Effects Of Organic Remediation Agents On The Remediation Of Cd Polluted Soil And Rape Growth Under High Nitrogen Conditions

As the primary pollutant affecting soil quality,cadmium is an environmental problem that needs to be solved urgently.Especially in the case of excessive nitrogen fertilizer,the adsorption and accumulation of cadmium by plants will increase to varying degrees,which brings huge risks to vegetable production and human health.In-situ passivation technology based on organic repair agents has received extensive attention in recent years because of its low cost,convenient operation,and obvious effects.This study firstly explored the effect of high nitrogen on the growth and cadmium uptake of rape through hydroponic experiments.Then,using spent mushroom substrateas（SMS）the main raw material to study the effects of remediation agent combinations on the remediation of cadmium-contaminated soil,the effects of ammonia oxidizing microorganisms（AOA and AOB）during nitrification and rape growth under high nitrogen conditions.The main research results are as follows:（1）The increase of nitrogen concentration will promote the increase of nitrate transporter and cadmium content in rape.The relative expression of nitrate transporter（NRT1.1）increased with the increase of nitrogen concentration,and NRT1.1 in high nitrogen was 15.31%,which was53.57%higher than that under medium and low nitrogen conditions.Under cadmium stress,NRT1.1 in low nitrogen,medium nitrogen and high nitrogen decreased by 18.68%,19.28%,12.24%,and NRT2.1 decreased by 14.78%,15.91%,25.71%respectively.The cadmium content of rape under high nitrogen conditions was 1.81mg·kg^-1,while under medium nitrogen and low nitrogen conditions the content were 1.40 and 0.96mg·kg^-1,respectively.（2）The combination of organic remediation agents has a better effect on reducing the available cadmium content of the soil and increasing the nutrients such as organic matter in the soil.The soil simulation remediation test shows that SMS can reduce the available cadmium content of the soil,and the content was reduced by 44.80%.The combination of SMS and alkaline materials had a better repair effect.Cd content was reduced by 48.32%-66.58%with the combination with biochar,and pH value was increased by 0.78 to 1.2 units,the soil enzyme activity was increased by5.74%-47.29%.Lime reduced the available cadmium content by 63.10%-66.47%,the pH value increased by 0.86 to 1.33 units,and the enzyme activity increased by 3.28%to 37.86%.The best combination of organic remediation agent is SMSB3（0.5%SMS+2%biochar）,the available cadmium in the soil can be reduced to 0.20mg·kg-1,and the organic matter content can be increased to 16.98%（28.54%higher than SMS）.（3）SMSB3 can reduce the proportion of exchangeable cadmium and prevent soil acidification caused by nitrification.The results of the remediation test of cadmium-contaminated soil under high nitrogen conditions showed that the pH of the soil with(NH₄2)SO₄decreased to5.92-6.07,and the pH decreased to the lowest when the nitrogen concentration was 0.8mg·kg^-1,and the exchangeable cadmium content accounted for 43.83%.The gene copy numbers of ammonia oxidizing archaea（AOA）and ammonia oxidizing bacteria（AOB）are 4.26×10⁷and6.15×10⁶.After the addition of SMSB3,the exchangeable cadmium content decreased to 14.00%,while the residual cadmium increased to 25.67%,the pH increased to 6.83-6.98,the highest AOB could be increased to 3.65×10⁸,and the AOA decreased to 0.11×10⁷.The pH of the soil added with Ca（NO₃）₂increased to 7.17-7.24 after remediation,when the nitrogen concentration was 0.1mg·kg^-1the pH was highest,and the exchangeable cadmium decreased to 6.67%,and the residual cadmium accounted for up to 51.33%.（4）SMSB3 can reduce the cadmium content in the edible part of rapeseed and promote its growth.The exchangeable cadmium content in potting soil with a nitrogen concentration of 0.1mg·kg^-1was the lowest.After restoration,exchangeable cadmium accounts for 6.67%and residual cadmium accounts for 52.50%,which was 5.48%-6.43%higher than non-potting soil.Without the addition of SMSB3,the cadmium content in all rapes exceeded the limit value of the national food safety standard(0.2mg·kg^-1),and the cadmium content increased with the increase of nitrogen concentration.Rape grown in soil with Ca（NO₃）₂had the most cadmium,which can reach 0.32mg·kg^-1.Rape grown in soil with added（NH₄）₂SO₄had lower cadmium,in the range of 0.23-0.28mg·kg^-1.After the addition of SMSB3,the cadmium in rapes were reduced to below the food safety standard,which was reduced by 40.00%at most.