| Heavy metal pollution in agricultural soils in China is currently one of the key problems,which seriously restricts the sustainable development of agricultural economy and the ecological civilization.Chemical immobilization,which has an outstanding effect on heavy metal mitigation for agricultural soils,has been widely applied in recent years.However,there are many types of amendment materials with various effects,thus it is difficult to choose the optimal one in practical applications.Moreover,there is little quantitative and comprehensive evaluation on the effects of different amendment materials on the bioavailability of heavy metals.There is also lack of research to quantify the effects of amendments on soil physical and chemical properties,and the bioavailability of heavy metals under the interaction of multiple factors.This study firstly synthetized 234 publications and 3116 observations across the globe.And meta-analysis method was used to quantitatively and systematically explore the comprehensive effects of amendment materials(clay mineral,calcium silicon material,phosphorous material,industrial by-products,organic material,biochar and their composites)on soil physical and chemical properties(p H,SOC and CEC,etc.),availability of heavy metals(Cd,Pb,Cu,Zn,As)and the metal uptake by plants.Meanwhile,the main variables that affecting the effect of immobilizing amendments,such as soil p H,SOC,CEC,trial type and duration,application rate,plant type and etc.,were used to perform sub-group analysis to determine the sources of heterogeneity and key variables.The main findings were as follows:(1)Meta-analysis showed that amendments application increased the content of soil physical and chemical propreties to various degrees(p<0.05),among which the increase of soil p H,SOC and CEC was 11.1%,32.8% and 38.6%,respectively.The increasing effect of amendments on soil p H,SOC and CEC was influenced by the annual average rainfall,trial type,soil texture,amendment type and application rate.Industrial byproducts had the greatest increase effect for soil p H,while organic materials had the greatest increase in SOC and CEC.Under the conditions of sandy soil,acidic soil,higher SOC and low rainfall,amendments application had positive effect on reducing heavy metals.(2)The effect of immobilizing amendments on the bioavailability of heavy metals was restricted by soil conditions,trial conditions,amendment types and application rate,crop types and other factors.Among them,soil p H,SOC,soil texture,amendment type and plant type were the key variables.The mean decreases of soil available Cd,Pb,Cu and Zn contents after amendments application were 36.6%,40.8%,17.2% and 16.5%,respectively.Futhermore,the contents of Cd,Pb,Cu,Zn and As in plants significantly reduced with an average decreases of 38.7%,42.8%,34.1%,37.6% respectively.Although the effect of amendments on soil available As was not significant,the concentration of As in plants was significantly reduced by 34.2% on average.(3)Composite materials and industrial by-products had the best effect on the reduction of soil available Cd and Pb,while silicon-calcium substances and industrial by-products had the best effect on the reduction of Cu and Zn.Clay minerals and silicon-calcium substances could significantly reduce the available As.Silicon-calcium substances,composite materials and industrial by-products had a good effect on reducing Cd,Pb,Cu and Zn in plants.Composite materials on reducing As in plants was the most prominent(68.3%)and significantly higher than other materials,while phosphorus materials significantly increased As(20.8%).In addition,the effect of phosphorus materials on reducing Pb in plants was also relatively prominent.(4)Correlation analysis showed that soil available Cd,Pb,Cu and Zn was significantly negative correlation with soil p H,and significantly positive correlation with SOC(p<0.05).Soil available As was positively correlated with soil p H and SOC,but not significantly(p>0.05).Soil CEC was significantly positive correlated with available Cd,Cu and Zn(p<0.05),while positively correlated with available Pb and As but not significantly(p>0.05).Take Cd as an example,there was a very significant negative correlation between soil p H and available Cd under the application of industrial by-products,biochar,silicon-calcium substances,clay minerals,organic and composite materials(p<0.01),and the correlation between SOC,CEC and available Cd under different amendment materials was quite different.There was a significant positive correlation between soil heavy metal availability and plant uptake under biochar,industrial by-products organic and composite materials. |
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