| A large number of studies have shown that the application of biochar can significantly reduce the bioavailability of heavy metals in soil,but it is affected by soil properties,the characteristics and application amount of biochar,crop types,the types of heavy metals,the degree of heavy metal pollution and other factors.Therefore,it is important significance to quantify and evaluate effect of biochar application on availability of heavy metals in soil and heavy metals uptake by plants for the remediation of heavy metal contaminated soil by biochar.We selected relative independent 203 studies about soil heavy metal(Cd?Pb?Cu and Zn)remedied by biochar as of May 2018.By using a meta-anaysis method,the effect of soil properties,biochar characteristics,application rates of biochar,and crop types on the availability of heavy metal in soil,the absorption of heavy metals by plants and physical and chemical properties of soil were quantitatively analyzed and evaluated.,In addition,on the basis of meta-anaysis results,the effects of three typical factors on the availability of Cu and Cd in soil were compared.The main results of the meta-anaysis are as follow:(1)The application of biochar can reduce the available Pb Cu and Zn in soil by 39.26%,59.40%,47.26% and 39.97% respectively.Sandy soil is beneficial to the reduction of available Cd and Cu,while loamy soil is beneficial to the reduction of available Pb and Zn.The availiable Cd,Pb,Cu and Zn decreased significantly in acidic soils.Among all the biochar types,the sludge derived biochars were the most effective one in reducing available Cd in soils(78.98%);the shell and residue derived biochars were the most effective one in reducing available Pb and Zn in soils(70.21% and 59.01%);the manure derived biochars were the most effective one in reducing available Cu in soils(57.23%).The reduction of available Cd,Pb,Cu and Zn in soil was enhanced with increased biochar dosage.(2)The application of biochar reduced acid soluable Cd,Pb,Cu and Zn in the soil by 18.84%,15.58%,28.04% and 23.65%,respectively,and increased the residual Cd,Pb and Cu by 35.98%,18.72% and 23.48%,respectively.(3)The application of biochar can significantly reduce the content of Cd,Pb,Cu and Zn in plant,with an average decrease of 39.16%,30.72%,19.78% and 28.16%,respectively.The decrease of Cd,Pb and Cu in grain crops is higher than that in economic crops,while the decrease of Zn in grain crops is lower than that in economic crops.(4)Application of biochar can significantly increase the biomass of economic crops and grain crops.The increase of economic crop biomass is generally higher than that of grain crop biomass,with the average increase of 52.65% and 23.07%,respectively.(5)The application of biochar can significantly increase the soil pH,soil total organic carbon,cation exchange capacity and soil microbial biomass carbon.The soil pH,total organic carbon and cation exchange capacity presented an overall increasing trend with increased biochar dosage.(6)The results of the orthogonal experimental design were as follows: Among the three factors(biochar doage,soil pH and plant planting),the plant planting has the largest influence on the effective Cu and Cd contents in the upper soil,while the soil pH has the largest influence on the effective Cu and Cd contents in the lower soil.In the upper soil,the optimal treatment of reducing available Cu and Cd soil was that 5% biochar was applied to soil with a pH of 7.11,and planted by pakchoi.In the lower soil,the optimal treatment was that 2.5% biochar applied to soil with a pH of 6.47,and planted by alfalfa.Application of biochar in soil with different pH can significantly increase the biomass of pakchoi and alfalfa,and reduce the content of Cu in pakchoi and alfalfa. |
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