利用中轻度镉污染农田提供理论依据，研究结果如下：

83份原位农田镉污染土壤采集自我国江苏南京、湖南株洲、广西桂林、广东韶关、河南新乡和天津等地区。其中单一镉污染土壤占26.14％，镉、铅和锌复合污染土壤占73.86％。土壤镉、铅和锌含量分别为0.19 ~ 12.22 mg/kg、10.18 ~ 1264 mg/kg和34.87 ~ 1775 mg/kg，超标率分别为90.36％、54.22％和48.19％。有效态镉为0.12 ~ 7.61 mg/kg。土壤pH为5.19 ~ 7.72，阳离子交换量（CEC）为2.06 ~ 35.38 cmol+/kg，有机质（SOM）含量为4.58 ~ 69.73 g/kg，电导率（EC）为19.60 ~ 875.00 μs，速效磷（AP）、全氮（TN）含量丰富，速效钾（AK）含量较少，土壤质地类型多为壤土。层次聚类分析和Pearson相关性分析结果表明，土壤中镉、铅和锌污染程度具有高度相似性（p<0.01）。土壤pH值与重金属含量呈负相关。

为研究不同农作物的土壤镉阈值，本文采用盆栽实验分别种植了小麦（Triticum aestivum L.）、青菜（Brassica rapa var. chinensis L.）、杨花萝卜（Raphanus sativus L. ）和胡萝卜（Daucus carota L. var. Sativa Hoffm）等四种作物。结果表明，可食部位镉含量超标率分别为小麦（72.34％）、青菜（46.81％）、杨花萝卜（55.56％）和胡萝卜（61.11％），污染土壤中种植四种作物的安全性高低为青菜>杨花萝卜>胡萝卜>小麦。四种作物不同部位的镉积累情况为小麦（籽粒<稃<秸秆）、青菜（茎叶<根）、杨花萝卜（块根木质部<块根韧皮部<须根<茎叶）和胡萝卜（块根<茎叶）。相关性分析表明，12项土壤理化指标中，与作物可食部位镉含量相关性最高的是土壤有效态镉，呈显著正相关（p<0.01），与pH呈显著负相关（p<0.01）。基于土壤有效态镉与pH构建四种作物镉含量的预测方程，R2均达到了0.84以上。推导出在不同pH条件下对应的不同土壤有效态镉阈值，镉阈值随pH升高而升高。如当土壤pH为6.5时，土壤有效态（DTPA提取态）镉阈值分别为小麦（0.44 mg/kg）、青菜（1.15 mg/kg）、杨花萝卜（0.61 mg/kg）和胡萝卜（0.55 mg/kg）。

本文从江苏南京栖霞山镉污染农田土壤中，分离纯化了一株耐镉且具有促生性质的芽孢杆菌菌株Bacillus sp.QX2。盆栽条件下施加QX2菌液可以使青菜生物量增加21.10％，镉含量降低26.60％，土壤有效态镉降低16.41％。对QX2菌株生理生化特性的测定发现，QX2菌株繁殖速度快，且能分泌碱性物质，可以耐受5 ~ 11的pH范围。镉最小抑菌浓度（MIC）为0.506 mmol/L。0.1 mmol/L氯化镉溶液培养下，QX2与3％生物炭（BC）对镉的去除率达71.74％，且使溶液pH显著升高（p<0.01）。

通过盆栽试验研究了不同处理（对照（Control）、施加QX2菌液（QX2）、施加3％稻壳生物炭（BC）、施加QX2菌液和3％生物炭（QX2+BC））对小麦、青菜、杨花萝卜和胡萝卜镉积累的阻控效果。结果表明，相比于对照，三种施加处理后四种作物生物量增加了6.27 ~ 52.89％，可食部位镉含量降低了9.87 ~ 44.29％，其中QX2+BC处理组的青菜和杨花萝卜达到食品安全标准。四种作物的光合速率、叶绿素含量、可溶性糖含量等也有不同程度地提高，同时叶片中丙二醛（MDA）含量和抗氧化酶活性降低。土壤有效态镉降低了6.70 ~ 32.16％，pH值提高了0.12 ~ 0.38，土壤脲酶活性也显著增加（p<0.05）。三种施加处理中，QX2+BC处理组的促生效果和对作物镉积累的阻控效果最好。

综上所述，在12种土壤理化指标中，有效态镉与pH是决定作物镉积累的关键土壤因素，无污染或低污染的偏碱性土壤更符合农作物安全种植的要求。降低土壤pH是修复及安全利用重金属污染土壤的有效手段。QX2菌株联合生物炭作用的镉固定化修复技术，一方面能提高土壤pH，显著降低土壤有效态镉，能有效降低作物的镉积累量，使其符合食品安全标准。另一方面能显著提高作物生物量，增加经济效益。为提高农作物安全性以及酸性轻度镉污染农田的“边修复边利用”，提供了重要的理论依据。

Cadmium(Cd) contamination in agricultural fields has been one of the serious environmental problems facing agricultural production in China, and China has promulgated three editions of Environmental Quality Standards to regulate the safety thresholds of heavy metals in agricultural fields, but the existing soil Cd thresholds still have great limitations due to the complex soil environment and the variability of Cd enrichment capacity of different plants. In this paper, soil Cd thresholds for four crops were investigated for 83 in situ Cd-contaminated soils from six regions in China. Meanwhile, the effect of Cd-immobilizing strain QX2 in blocking Cd accumulation in crops was investigated to provide a theoretical basis for the safe and reasonable use of medium to light Cd-contaminated farmland, and the results of the study are as follows.

83 in-situ Cd-contaminated farmland soils were collected from Nanjing, Jiangsu, Zhuzhou, Hunan, Guilin, Guangxi, Shaoguan, Guangdong, Xinxiang, Henan and Tianjin, China. The contents of Cd, Pb and Zn are 0.19 ~ 12.22 mg/kg, 10.18 ~ 1263.63 mg/kg and 34.87 ~ 1775.10 mg/kg, respectively, and the exceedance rates were 90.36%, 54.22% and 48.19%, respectively. The content of available Cd is 0.12 ~ 7.61 mg/kg. There is a serious situation of heavy metal pollution. Soil pH was 5.19 ~ 7.72, CEC was 2.06 ~ 35.38 cmol+/kg, EC was 19.60 ~ 875.00 μs, SOM content was 4.58 ~ 69.73 g/kg, AP and TN were abundant, AK content was low, and the soil texture type was mostly loamy. The results of hierarchical cluster analysis and Pearson correlation analysis showed that the degree of Cd, Pb and Zn contamination in the soil was highly similar (p<0.01), pH was negatively correlated with heavy metal content.

To study the soil Cd thresholds of different crops, wheat (Triticum aestivum L.), cabbage (Brassica rapa var. chinensis L.), radish (Raphanus sativus L.) and carrot (Daucus carota L. var. Sativa Hoffm). The exceedances of Cd in edible parts of the four crops were wheat (72.34%), cabbage (46.81%), radish (55.56%) and carrot (61.11%), and the safety of growing the four crops in contaminated soil was cabbage > radish > carrot > wheat. The correlation analysis of 12 soil physicochemical indicators with Cd accumulation in crops showed that the highest correlation between Cd content in edible parts of the four crops and the available of Cd in soil was significant positive correlation (p<0.01) and a significant negative correlation with pH (p<0.01). The prediction equations of Cd content of the four crops were constructed based on the available of soil Cd and pH, and the R2 reached above 0.84 in all cases. Different soil available Cd thresholds corresponding to different pH conditions were deduced, and the Cd threshold increased with increasing pH. When the soil pH was 6.5, the soil available Cd (DTPA extracted state) thresholds were wheat (0.44 mg/kg), cabbage (1.15 mg/kg), radish (0.61 mg/kg) and carrot (0.55 mg/kg), respectively.

In this paper, a Cd-tolerant and probiotic strain of Bacillus sp. QX2 was isolated and purified from Cd-contaminated farmland soil in Qixia Mountain, Nanjing, Jiangsu, China. application of QX2 bacterial solution under pot conditions increased the biomass of cabbage by 21.10%, reduced the Cd content by 26.60%, and reduced the available of soil Cd by 16.41%. Determination of physiological and biochemical characteristics of QX2 strain revealed that QX2 strain multiplied rapidly and could secrete alkaline substances and tolerate pH range of 5 ~ 11. The Cd MIC of QX2 was 0.506 mmol/L. The removal of Cd by QX2 with 3% biochar (BC) under 0.1 mmol/L Cd chloride solution incubation reached 71.74% and increased the pH of the solution significantly (p<0.01).

The blocking effect of different treatments (control (Control), application of QX2 bacterial solution (QX2), application of 3% biochar (BC), application of QX2 bacterial solution and 3% biochar (QX2+BC)) on Cd accumulation in wheat, cabbage, radish and carrot was investigated by pot experiments. The results showed that compared with the control, the biomass of the four crops increased by 6.27 ~ 52.89% after the three applied treatments, and the Cd content in edible parts decreased by 9.87 ~ 44.29%, among which the cabbage and radish in the QX2+BC treatment group met the food safety standard. Photosynthetic rate, chlorophyll content, and soluble sugar content of the four crops also increased, while MDA and antioxidant enzyme activities decreased. Available Cd decreased by 6.70 ~ 32.16%, pH increased by 0.12 ~ 0.38, and soil urease activity also increased significantly (p<0.05). Among the three applied treatments, the QX2+BC treatment group showed the best growth-promoting effect and deterrent effect on crop Cd accumulation.

In conclusion, among the 12 soil physicochemical indicators, available Cd and pH are the key factors determining Cd accumulation in crops, and unpolluted or low-polluted alkaline soils are more compatible with the requirements for safe crop cultivation. Reducing soil pH is an effective means to remediate and safely utilize heavy metal contaminated soil. Cd immobilization remediation technology by QX2 strain combined with biochar can, on the one hand, increase soil pH and significantly reduce soil Cd in its active state, which can effectively reduce Cd accumulation in crops and make them meet food safety standards. On the other hand, it can significantly improve crop biomass and increase economic benefits. It provides an important theoretical basis for improving the safety of crops and the "use while remediation" of acidic and lightly Cd-contaminated farmland.

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