Effect Of Long-term Lime On Cd-contaminated Soil Quality And Cadmium Accumulation In Rice And Wheat Issuses

Cd in soil can be easily assimilated by plants through the food chain and threaten human health.Therefore,it is necessary to restore Cd contaminated to ensure the security of grain.The Chengdu plain is the main producing area of rice and wheat,and paddy-upland rotation is the main farming pattern.With the development of industry,Cdcontamination in the area is becoming more and more serious.The accumulation of Cd in rice and wheat grains is high,which is detrimental to food safety.Therefore,reducing the heavy metal content in rice and wheat grain in Cd contaminated soil ensuring the safety of grain production has become the focus of scientific research workers.Recently,there are many methods to restore Cd-contaminated soil.There are many types of remediation techniques,such as physical technologies including soil solidification,electrodynamics,soil exchange etc.chemical techniques,i.e.Chemistry,leaching,solvent extraction,soil performance improvement,as well as microbial and phytoremediation technology.Among those remediation technologies,in-situ soil restoration has become a research hotspot due to its high efficiency and low price,etc.Lime application is a traditional method of acid soil amelioration.Because lime can reduce the acidity of the soil and increase the pH of the soil,it can significant reduce the toxicity of heavy metals to crops.In order to explore effective treatment measures for Cd moderate-contaminated soil in Chengdu Plain,in this study,we a three-year rice-wheat rotation experiment was conducted with a control(A)and 4 lime treatments at different dosages,referred as B,C,D and E.To explore the effects of long-term application of lime on the physical and chemical properties of soil,the availability of trace elements,Cd speciation,and Cd accumulation in rice and wheat grains which benefit to reasonably use lime to restore Cd-contaminated soil and to ensure food safe production,providing scientific date.The main results are as follows.(1)The application of lime significantly reduced the availability of Cd and reduced the Cd accumulation in rice and wheat grains.The application of lime could increase the yield of rice and wheat within a certain dosage.Compared with the control,D and E reduced the accumulation of Cd in rice grain and decreased to 0.12 mg/kg and 0.08 mg/kg,respectively.D and E reduced the Cd content of wheat grain to 0.097 mg/kg and 0.075mg/kg,respectively,which meet the national standard of"national standards for food safety-the limits of contaminants in food"(GB 2762-2012).Lime addition benefit to the safety of grain production.By consideration of the Cd accumulation the yield of rice and wheat,the addition rate should be higher than 2250 kg/hm~2 per season,which could make sure the safety of production in Cd moderate-contaminated soil.(2)The content of available microelements is very important for crop growth.The results showed that the content of available Fe,Mn,Zn and Mo in soil significant decreased with the increase of lime dosage,indicating a linear correlation.Compared with the control,the decline range was 26.7%~75.5%,63.2%~80.2%,41.4%~51.0%,and16.1%~35.5%,respectively,with significant different(P<0.05);but there was no significant difference in available Cu and B content among the treatments.It shows that soil available Fe,Mn,Zn and Mo are sensitive to different amounts of lime.Therefore,lime can be used to restore the acid soil polluted by heavy metals.The microelement should be supplemented to ensure crop production.(3)The application of lime can improve soil acidity and increase soil acidification buffering capacity.The pH value of the treatments A,B,C,D and E were 5.3,6.4,7.4,8.1and 8.4,respectively,consequently,the soil become alkaline.The effective P content in soil increased by 59.6%with the increasing of lime application compared with the control.The soil available K,organic matter and CEC increased first and then decreased with the increasing of the amount of lime application.Compared with the control,the content of available K increased by 31.4%and then decreased by 11.1%,the content of organic matter increased first by 9.90%and keep consenst as control,the content of CEC increased by 5.27%and slightly lower than that of the control.The content of TN and AN in soil did not change significantly as comparison with the control,;the dosages of lime application has insignificant effect on TN and AN.The lime addition benefit to the formation of residue Cd by the decrease of exchangeable Cd in soil with a decrease of the proportion of exchangeable Cd from 44.1%to 29.0%and with a increment of the proportion of residual Cd from 18.8%to 34.3%.OxidizableCd content remains a lower level,which has little effect by the lime addition at different dosages.According to the national second soil survey nutrient classification standards,the lime addition has insignificant effect on the changes in soil nutrients by grade when the lime dosage was lower than 3000 kg/hm~2 per season.In this study,the amount of lime application is in the range of 1500 kg/hm~2~3000 kg/hm~2,which is benefit to reduce soil acidity as well as soil nutrients.Above all,a grey correlation analysis was used to analyze the different lime application rates to evaluate the soil-plant system comprehensively in order to select the best application amount.Twenty-three were selected as the evaluation system.The grey relation degree of each treatment is obtained by selecting ideal model,dimensionless,absolute deviation calculation and grey relational system calculation.The grey correlation coefficient shows that by consideration of the accumulation of Cd in plant issues,available Cd and soil fertility,the lime application at a dosage of 3000 kg/hm~2 is more suitable for the system in the soil with a pH was 5.3 and Cd content was 0.7600 mg/kg.