| Remediation of heavy metal contaminated farmlands,especially safe utilization of polluted soils of light to moderate levels,is an important measure to respond to the national ecological civilization construction strategy and to ensure food security.Among those heavy metal pollutants in farmlands,cadmium is one major concern,which has the highest ratio of sites exceeding standard,high toxicity,and could be easily uptaken by a stable food in China–rice.Common safe utilization techniques for light/moderate contaminated soil includes:in-situ passivation,foliar fertilizer,and microbid-remediation technology,etc.These techniques all have strong stability,good environmental compatibility,and could be applied while planting.However,the exact application effect depends on the actual field data.The present study assessed the long-term field application effect of one pre-developed silic passivant.Besides,in order to deal with current complex source and status of soil pollution,soil passivation measure was integrated along with foliar fertilizer or microbio-remediation technology to explore their application effect in field scale.This study aimed to assess the reduction effect on cadium in rice by several integrated technologies,and to select suitable integrative technique（s）for soil remediation in Pearl River Delta.This study is of great significance toward providing data support for establishment of safe utilization technology system for heavy metal contaminated soils in China.The main results are as follows:（1）As frequently used soil remediation materials,the application effects and longevities of silic passivants need to be assessed.This study evaluated the efficiency and longevity of our pre-developed silic amendment in regulating cadium phyto-availability in an acid cadium contaminated soil under field conditions in Zhangshi,Shaoguan.The results indicated that the experimental amendment could significantly increase soil p H（p<0.05）,and reduce cadium accumulation in rice grain（p<0.05）.In the first year of application,the decrease range of cadium in rice grain were 21%～56%.Cadium concentrations in rice grain after remediation measures were all below the national food safety standards of the rice cadmium.In the second year of application,the decrease range of cadium in rice grain were 15%～35%.According to the short-term application effect,the relative low addition amount of amendment(1500 kg·hm-2)would achieve safe productuin purpose.However,larger quantity of amendment(>3000 kg·hm-2)would be needed for long-term remediation.In summary of two-years field experiment,the longevity of the studied silic amendment lasted for at least two seasons,however,the immobilization efficiency decreased by 7～17 percent in the second season.（2）During the application of passivators in cadmium contaminated soils,soil nutrients were immobilized simultaneously due to the lackness of specific selectivity of generally-used passivators,which may have a negative effect on plant growth.Field experiments were conducted to investigate the effect of single/combined application of silicic passivator,Fe modified mudstone,and growth-promoting microbial inoculum on cadium accumulation in different parts of rice plant,as well as on plant growth.The results indicated that all treatment,except for single microbial inoculum application,could reduce cadium concentrations in all parts of rice,and meanwhile promote rice growth.cadium concentrations in rice grain after these remediation measures were all less than the national food safety standards of the rice cadmium 0.2 mg·kg-1.In treatments with microbial inoculum,soil available phosphorus increased significantly.By comprehensive consideration of remediation and growth promoting effects,an integrative technique of alkalescent modified mudstone and growth-promoting microbial inoculum is an effective passivation method and suitable for extensive application in Pearl River Delta.（3）In order to deal with current complex source and status of soil pollution,field experiments were conducted to investigate the effect of integrative remediation measures of foliar fertilizer and different passivation materials on cadium accumulation in rice.The results indicated that‘foliar fertilizer’had no significant impact on cadium accumulation in different parts of rice plant.The integrative techniques‘foliar fertilizer+silicon passivation’,‘foliar fertilizer+lime’,‘foliar fertilizer+hydroxyapatite’could reduce cadium concentrations in all parts of rice,while had no significant effect on rice yields.The decreased range in root cadium were 23.1%～33.5%,in straw cadium were 37.5%～64.3%,and in rice grain were 25.2%～48.8%.Cadium concentrations in rice grain after integrative remediation measures were all less than the national food safety standards of the rice cadmium 0.2 mg·kg-1.‘Foliar fertilizer+silicon passivation’and‘foliar fertilizer+lime’measures significantly reduced cadium availability（BCF）,while‘foliar fertilizer’and‘foliar fertilizer+hydroxyapatite’measure had no significant effect.All integrative remediation treatments could reduce cadium transfer from root to rice grain,however,this decrease effects were not significant.By comprehensive consideration of remediation effect,additive amount and potential long-term effect,‘foliar fertilizer’is a recommended remediation technique for farmland soils with light level of cadium risks.However,to deal with complex soil pollutions,a systematic measure of‘foliar fertilizer+silicon passivation’is an effective passivation method and an eco-friendly integrative technology of safe utilization. |
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