| 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 and34.87～1775.10 mg/kg,respectively,and the exceedance rates were 90.36%,54.22%and48.19%,respectively.The content of available Cd is 0.12～7.61 mg/kg.There is a serious situation of heavy metal pollution.Soil p H was 5.19～7.72,CEC was 2.06～35.38cmol~+/kg,EC was 19.60～875.00μs,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),p H 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 p H(p<0.01).The prediction equations of Cd content of the four crops were constructed based on the available of soil Cd and p H,and the R~2reached above0.84 in all cases.Different soil available Cd thresholds corresponding to different p H conditions were deduced,and the Cd threshold increased with increasing p H.When the soil p H was 6.5,the soil available Cd(DTPA extracted state)thresholds were wheat(0.44mg/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 QX2strain revealed that QX2 strain multiplied rapidly and could secrete alkaline substances and tolerate p H 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 reached71.74%and increased the p H of the solution significantly(p<0.01).The blocking effect of different treatments(control(Control),application of QX2bacterial 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%,p H 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 p H 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 p H 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 p H 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. |
